The ancestry of Homo floresiensis

Did ‘hobbit people’ of Flores evolve from Homo erectus or a more primitive hominin?

Homo floresiensis is an extinct Late Pleistocene hominin species known only from the Indonesian island of Flores. The type specimen LB 1 is a diminutive 30-year-old female who stood just 1.06 m (3 ft. 6 in.) tall. Nicknamed ‘Flo’, she had a cranial capacity initially estimated to be just 380 cc, comparable to that of an australopithecine. Her weight was estimated to be somewhere between 16 and 36 kg (35 and 79 lb.). Yet she was apparently human: she lacked the large back teeth of an australopithecine, the proportions of her facial skeleton were those of a human, and she appeared to be a humanlike fully-committed biped (Brown, et al., 2004; Morwood, et al., 2004).

The extremely small cranial capacity has been hotly debated since the species was first described in 2004. Some have claimed that Flo was a modern human suffering from microcephaly, a developmental disorder leading to a smaller brain (Jacob, et al., 2006; Martin, et al., 2006), but the majority reject this view and recognise Homo floresiensis as a new human species with a long, low cranial vault and other features characteristic of archaic humans (Argue, et al., 2006; Falk, et al., 2005; Falk, et al., 2007; Tocheri, et al., 2007; Lyras, et al., 2008).

Two principle theories have emerged as to the origin of these hominins. The first is that Homo floresiensis was a dwarf form of Homo erectus (approximate cranial capacity 1,000 cc) which underwent a dramatic reduction in size as a result of a phenomenon known as insular dwarfism. Animals living on an island where food is relatively scarce and predators are few or absent will ‘downsize’ over many generations, in order to reduce calorific requirements. What is actually happening is that evolution is favouring the smaller offspring in each generation. If predators do not pose a threat, any advantages in being large will be outweighed by poorer fuel-economy. What has to be questioned is whether insular dwarfism could lead to brain size reduction of the extent seen in Homo floresiensis.

The second theory is that Homo floresiensis is derived from a hominin species more primitive and smaller-brained than Homo erectus such as Homo habilis (approximate cranial capacity 600 cc) or even an australopithecine (approximate cranial capacity 400 cc). This second model implies that Homo erectus was not the first hominin species to leave Africa, contrary to the widely-accepted Out of Africa 1 hypothesis.

Two studies, one published in 2010 and the other earlier this year, have focussed on decreases in brain size. The first study considered the decreases in body mass and brain size that have been documented for a number of other primate lineages. Researchers tested putative ancestors for Homo floresiensis against these, using the high, medium and low estimates of its body mass. Results suggested that Homo erectus is only feasible as an ancestor for Homo floresiensis if the low estimate of 16 kg (35 lb.) is accepted. For the medium estimate of 24 kg (53 lb.), Homo habilis or the Dmanisi hominins are more feasible as ancestors. The high estimate is not compatible with any proposed scenario (Montgomery, et al., 2010).

The second study re-evaluated the brain size of Homo floresiensis using micro-CT scanning, and obtained an upwardly-revised estimate of 425 cc. This revised figure means that the degree of brain size reduction in relation to body mass is less, and therefore easier to explain. Nevertheless, if Homo erectus was the ancestor, the decrease in brain size is still too great to be explicable solely as a scaling downwards as body mass is downsized, and some other factor must have come into play. In an environment where food is scarce, and given that brain tissue is ‘expensive’ in metabolic terms, further reduction in brain size might have been advantageous. However, this could not be achieved without some loss of cognitive ability. The authors of the report suggested that cognitive abilities comparable to Homo habilis might have sufficed in an island habitat lacking dangerous predators (Kubo, et al., 2013).

While this ‘dumbing down’ scenario cannot be dismissed, it seems implausible. According to the widely-accepted ‘social brain hypothesis’ (Byrne & Whiten, 1988), the large primates of primates evolved in response to a need to predict the likely future social behaviour of their fellows, and base relationships upon these predictions. While Homo floresiensis would not have had to face dangerous predators, individuals would still need to interact with other group members. It is difficult to see that being less smart than one’s fellows could be anything over than a severe disadvantage, regardless of other circumstances.

I would therefore be inclined to the view that Homo erectus was not the ancestor of Homo floresiensis, and this view is supported by a number of studies considering the skeletal evidence. These have noted that while the cranial metrics were consistent with Homo erectus, the limb proportions of Homo floresiensis had more in common with Australopithecus garhi (Argue, et al., 2006) and the feet were a mosaic of primitive apelike and derived humanlike features. The big toe was fully in-line, albeit short, and the metatarsals followed a humanlike sequence in which the 1st (innermost) was the most robust (sturdily-built), followed by the 5th (outermost), then 4th, 3rd, and finally 2nd. The foot, though, was disproportionately long in comparison to that of a modern human; the lesser metatarsals (2nd to 5th) were long; and the outer toes were long and curved, unlike the short, straight toes of a modern human (Jungers, et al., 2009). The fact that the feet and limb proportions of Homo erectus were modern suggests that Homo floresiensis evolved from a species that was more primitive, such as Homo habilis.

References:

1. Brown, P. et al., A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature 431, 1055-1061 (2004).

2. Morwood, M. et al., Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature 431, 1087-1091 (2004).

3. Jacob, T. et al., Pygmoid Australomelanesian Homo sapiens skeletal remains from Liang Bua, Flores: Population affinities and pathological abnormalities. PNAS 103 (36), 13421–13426 (2006).

4. Martin, R. et al., Comment on ‘‘The Brain of LB1, Homo floresiensis’’. Science 312, 999b (2006).

5. Argue, D., Donlon, D., Groves, C. & Wright, R., Homo floresiensis: Microcephalic, pygmoid, Australopithecus, or Homo? Journal of Human Evolution 51, 360-374 (2006).

6. Falk, D. et al., The Brain of LB1, Homo floresiensis. Science 308, 624-628 (2005).

7. Falk, D. et al., Brain shape in human microcephalics and Homo floresiensis. PNAS 104 (7), 2513–2518 (2007).

8. Tocheri, M. et al., The Primitive Wrist of Homo floresiensis and Its Implications for Hominin Evolution. Science 317, 1743-1745 (2007).

9. Lyras, G., Dermitzakis, M., Van der Geer, A., Van der Geer, S. & De Vos, J., The origin of Homo floresiensis and its relation to evolutionary processes under isolation. Anthropological Science (2008).

10. Montgomery, S., Capellini, I., Barton, R. & Mundy, N., Reconstructing the ups and downs of primate brain evolution: implications for adaptive hypotheses and Homo floresiensis. BMC Biology 8 (9), 1-19 (2010).

11. Kubo, D., Kono, R. & Kaifu, Y., Brain size of Homo floresiensis and its evolutionary implications. Proceedings of the Royal Society B 280 (1760) (2013).

12. Byrne, R. & Whiten, A., Machiavellian Intelligence (Oxford University Press, Oxford, 1988).

13. Jungers, W. et al., The foot of Homo floresiensis. Nature 459, 81-84 (2009).

The Mesoamerican ballgame

A matter of life and death

The term ‘ballgame’ refers to a wide variety of ball sports widely played in ancient Mesoamerica. These sports were a key element in ancient Mesoamerican religious, ritual and political life. Rubber balls have been found at the early Olmec site of El Manatí, dating to around 1600 BC (Ortíz & Rodríguez, 1999), and are the earliest known example of the use of rubber in Mesoamerica. Rubber was also used to make figurines and bands for hafting axe heads, and in liquid form it was used for painting and for medicinal purposes. It was obtained by tapping the indigenous Castilla elastica tree. Latex from this source dries into a brittle substance that is of little use, but Mesoamerican people improved its elastic properties by adding an extract from the vine Ipomoea alba to produce a solid white mass. Modern researchers have found that a ball formed from such material exhibits typical rubbery behaviour, and can bounce to a height of 2 m (6 ft. 6 in.). The technique alters the mechanical properties of latex and predates the modern vulcanising process by 3,500 years (Hosler, et al., 1999).

At least three variants of the ballgame continue to be played to this day, although the rules of the original pre-Columbian games are not known. The pivotal role of the ballgame in ancient Mesoamerican life is reflected in the large number of known ball courts – almost 1,300 – located at around 1,000 sites. The earliest-known ball court is that at Paso de la Amada, Chiapas, dating to 1600 BC (Blomster, 2012).

Much early evidence of the game comes from ceramic figurines and other images of ballplayers. The earliest such figurines date to 1700 BC and were recovered from a tomb at El Opeño, Michoacan. An arranged scene portrays five male ballplayers and three female spectators. Three of the ballplayers are equipped with bats. All five are wearing shin-pads and short helmets, and some wear mitts over their hands. A small yoke-shaped basalt piece was also found in the tomb. It was probably worn on the hand to protect it or for hitting the ball, and is the earliest example of ballgame equipment found so far. However, the ballplayers lack the elaborate costumes that characterise later depictions. It is likely that the ballgame had yet to assume its later significance (Blomster, 2012). Notably, the ball court at Paso de la Amada predates the emergence of a hierarchical society there by about a century (Lesure, 1997). Perhaps at this stage, the ballgame was still primarily a recreational activity.

Soon however, it would become linked to conflict, completion, hereditary leadership, and emerging political inequality. Ballgame costume is present on several pieces of monumental sculpture from the Olmec site of San Lorenzo. Monument 34 depicts a half-kneeling male figure wearing shorts, with a thick protective belt and loincloth. The monument has been interpreted as an Olmec ruler in his role as a ballplayer. At one of the San Lorenzo satellite towns, a monument features a similarly-clad ballplayer straddling a bound captive probably destined for sacrifice. Figurines from San Lorenzo depict ballplayers equipped with headdresses and helmets that mask the whole of the face, except for the eyes. They are wearing wide, thick padded belts and loincloths, and round pendants interpreted as mirrors. Similar imagery is seen with ballplayer figurines recovered at Etlatongo in Oaxaca and Cantón Corralito in Chiapas (Blomster, 2012). The latter site has been interpreted as an Olmec colony due to the similarity of its ceramic assemblage with that of San Lorenzo (Cheetham, 2007).

Figurines from the central Mexican sites of Tlatilco and Tlapacoya show distinct differences to Olmec figurines. The differences may reflect regional variations of either the game itself or the attendant rituals. Some examples from Tlapacoya wear a protective yoke supported by vertical or crossed suspenders on the front torso, probably related to the thick padded belts from San Lorenzo and Cantón Corralito. Some central Mexican figurines also wear tall, elaborate headdresses and ear flares, again distinct from their Olmec counterparts. These elaborate costumes were probably worn during ceremonies taking place before or after the game, rather than during the game itself (Blomster, 2012).

It is generally accepted that the ballgame was closely associated with elite power. It represented institutionalised ritual combat, possibly serving as an alternative to actual warfare. The ballgame might also have served a role in local dispute resolution. Some versions of the game were associated with human sacrifice and others were of great cosmological significance. The Maya text Popol Vuh describes the ballgame as a contest between mortals and sinister underworld deities. The play of the ball in the court symbolised the movements of the sun and moon, in turn representing the regeneration of life and the maintenance of cosmic order; the ball court itself represented a portal to the underworld (Blomster, 2012). The former Liverpool F.C. manager Bill Shankly allegedly described football as much more important than life and death: the same, apparently, was true of the Mesoamerican ball game.

References:

1. Ortíz, M. & Rodríguez, M., in Social Patterns in Pre-Classic Mesoamerica, edited by Grove, D. & Joyce, R. (Dumbarton Oaks Research Library and Collection, Washington, DC, 1999), pp. 225-254.

2. Hosler, D., Burkett, S. & Tarkanian, M., Prehistoric Polymers: Rubber Processing in Ancient Mesoamerica. Science 284, 1988-1991 (1999).

3. Blomster, J., Early evidence of the ballgame in Oaxaca, Mexico. PNAS 109 (21), 8020–8025 (2012).

4. Lesure, R., Early Formative Platforms at Paso de la Amada, Chiapas, Mexico. Latin American Antiquity 8 (3), 217-235 (1997).

5. Cheetham, D., Cantón Corralito: Objects from a Possible Gulf Olmec Colony, Crystal River, FL:Foundation for the Advancement of Mesoamerican Studies Inc. (2007).

The Mesoamerican Calendar

Mesoamerican calendrical systems have become well-known to the general public in recent years as a result of the Maya Long Count, which ended on 21 December 2012. There is no reason to suppose that the Maya expected anything untoward to occur on that day, but that did not stop the doomsday industry from working overtime. As the supposed day of reckoning approached, groups camped out by a mountain in the south of France to await rescue by flying saucer. There was much nonsense about a rogue planet called Nibiru and other supposed perils. The cinema industry cashed in on the hoo-hah with the disaster movie 2012 and the rather more thoughtful Melancholia. Neither movie paid much heed to the laws of physics. In fact, periodic end of the world ‘scares’ are nothing new, and go back at least a thousand years (Moore, 1999).

The Long Count was actually only one of three calendars in use in pre-Columbian Mesoamerica. For day-to-day reckoning, there was a solar calendar or haab cycle of 365 days, and there was a ritual calendar of 260-days known as the tzolkin or sacred almanac. All three calendars made use of the vigesimal or base-20 system of counting, rather than our familiar decimal or base-10 system. The system employed a place-value notation and a zero, long before the Hindu-Arabic system introduced these concepts. It may have come about through the practice of counting the digits on the feet as well as on the hands. Numbers were represented by combinations of ones (dots), fives (bars) and zeros (various characters) stacked vertically, with place value increasing from bottom to top (Aventi, 2001). The other number that featured prominently in Mesoamerican calendrical systems was 13, representing the number of levels of heaven in Mesoamerican cosmology (cf. the seven levels of heaven in the Jewish, Islamic and Hindu traditions).

The haab cycle comprised 18 ‘months’ of 20 days each, plus 5 intercalary days. Each date denoted by one of 20 day names paired with one of 18 month names. Like the pre-Ptolemaic Egyptian calendar, it did not take leap years into consideration, and thus did not accurately track the solar year. Days in the tzolkin were denoted by a number from 1 to 13 and one of 20 names, for a total of 260 days. The haab and tzolkin cycles were combined into the Calendar Round, which repeats every 18980 days (52 years). The 52-year cycle was a period of great significance throughout Mesoamerica. The termination was celebrated by the New Fire ceremony, in which fires everywhere were extinguished and domestic implements and statues were discarded (Aventi, 2001).

The Long Count calendar generated dates from a fixed start point that were to all intents and purposes unique (as are Gregorian dates). The basic unit of time was the tun of 360 days, which was subdivided into 18 uinals of 20 kins (days) each. The tun was multiplied by successive powers of 20 (the vigesimal equivalent of decades and centuries) named katuns and baktuns. Thus a katun is 360 x 20 = 7200 days and a baktun is 360 x 20 x 20 = 144,000 days or just over 394 years. The Maya did not invent the Long Count, but by Classic times (AD 250 – 800), only they were using it (Webster & Evans, 2005). The Maya implementation of the Long Count began on a date corresponding to 11 August 3114 BC in the Gregorian calendar. The 13th baktun from that date ended on 21 December 2012. There is some dispute as to what is supposed to follow. The usual view is that 13 baktuns (just over 5125 years) represents a creation epoch and the count returns to zero (Aventi, 2001). However, there is some evidence that the Maya intended the count to continue. There may be higher-order units beyond the baktun which scholars (in the absence of the original Maya terms) have named the piktun, kalabtun, kinchiltun and alautun.

The reason for a 260-day ritual count remains uncertain. One suggestion is that it originated at a location between 14°42' and 15 N., where the Sun crosses the zenith at 260 and 105-day intervals. A possible candidate is the Late Formative Period site of Izapa, which is located on the Pacific Coast of Mexico (Malmstrom, 1973). One objection to this interpretation is that the 260-day cycle simply repeats and does not factor in the concomitant 105-day cycle (Henderson, 1973). Another problem is that the 260-day cycle may have been in use at Monte Albán around 500 BC, considerably earlier than Izapa (Henderson, 1973; Marcus & Flannery, 2004). There are also Olmec inscriptions that suggest that the cycle might date to as early as 650 BC (Pohl, Pope, & von Nagy, 2002; Stokstad, 2002).

Other suggestions are a link to the average human gestation period of 266 days, or to various astronomical cycles. Two tzolkin (520 days) corresponds closely to three eclipse half-years (519.93 days). The eclipse half-year of 173.31 days is the period between successive eclipse seasons, i.e. a period of around 33 days when the Earth, Moon and Sun can line up to produce an eclipse. There is also a close correspondence between the tzolkin and the average of 263 days that Venus remains visible as either a morning or evening star, before it disappears into the dawn or twilight skies. Links to Mars have also been suggested. The synodic period of the Red Planet (i.e. the interval between successive close approaches to Earth) is almost exactly three tzolkin, or 780 days (Aventi, 2001).

References:
Aventi, A. (2001). Skywatchers. Austin, TX: University of Texas Press.
Henderson, J. (1973). Origin of the 260-Day Cycle in Mesoamerica. Science, 185, 542.
Malmstrom, V. (1973). Origin of the Mesoamerican 260-Day Calendar. Science, 181, 939-940.
Marcus, J., & Flannery, K. (2004). The coevolution of ritual and society: New 14C dates from ancient Mexico. PNAS, 101(52), 18257–18261.
Moore, P. (1999). Countdown!... or how nigh is the end? London: Pan.
Pohl, M., Pope, K., & von Nagy, C. (2002). Olmec Origins of Mesoamerican Writing. Science, 298, 1984-1987.
Stokstad, E. (2002). Oldest New World Writing Suggests Olmec Innovation. Science, 298, 1873-1874.
Webster, D., & Evans, S. (2005). Mesoamerican civilization. In C. Scarre, The human past (pp. 594-639). London: Thames & Hudson.

Archaeological evidence for carcass processing at Kanjera, Kenya, 2 million years ago.

Earliest unambiguous evidence for meat-eating by early hominins.

Modern humans are the only existent primates anatomically adapted for the regular consumption of significant quantities of meat. The human gut is reduced compared with that of other primates, a configuration more suited to a meat-eating diet than the predominantly vegetarian diet of other primates. Although crucial to many models of hominin evolution, however, the timing of and circumstances in which early hominins began to include significant quantities of meat in their diet remain poorly understood.

The earliest-known stone tools, from Gona, Ethiopia, are 2.6 million years old and are often taken to be early evidence for meat eating (Semaw, et al., 1997; Semaw, 2000). No hominin remains were recovered in association with the tools, but in 1999, anthropologists working at the nearby Bouri Formation reported the discovery of large mammal bones bearing cut-marks apparently made by stone tools, possibly as a result of dismembering and filleting carcasses. Animals appeared to have been defleshed, and their long bones broken open, presumably to extract marrow. The bones were found in association with 2.5 million-year-old australopithecine remains, thought to be of Australopithecus garhi (de Heinzelin, et al., 1999).

It has also been claimed that 3.39 million-year-old animal bones from Dikika, Ethiopia, show stone tool cut-marks for flesh removal, and signs of having been struck with hammerstones to extract bone marrow (McPherron, et al., 2010). In the absence of any associated tools, there is no way to tell whether the cut-marks were produced with specially-made tools or naturally-sharp pieces of stone. Some are sceptical and argue that as the bones were buried in coarse-grained, sandy deposits, it is likely that trampling by animals produced the marks (Domınguez-Rodrigo, et al., 2011).

Even if the above is accepted as evidence of carcass-processing by early hominins, it is too insubstantial to show whether these were one-off forays into meat-eating or part of a more substantial shift in hominin dietary adaptations. To demonstrate ‘persistent carnivory’ requires a geologically-stratified series of relatively large assemblages of animal remains, each showing extensive signs of persistent hominin activity. The sum of the assemblages must demonstrate that this activity persisted over the course of at least a thousand years (Ferraro, et al., 2013).

Although rather more recent than the above dates, such evidence has now been reported from Kanjera South, a small site located on the shores of Lake Victoria, southwestern Kenya (Ferraro, et al., 2013). Three excavations along 50 metres have yielded several thousand well-preserved animal remains, approximately 2 million years old, and associated with stone tools. There is a consistent record of hominin activities throughout the stratified sequence, which spans hundreds or possibly thousands of years.

The animal remains included gazelle and other small bovids, together with a smaller number of medium-sized bovids. The remains showed clear evidence of butchery by hominins in the form of cut-marks and damage caused by hammerstones. Patterns of tooth-marks made by carnivores such as lions and hyenas suggest that these animals only had access to the carcasses after the hominins had removed the bulk of the meat and bone marrow. Carnivores typically chew on the mid-shafts of long bones, but the percentage of bones that were so marked was low.

Small bovids are invariably wholly consumed by carnivores within hours of death, implying that the hominins acquired and butchered them very soon after death. A possible implication is that these animals were hunted rather than scavenged, and that Kanjera represents the earliest archaeological record of hunting activities by hominins.

The skeletal remains of the small bovids suggest that they were transported to the site for butchery more or less intact. However, in the case of the medium-sized bovids, head and limb parts predominate. These animals were too large to transport intact, so the hominins removed the limb parts, leaving the rest of the body behind. Although head contents are nutritious, they are difficult to exploit and would thus be ignored by other scavengers. They therefore represent a niche that tool-using hominins could exploit. It is therefore likely that hominins scavenged leftover head parts from carnivore kills and transported them to the site for processing.

The Kanjera data not only provides the required evidence of hominin meat-eating over a period of many centuries: it also provides clues about specific activities. Thus, it seems, the hominins obtained much of their meat by hunting small bovids, but they also scavenged medium-sized bovid heads as a separate by complimentary activity. The date of 2 million years ago is somewhere between 200,000 and 500,000 years earlier than the previous earliest evidence for persistent hominin carnivory.

References:

1. Semaw, S. et al., 2.5-million-year-old stone tools from Gona, Ethiopia. Nature 385, 333-336 (1997).

2. Semaw, S., The World’s Oldest Stone Artefacts from Gona, Ethiopia: Their Implications for Understanding Stone Technology and Patterns of Human Evolution Between 2.6–1.5 Million Years Ago. Journal of Archaeological Science 27, 1197–1214 (2000).

3. de Heinzelin, J. et al., Environment and Behavior of 2.5-Million-Year-Old Bouri Hominids. Science 284, 625-629 (1999).

4. McPherron, S. et al., Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature 466, 857-860 (2010).

5. Domınguez-Rodrigo, M., Pickering, T. & Bunn, H., Reply to McPherron et al.: Doubting Dikika is about data, not paradigms. PNAS 108 (21), E117 (2011).

6. Ferraro, J. et al., Earliest Archaeological Evidence of Persistent Hominin Carnivory. PLoS One 8 (4) (2013).

Australopithecus sediba: a possible human ancestor

Australopithecus sediba is a possible human ancestor discovered in South Africa in 2010. The discovery was made at Malapa, a fossil-bearing cave located about 15 km (9.3 miles) NE of the well-known South African hominid-bearing sites of Sterkfontein and Swartkrans and about 45 km (28 miles) NNW of Johannesburg  (Berger, et al., 2010). It is situated within the Cradle of Humankind World Heritage Site. The recovery effort was led by Lee Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg. The find was made when Matthew, Lee’s 9 year old son, discovered hominin collar bone embedded in a rock (Balter, 2010).

The find comprised two extremely well-preserved partial skeletons that were initially thought be somewhere between 1.78 and 1.95 million years old (Dirks, et al., 2010), later revised to 1.977 million years (Pickering, et al., 2011). These belonged to a juvenile male (MH1) aged 12 to 13 at time of his death and an adult female (MH2) (Berger, et al., 2010). They were found together buried in alluvial sediment, deep within the Malapa cave, part of an eroded cave system. Also found were the remains of wildcats, hyenas and a number of other mammals. On the ground above the cave are a number of ‘death traps’, or long vertical shafts. The smell of damp issuing from the shaft would have attracted animals. The pair – possibly mother and son – may have fallen to their deaths while searching for water. The sediments imply that subsequent high-volume water inflow, perhaps the result of a large storm, caused a debris flow. This carried the still partially articulated bodies deeper into the cave, to deposit them along a subterranean stream (Dirks, et al., 2010).

MH1 and MH2 were assigned to a new australopithecine species, Australopithecus sediba. The word ‘sediba’ means ‘fountain’ or ‘wellspring’ in the Sotho language. The more complete cranium of the juvenile MH1 has a capacity of 420cc, probably at least 95 percent of adult size. The remains share numerous similarities with Australopithecus africanus in the cranial vault, facial skeleton, lower jawbone and teeth, but there are also significant differences in the cranial, dental and postcranial anatomy. Homo-like features include smaller molars and premolars and less pronounced cheekbones. Certain features of the pelvis are similar to those seen in Homo erectus. The lower-to-upper limb bone proportions are also similar to those of later Homo, and unlike the more apelike proportions of Homo habilis. The anatomy of its hip, knees and ankles suggest that Australopithecus sediba was a habitual biped. Overall, it was claimed that Australopithecus sediba shares more derived features with early Homo than it does with other australopithecines. However, Berger was reluctant to place the new discovery within Homo, preferring to classify it as an australopithecine (Berger, et al., 2010).

The initial announcement of Australopithecus sediba attracted extensive news coverage, but not everybody was convinced by the claims made for it. Australian anthropologist Darren Curnoe was reported (MacKnight, 2010) as claiming that Australopithecus sediba is in the wrong place at the wrong time to be a human ancestor. He noted that Homo habilis emerged in East Africa well before the time of Australopithecus sediba. However, his argument does assume that Homo habilis is indeed an early human.  This may not be the case. It is also possible that at least some of Australopithecus sediba’s humanlike features could have evolved independently, and may not necessarily imply shared ancestry (Wood & Harrison, 2011).

Nevertheless, subsequent studies do support Berger’s initial claims. They suggest that aspects of the brain, dental morphology, pelvis, hand and foot of Australopithecus sediba could be interpreted as incipient humanlike features. A virtual endocast of the brain, obtained from synchrotron scanning, revealed an australopithecine-like size and pattern of convolutions. However, the orbitofrontal region showed possible development towards a humanlike frontal lobe. Possibly some neural reorganization of the brain preceded its later size increase in early humans (Carlson, et al., 2011).

The teeth of MH1 and MH2 are a mosaic of primitive and derived traits. Cladistic analysis of 22 dental traits suggest that Australopithecus sediba was a sister species of Australopithecus africanus (i.e. the two shared a common ancestor) and that the two were further evolved in the direction of Homo than were the australopithecines from East Africa (Irish, Guatelli-Steinberg, Legge, de Ruiter, & Berger, 2013). The lower jawbone morphology reduced dentition (especially canines and premolars) confirms that Australopithecus sediba was a distinct species to Australopithecus africanus and not merely a late-surviving form of that species (de Ruiter, et al., 2013).

The upper ribcage of Australopithecus sediba exhibits an apelike funnel shape, unlike the barrel shape associated with Homo. The funnel shape, as noted above, may be an adaptation to under-branch suspensory locomotion. The barrel shape may be associated with the increased chest volume and lung function necessary for endurance walking and running. The lower thorax, however, appears less flared than that of apes and more closely approximates the morphology found in humans (Schmid, et al., 2013). The spine is long and flexible, a form that has more in common with early Homo than with other australopithecines. Curvature of the lower spine is a hallmark of walking upright (Williams, Ostrofsky, Frater, Churchill, Schmid, & Berger, 2013).

The upper limbs were still predominantly apelike, suggesting the retention of substantial climbing and suspensory abilities (Churchill, et al., 2013). The hands show a mixture of australopithecine and human features. They retained adaptations for tree-climbing, but there was also a long thumb and shorter fingers. These suggest precision gripping of the type associated with tool manufacture and use (Kivell, Kibii, Churchill, Schmid, & Berger, 2011).

The pelvis and foot presented a mosaic of apelike and humanlike characteristics. These suggested adaptations to a more efficient (albeit not entirely human) form of bipedalism, at the expense of reduced arboreal efficiency (Kibii, et al., 2011; Zipfel, DeSilva, Kidd, Carlson, Churchill, & Berger, 2011). The bipedal mechanics differed from those reconstructed for other australopithecines, suggesting that there may have been several forms of hominin bipedalism at this time. The adaptations of Australopithecus sediba may have enabled it to both walk and climb reasonably well and thus survive in a dual arboreal/terrestrial world (DeSilva, et al., 2013).

References:

Balter, M. (2010, April 9). Candidate Human Ancestor From South Africa Sparks Praise and Debate. Science, 328, 154-155.

Berger, L., de Ruiter, D., Churchill, S., Schmid, P., Carlson, K., Dirks, P., et al. (2010, April 9). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science, 328, 195-204.

Carlson, K., Stout, D., Jashashvili, T., de Ruiter, D., Tafforeau, P., Carlson, K., et al. (2011, September 9). The Endocast of MH1, Australopithecus sediba. Science, 333, 1402-1407.

Churchill, S., Holliday, T., Carlson, K., Jashashvili, T., Macias, M., Mathews, S., et al. (2013, April 12). The Upper Limb of Australopithecus sediba. Science, 340.

de Ruiter, D., DeWitt, T., Carlson, K., Brophy, J., Schroeder, L., Ackermann, R., et al. (2013, April 12). Mandibular Remains Support Taxonomic Validity of Australopithecus sediba. Science, 340.

DeSilva, J., Holt, K., Churchill, S., Carlson, K., Walker, C., Zipfel, B., et al. (2013). The Lower Limb and Mechanics of Walking in Australopithecus sediba. Science, 340.

Dirks, P., Kibii, J., Kuhn, B., Steininger, C., Churchill, S., Kramers, J., et al. (2010, April 9). Geological Setting and Age of Australopithecus sediba from Southern Africa. Science, 328, 205-208.

Irish, J., Guatelli-Steinberg, D., Legge, S., de Ruiter, D., & Berger, L. (2013, April 12). Dental Morphology and the Phylogenetic “Place” of Australopithecus sediba. Science(340).

Kibii, J., Churchill, S., Schmid, P., Carlson, K., Reed, M., de Ruiter, D., et al. (2011, September 9). A Partial Pelvis of Australopithecus sediba. Science, 333, 1407-1411.

Kivell, T., Kibii, J., Churchill, S., Schmid, P., & Berger, L. (2011, September 9). Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities. Science, 333, 1411-1417.

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Pickering, R., Dirks, P., Jinnah, Z., de Ruiter, D., Churchil, S., Herries, A., et al. (2011, September 9). Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo. Science, 333, 1421-1423.

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Fossil evidence for interbreeding between Neanderthals and modern humans

Lower jawbone discovered in 1957 could be from Neanderthal/modern human hybrid.

Although interbreeding between Neanderthals and modern humans has been inferred from genetic data, convincing fossil evidence for hybridisation has hitherto been lacking. Claims that the 24,500-year-old skeleton of a 4-year-old child found at Abrigo do Lagar Velho, Portugal in 1998 is an example of a hybrid (Duarte, et al., 1999) have not been widely accepted. Notably, the burial was typical of the Gravettian, a culture that is firmly associated with modern humans. It is possible that the infant was simply an unusually stocky modern human juvenile, or a ‘chunky child’ as one critic put it (Tattersall & Schwartz, 1999).

A newly-published report has claimed that a better case may be made for a lower jawbone from the Riparo Mezzena rocks helter, northern Italy. The jawbone was found in 1957, along with stone artefacts of the Mousterian tradition. As this tradition is firmly associated with Neanderthals in Europe, the 34,500-year-old jawbone was assumed to be Neanderthal. Mitochondrial DNA extracted from the jawbone confirms it to be Neanderthal. However, the Mezzena lower jawbone shows a number of modern features, including an incipient chin, which contrasts with the chinless jaws of ‘classic’ Neanderthals. The authors of the report claim that these features demonstrate hybridisation between Neanderthals and modern humans (Condemi, et al., 2013).

It is possible that the interbreeding occurred with a modern population thought to have been living at the nearby site of Grotta di Fumane (Longo, et al., 2012).

References:

1. Duarte, C. et al., The Early Upper Paleolithic Human Skeleton from the Abrigo do Lagar Velho (Portugal) and Modern Human Emergence in Iberia. PNAS 96, 7604–7609 (1999).

2. Tattersall, I. & Schwartz, J., Hominids and hybrids: The place of Neanderthals in human evolution. PNAS 96, 7117–7119 (1999).

3. Condemi, S. et al., Possible Interbreeding in Late Italian Neanderthals? New Data from the Mezzena Jaw (Monti Lessini, Verona, Italy). PLoS One 8 (3) (2013).

4. Longo, L. et al., Did Neandertals and anatomically modern humans coexist in northern Italy during the late MIS 3? Quaternary International 259, 102–112 (2012).

Study highlights differences in brain organisation between Neanderthals and modern humans

Neanderthals focussed on vision at expense of social networking.

A new study has suggested that there were significant differences in the neurological organisation of Neanderthals and modern humans, reflecting physiological differences between the two species. Neanderthals, as has long been known, were larger and more powerfully-built than modern humans. Consequently, it is suggested that they required proportionately more ‘brain power’ to carry out body maintenance ‘housekeeping’ tasks and control functions. In addition, it is suggested that Neanderthals had larger eyes than modern humans, which also used up brain power. They lived at high latitudes in Eurasia, where they experienced lower light levels than people living in the tropics.

Researchers considered the remains of 21 Neanderthals and 38 modern humans dating from between 27 to 200 thousand years ago. They adjusted brain sizes to compensate for the greater Neanderthal body size, and estimated the size of the visual cortex from eye socket measurements. The average size of the Neanderthal eye socket was found to 44 by 36 mm (1.73 by 1.42 in.) compared with 42 by 30 mm (1.65 by 1.18 in.) for the modern humans. This equates to an eyeball volume of 34 cc against 29.5 cc; a 15 percent difference.

With more brain power required for housekeeping and visual functions, less would have been available for social interactions, and it has been suggested the Neanderthal maximum social group size was smaller than the ‘Dunbar Number’ of 150 associated with modern humans. The area covered by extended Neanderthal communities would have been smaller than those of modern humans. Their ability to trade would have been reduced, as would their capacity to learn of distant foraging areas potentially unaffected by local shortages. Furthermore, their ability to acquire and pass on innovations may have been limited in comparison to modern humans.

In the high latitudes of Eurasia, far from their African homeland, modern humans were disadvantaged in as much as they lacked the enhanced visual acuity, as well as other Neanderthal adaptations to the colder climate. Unable to adapt their bodies, modern humans adapted their technology, and thus became more reliant on it than were the Neanderthals. However, technological change can greatly outpace evolutionary change. The combination of adaptable technology and enhanced social networks gave the first modern humans in Europe a competitive advantage over the physically-adapted Neanderthals, eventually bringing about the demise of the latter.

References:

1. Pearce, E., Stringer, C. &  Dunbar, R., New insights into differences in brain organization between Neanderthals and anatomically modern humans. Proceedings of the Royal Society B 280 (1758) (2013).

Higher levels of Neanderthal ancestry in East Asians than in Europeans

A new study published in the journal Genetics (Wall et al, 2013) has concluded that East Asians have a higher level of Neanderthal DNA than do Europeans. The result implies that there was more than one episode of interbreeding between modern humans and Neanderthals.  After the ancestors of modern Europeans and East Asians separated, the latter population continued to interbreed with Neanderthals.

Given that the Neanderthals are thought to have been a predominantly Western Eurasian species, this result is unexpected. It is becoming clear that the history of interbreeding between modern humans and Neanderthals was rather more complex than was originally thought.

The paper is available open access - see this link on the Genetics website

References:
Wall, J., Yang, M., Jay, F., Kim, S., Durand, E., Stevison, L., Gignoux, C., Woerner, A., Hammer, M., and Slatkin, M. (2013) Higher levels of Neanderthal ancestry in East Asians than in Europeans, Genetics, Early Online

To Mars in a nutshell

‘O God, I could be bounded in a nutshell and count myself a king of infinite space…’

Hamlet was not alluding to space travel, but he might as well have been. An audacious proposal announced by American millionaire Dennis Tito calls for a man and woman to make a 501-day round trip to Mars in a spacecraft half the size of a camper van. There will be no landing – the spacecraft will simply make a fly-by, skimming past the Red Planet at a minimum altitude of 100 miles. The crew are likely to be a middle-aged married couple.

Dennis Tito first made the headlines in 2001, when over the objections of NASA he paid for a seat on the Russian Soyuz TM-32 mission to the International Space Station. He was subsequently described as the first ‘space tourist’, a rather unfortunate label in my view. Tito, now 72, shares the frustration of all space enthusiasts at the complete lack of progress with the manned exploration of space since Project Apollo. It is now four decades since Cernan and Schmitt blasted off from the surface of the Moon. Nobody has been back; no manned spacecraft has left Earth orbit since.

There have been innumerable proposals for an expedition to Mars, but none have got off the ground even metaphorically. It is of course much harder to mount an expedition to Mars than it is to the Moon. The most obvious problem is that Mars is very much further away than the Moon. The Apollo missions typically lasted under ten days; the duration of Tito’s mission will be fifty times longer. The next problem is that Mars, though small in comparison to Earth, is still much larger than the Moon. Furthermore, unlike the Moon, it has a significant atmosphere. To land on Mars and take off again, you need a craft that is not only built for re-entry, but is also able to escape the higher Martian gravity on take-off. This means a craft that is considerably larger and more complex than the Apollo lunar module. The fuel requirements for the mission are immense. Assuming an Apollo-type lander-orbiter configuration, you need sufficient fuel for 1) the spacecraft to launch and leave Earth orbit, 2) achieve Martian orbit, 3) the lander to land and take-off, 4) the orbiter to leave Martian orbit, 5) make any required mid-course alterations.

The crucial difference between a manned expedition and the innumerable unmanned landers and rovers sent to Mars since the 1970s is that the latter don’t have to return to Earth. To date, no unmanned sample return mission to Mars has ever been attempted, and even attempts to return samples from its moons have failed. To get round the problem, some have suggested a one-way trip to Mars. Unlike the Moon, there are sufficient raw materials on Mars to allow colonists to keep themselves alive indefinitely. 

The Tito proposal involves a fly-by rather than a one-way trip. There’s no landing, but the crew don’t have to spend the rest of their lives on Mars. The spacecraft will be launched on a so-called free return trajectory, which will return it to Earth without the expenditure of fuel. Very little fuel will be needed after leaving Earth orbit. The result is a far simpler mission profile, though this term is relative. Unlike the International Space Station, which is periodically resupplied from Earth, the spacecraft will need to carry oxygen and supplies for the whole of the 501 day round trip. Even items such as toilet paper will amount to 28kg (62 lb.) in the supplies manifest. A major complication is that the spacecraft will be travelling at 51,000 km per hour (32,000 mph) when it returns to Earth. No manned spacecraft has ever attempted re-entry at such speed. It is likely that the spacecraft will have to slow down by aerobraking in the Earth’s outer atmosphere. The technique has been used for twenty years to slow unmanned space probes, but has never been attempted with a manned craft.

Another factor is radiation from the Sun and from interstellar space. A vehicle in Low Earth Orbit, such as the International Space Station or a shuttle, is largely protected by the Earth’s magnetic field. On a short-duration mission beyond Earth orbit – such as Apollo – the dosage is not large enough to be a problem. The possible effects of exposure on a long-duration mission include sterility and an elevated risk of developing cancer in later life. That is the reason for selecting a middle-aged crew. It is further assumed that a married couple could better endure the psychological stresses of long-term confinement.

There is also the risk of a coronal mass ejection from the Sun – a massive burst of radiation occurring during a solar flare. The proposed mission will take place during a period of low solar activity, but the risk isn’t entirely absent. The radiation could seriously harm or even kill the crew. Unfortunately, there is very little that can be done with present-day technology to shield a spacecraft against radiation. Finally, there is the stark reality that if something goes wrong with the spacecraft or if there is a medical emergency on-board, there will be absolutely nothing that can be done to abort the mission.

No concrete proposals yet exist for the mission. A possible configuration would involve a Dragon spacecraft from the private US space company Space X. The Dragon is a re-usable capsule-type craft that has already carried out an unmanned resupply mission to the International Space Station. The Dragon would be coupled to an inflatable habitat module of the type under development by Bigelow Aerospace, another private US space company. The mission would be launched with a Space X Falcon heavy-lift launch vehicle. First launch of the Falcon Heavy is expected either late this year or early next year.

The next launch window for the 501-day flight occurs in January 2018. After that, Mars will not be in the right position again until 2031. This gives Tito 5 years to get his mission off the ground. At the glacial speeds which NASA has operated since Apollo, this might not seem possible. However, it should be remembered that little over eight years passed from Alan Shepard’s sub-orbital spaceflight in 1961 to the late Neil Armstrong’s ‘giant leap for mankind’. The entire history of powered flight from Kittyhawk to the Sea of Tranquillity took place within the lifetime of many, including my grandparents.

The cost of the mission has been estimated at between $1 to 2 billion (£660 – 1200 million). This might sound like a lot of money, but it is actually less than Russian oligarch Roman Abramovich is alleged to have spent on Chelsea FC over the last decade. In space terms, it’s peanuts. In terms of actual Mars science, the value of the mission will be far less than can be achieved with unmanned orbiters and rovers. The scientific value of the mission will be in terms of what can be learned about the physiological and psychological effects of long-term spaceflight beyond Earth orbit.

The real worth of the mission, however, will be in its inspirational rather than scientific value. Nobody much under the age of 50 can remember the Moon landings. The current President of the United States was a few days short of his eighth birthday when Armstrong and Aldrin landed on the Moon; UK Prime Minister David Cameron was a 2 ½ year-old toddler. I think we’ve been waiting long enough for mankind’s next giant leap.

© Christopher Seddon 2013

Isotope analysis documents transition to agriculture in the Balkans

Mesolithic foragers were gradually assimilated into farming communities.

The Iron Gates are a series of gorges situated on the Danube between the Carpathian Mountains and the Dinaric Alps. In the early millennia after the last Ice Age, the region supported a number of sedentary or near-sedentary Mesolithic communities. At the sites of Lepenski Vir, Padina and Vlasac, fishers exploited migratory sturgeon, catfish, carp and other species (Borić, 2002).

There is no evidence for long-distance interactions during the early Mesolithic period from 9500 to 7400 BC, but these increased during the period from 7400 to 6200 BC. Archaeological evidence is based on the presence of the marine gastropods Columbella rustica and Cyclope neritea, which must have come from coastal regions more than 400 km (250 miles) away. This period was characterised by long-lasting and evidently successful communities. A large number of burials have been excavated, with bodies typically in the extended supine position characteristic of Mesolithic inhumations (Borić & Price, 2013).

The period between 6200 and 6000 BC saw a Mesolithic to Neolithic transition in the region, and was characterised by cultural hybridity (Borić & Price, 2013). At Lepenski Vir, remarkable trapezoidal, semi-subterranean, flat-roofed dwellings were constructed on the banks of the Danube (Borić, 2002). They varied in size from 5 to 30 sq. m. (54 to 320 sq. ft.), with the wider ends facing the river. The floors were dug 0.5 to 1.5 m (1 ft. 8 in. to 3 ft. 3 in.) into the terraced slopes of the river bank, and were surfaced with reddish limestone plaster. Inside, elongated pits lined with limestone blocks served as hearths (Mithen, 1994; Borić, 2002). Many houses contained burials, although burials were also placed outside houses (Radovanovic, 2000). Human/fish anthropomorphic sculptures carved from boulders were also found in many of the houses. These have been interpreted as evidence of a belief system characterised by a totemic relationship between humans and the fish that were so vital to their subsistence economy (Borić, 2005). In addition to these indigenous elements, Neolithic elements including pottery and polished stone axes appeared at Lepenski Vir (Borić & Price, 2013).

At this stage, the lack of domesticated animals at suggests that subsistence patterns remained unchanged. Mortuary practices were still characterised by typical Mesolithic extended supine burials during this period. However, the Early Neolithic site of Ajmana, in the downstream area of the gorges, was contemporary with these indigenous forager communities. By 6000 BC, further changes were evident in the region with the first appearance of crouched/flexed burials characteristic of the Neolithic period. The trapezoidal buildings of Lepenski Vir were replaced by more typical Neolithic constructions, and there was an increase in the number of settlements across the region as a whole (Borić & Price, 2013).

In total, over 500 graves have been excavated from the Mesolithic and Early Neolithic periods in the Danubian Iron Gates, and stable isotope analysis of the remains has provided considerable insight into the transition to agriculture in the region. Dietary data inferred from carbon and nitrogen isotope analysis of bone collagen suggests that after around 6200 BC, there was a shift from the Mesolithic reliance on the locally-abundant fish to a cereal-based diet. Strontium isotope data from dental enamel indicate that at the same time, burials of non-local first-generation migrants increased significantly. These burials are predominantly of the crouched/flexed type. Notably, ⁸⁷Sr/⁸⁶Sr ratios of these migrants fall both above and below local values, suggesting that they originated from at least two geologically-distinct regions. The dating of remains suggests that they might have arrived in several waves (Borić & Price, 2013).

Paradoxically, it appears that during the earliest stages of the Neolithic in southeastern Europe, Neolithic farmers were more mobile than the indigenous foragers, who remained tied to their Danubian fishing niche. The data from Lepenski Vir shows that during the transitional period, more nonlocal women than men were buried at the site. The suggestion is that women came to the site from Neolithic communities as part of an ongoing social exchange. At the same time, the numbers of Neolithic-type artefacts at the site testify to an increasing Neolithic presence in the region, and the Mesolithic way of life came under growing pressure. The period of co-existence lasted for two centuries between 6200 and 6000 BC, but in the centuries thereafter the foragers were completely absorbed into the farming communities and their way of life finally vanished (Borić & Price, 2013).

References:

1. Borić, D., The Lepenski Vir conundrum: reinterpretation of the Mesolithic and Neolithic sequences in the Danube Gorges. Antiquity 76 (294), 1026–1039 (2002).

2. Borić, D. & Price, D., Strontium isotopes document greater human mobility at the start of the Balkan Neolithic. PNAS 110 (9), 3298–3303 (2013).

3. Mithen, S., in Prehistoric Europe, edited by Cunliffe, B. (Oxford University Press, Oxford, 1994), pp. 79-135.

4. Radovanovic, I., Houses and burials at Lepenski Vir. European Journal of Archaeology 3 (3), 330-349 (2000).

5. Borić, D., Body Metamorphosis and Animality: Volatile Bodies and Boulder Artworks from Lepenski Vir. Cambridge Archaeological Journal 15 (1), 35–69 (2005).

Did Neanderthals die out before modern humans reached southern Europe?

Study casts doubt on late Neanderthal survival in Iberian Peninsula.

Until fairly recently, it was believed that Neanderthals and modern humans coexisted in Europe for up to 10,000 years, but recent improved radiocarbon dates suggest that this period was far shorter – possibly no more than 1,000 or 2,000 years (Mellars, 2006). Many supposedly-late Neanderthals have now been shown to be much older than first believed. For example, two specimens from Vindija Cave in Croatia were originally thought to be from 32,000 to 33,000 years old (28,000 to 29,000 radiocarbon years BP) (Smith, et al., 1999), but these dates are now thought to be nearer 36,000 to 37,000 years old (32,000 to 33,000 radiocarbon years BP) (Higham, et al., 2006). Similarly, an infant from Mezmaiskaya Cave in the northern Caucasus, once believed to be a late survivor from 29,000 years ago, is now believed to be have lived more like 40,000 years ago (Pinhasi, et al., 2011).

Another factor is that calendar dates from this period might have been systematically underestimated. Radiocarbon dates do not coincide exactly with calendar dates, and the latter must be estimated using calibration data. A recent re-evaluation suggests that the estimated calendar dates for this period should be older than was previously believed. The revised dates suggest that overall, the period of coexistence between Neanderthal and modern human populations within the individual regions of Europe such as western France was fairly brief, possibly no more than 1,000 or 2,000 years (Mellars, 2006).

At the peripheries of Europe, Neanderthals might have persisted for rather longer than elsewhere. Possible late survival is documented from two very different settings: Gorham’s Cave, Gibraltar, and Byzovaya, in the western foothills of the northernmost Urals. Gorham’s Cave seems to have been a favoured location that was visited repeatedly over many thousands of years. Natural light penetrates deep into the cave, and a high ceiling permits ventilation of smoke from the hearths that were repeatedly made there. Neanderthal occupation of the cave continued until 33,000 years ago (28,000 radiocarbon years BP), and possibly until as recently as 29,000 years ago (24,000 radiocarbon years BP), and the site was later used by modern humans right up until Phoenician and Carthaginian times. However, there was a 5,000 years hiatus after the last Neanderthal occupation before the first modern humans took up residence (Finlayson, et al., 2006; Finlayson, et al., 2008). At Byzovaya, a total of 313 stone artefacts have been collected over the years, all reflecting typical Middle Palaeolithic tool production techniques characteristic of Neanderthal Mousterian industries, and ranging from 31,000 to 34,000 years old (Slimak, et al., 2011).

However, a newly-published study has cast doubt on the late Neanderthal survival in the Iberian Peninsula. Researchers used a technique known as ultra-filtration to remove traces of modern contaminants (for example preservatives and glues) from fossil bone collagens (proteins making up the bone matrix) prior to radiocarbon dating. Without this process, it is claimed that the contaminants make samples appear younger than they actually are. For example, a carbon contamination of just one percent will make a 50,000-year-old sample appear to be just 37,000 years old. A total of 215 Neanderthal bones from 11 supposedly-late Neanderthal sites were screened for collagen. Unfortunately, only 27 bones were found to contain enough collagen for radiocarbon dating using the ultra-filtration technique. These were recovered from just two sites: Jarama VI and Cueva del Boquet Zafarraya. The results suggested that the Neanderthal remains from the two sites were at least 10,000 years older than previously believed (Wood, et al., 2013). Should other dates for the Iberian Neanderthals turn out to have been similarly understated, then it would suggest that they died out before modern humans arrived. However, it should be noted that the authors of the Gorham’s Cave report had previously considered and ruled out the possibility of contamination affecting their results (Finlayson, et al., 2008).

References:

1. Mellars, P., A new radiocarbon revolution and the dispersal of modern humans in Eurasia. Nature 493, 931-935 (2006).

2. Smith, F., Trinkaus, E., Pettitt, P., Karavanic, I. & Paunovic, M., Direct radiocarbon dates for Vindija G1 and Velika Pecina Late Pleistocene hominid remains. PNAS 96 (22), 12281–12286 (1999).

3. Higham, T., Ramsey, C., Karavanic, I., Smith, F. & Trinkaus, E., Revised direct radiocarbon dating of the Vindija G1 Upper Paleolithic Neandertals. PNAS 103 (3), 553–557 (2006).

4. Pinhasi, R., Higham, T., Golovanova, L. & Doronichev, V., Revised age of late Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus. PNAS 108 (21), 8611-8616 (2011).

5. Finlayson, C. et al., Late survival of Neanderthals at the southernmost extreme of Europe. Nature 443, 850-853 (2006).

6. Finlayson, C. et al., Gorham’s Cave, Gibraltar - The persistence of a Neanderthal population. Quaternary International 181, 74-71 (2008).

7. Slimak, L. et al., Late Mousterian Persistence near the Arctic Circle. Science 332, 841-845 (2011).

8. Wood, R. et al., Radiocarbon dating casts doubt on the late chronology of the Middle to Upper Palaeolithic transition in southern Iberia. PNAS 110 (8), 2781-2786 (2013).

BH-1 hominin mandible from Serbia suggests Neanderthals evolved in isolation in Western Europe

BH-1 is a left fragment of a human mandible (lower jawbone), complete with all three molar teeth. It was recovered in 2005 at Mala Balanica cave, Serbia, along with a number of quartz artefacts. Originally estimated to be around 115,000 years old (Roksandic, et al., 2011), it is now believed to be at least 400,000 years old. Newly obtained ages, based on electron spin resonance combined with uranium series isotopic analysis, and infrared/post-infrared luminescence dating, yielded a minimum age of between 397,000 to 525,000 years old. This date makes BH-1 one of the oldest hominins in Europe, and the most easterly European hominin of the Middle Pleistocene (Rink, Mercier, Mihailovic, Morley, Thompson, & Roksandic, 2013).

Middle Pleistocene hominins from the period 600,000 to around 200,000 years ago are conventionally lumped together as Homo heidelbergensis (or Archaic Homo sapiens). By the end of this period, the Neanderthals had emerged from the European populations and modern Homo sapiens from the African populations. In fact, the reality of the situation is far from understood and was almost certainly far more complicated.

The problem with Homo heidelbergensis is as Archaic Homo sapiens it came to be used as a kind of ‘wastebasket category’ for anything that wasn’t Homo erectus, a Neanderthal, or a modern human (Cameron & Groves, 2004). Consequently, it tends to be defined in terms of features intermediate between Homo erectus and later humans rather than unique traits, which are a prerequisite for properly defining a species (Harvati, 2007). The situation has been referred to by anthropologist Phillip Rightmire (1998) as the ‘muddle in the middle’ and there is much debate as to whether Homo heidelbergensis is indeed a single species.

BH-1 is a potentially important piece in the jigsaw. It differs significantly from European hominins generally classified as Homo heidelbergensis. It shows a complete lack of the incipient Neanderthal traits that are present for most Western European hominins of the Middle Pleistocene. Instead, it shows primitive Homo erectus-like traits (Roksandic, et al., 2011).

This data suggests that the Neanderthals may have arisen solely in Western Europe, only later spreading to Southeast Europe and Southwest Asia. During glacial periods, Western Europe was cut off from the rest of Eurasia, and the distinctive morphology of the Neanderthals may have evolved in isolation. The process may have been driven by genetic drift impacting on small isolated proto-Neanderthal populations, rather than the effects of Darwinian natural selection (Weaver, 2009). Genetic drift refers to random changes in the relative frequency in which an allele occurs in a population. In small populations, over a number of generations, the effect can result in some alleles becoming fixed and others disappearing altogether, even if the prevailing alleles confer no particular selective advantage on their possessors. An analogy for genetic drift is seen in small isolated villages where everybody ends up with the same surname. If for example Mr and Mrs Smith are the only Smiths in the village and they have only daughters, then the surname Smith will disappear from the next generation. Over enough generations, the villagers will ‘drift’ to just one surname.

In contrast to Western Europe, the Balkan Peninsula was never isolated, and early humans there remained biologically similar to those from Southwest Asia. Accordingly, the population inhabiting the Balkan Peninsula could have retained a number of primitive non-Neanderthal traits, without precluding morphological changes associated with increased brain size and tooth reduction observed in Middle Pleistocene populations throughout Eurasia and Africa (Rink, Mercier, Mihailovic, Morley, Thompson, & Roksandic, 2013).

Rink et al (2013) New Radiometric Ages for the BH-1 Hominin from Balanica (Serbia): Implications for Understanding the Role of the Balkans in Middle Pleistocene Human Evolution may be downloaded from the open-access PLoS One website.

References:
Cameron, D., & Groves, C. (2004). Bones, Stones and Molecules: “Out of Africa” and Human Origins. Elsevier Academic Press.

Harvati, K. (2007). 100 years of Homo heidelbergensis - life and times of a controversial taxon. Mitteilungen der Gesellschaft für Urgeschichte, 16, 85-94.

Rightmire, P. (1998). Human Evolution in the Middle Pleistocene: The Role of Homo heidelbergensis. Evolutionary Anthropology, 6(6), 218-227.

Rink, W., Mercier, N., Mihailovic, D., Morley, M., Thompson, J., & Roksandic, M. (2013). New Radiometric Ages for the BH-1 Hominin from Balanica (Serbia): Implications for Understanding the Role of the Balkans in Middle Pleistocene Human Evolution. PLoS One, 8(2).

Roksandic, M., Mihailovic, D., Mercier, N., Dimitrijevic, V., Morley, M., Rakocevic, Z., et al. (2011). A human mandible (BH-1) from the Pleistocene deposits of Mala Balanica cave (Sicevo Gorge, Nis, Serbia). Journal of Human Evolution, 61(2), 186-196.

Weaver, T. (2009). The meaning of Neandertal skeletal morphology. PNAS, 106(38), 16028–16033.

Ice Age art: arrival of the modern mind

Astonishingly lifelike, the 21,000 year old mammoth-ivory bison sculpture (below) is unquestionably the work of a talented artist. Excavated at Zaraysk in Russia in 2002, it is one of 130 portable art objects from the European Upper Palaeolithic featured in Ice Age art: arrival of the modern mind, which opens at the British Museum on 7 February. These have been set alongside a small selection of works by Henry Moore, Henri Matisse, Piet Mondrian and other 20th Century artists. Exhibition curator Dr Jill Cook was unfortunately unable to include Genesis by Sir Jacob Epstein and works from the currently-closed Musée Picasso in Paris.

However, the modern art is not included for comparison but to emphasise that this exhibition is first and foremost about art rather than archaeology. Art is not merely a product of what we glibly term ‘civilisation’, it is a fundamental part of the modern human condition. We see the earliest evidence for its expression in South Africa over 70,000 years ago, but the earliest-known figurative art appears around 40,000 years ago in Europe. Modern humans entered Europe 46,000 years ago, and over the next 5000 years they dispersed across the continent. Although the last Ice Age had yet to reach its full extent, conditions were very different to those in the African homeland they had left tens of millennia earlier. Nevertheless, the richness of the art they produced over the next 35,000 years clearly demonstrates that there was far more to their lives than a grim battle for survival.

One of the earliest-known examples of what is unequivocally figurative art, the Löwenmensch (Lion-man) of Hohlenstein-Stadel (below) is a therianthropic (part human, part animal) figurine of a human figure with a lion’s head. It is 30 cm (11.8in) high and is carved from mammoth ivory. Hohlenstein-Stadel is an Aurignacian cave site in the Lone Valley of the Swabian Jura Mountains of south-western Germany. The Löwenmensch was discovered by archaeologists Otto Völzing and Robert Weitzel in 1939 in numerous fragments, but with the outbreak of World War II, it was forgotten for thirty years. Reconstruction of the figurine was begun in 1969 by Joachim Hahn, but was not completed until 1988. The Löwenmensch is over 36,000 years old. It may represent a shaman partially transformed into a lion, or a mythical being or a supernatural spirit. In 2003, a similar but much smaller figurine was discovered at Hohle Fels Cave in the Ach Valley, only a short distance from Lone Valley. The implication is that the people of the Ach and Lone Valleys were members of the same cultural group, and shared beliefs and practices connected with therianthropic images of felines and humans.

Among the most iconic objects of Ice Age Europe are the so-called 'Venus' figurines that have found at many Upper Palaeolithic sites. Most date to the Gravettian period from 28,000 to 22,000 years ago, though some are from the preceding Aurignacian. Typically lozenge-shaped, these figurines are characterised by exaggerated sexual characteristics, with very large breasts, accentuated hips, thighs and buttocks, and large, explicit vulvas. Other anatomical details tend to be neglected; especially arms and feet, and the heads generally lack facial detail. The contrast with the classical portrayal of Venus could not be greater. The figurines are carved from materials including mammoth ivory, serpentine, steatite or limestone and are often coloured with ochre. The Black Venus of Dolni Věstonice (below) was made from fired clay, and is among the earliest known ceramics.

The 'Venus' figurines are often interpreted as fertility figures, mother goddesses etc, but their real function is unknown. One novel suggestion, by anthropologists Leroy McDermott and Catherine Hodge McCoid, is that they may be self-portrayals of pregnant women. They note likenesses between a photograph of a “Venus” figurine viewed from above and one of a pregnant woman standing with her feet together, viewed from her own perspective looking down on her breasts and abdomen.

After visiting the newly-discovered Lascaux Caves in 1940, Pablo Picasso is said to have remarked “We have invented nothing” in reference to the cave’s 17,000 year old polychrome rock art. The story may be apocryphal, but the British Museum has offered the public a unique opportunity to judge for themselves.

Ice Age Art: Arrival of the Modern Mind is at the British Museum from 7 Feb until 26 May, 2013

Humans from the beginning

This blog has been largely defunct for nearly four years while I have been writing my book Humans: from the beginning which is now nearing completion. I have now resumed blogging on my new site, www.humanprehistory.com and may also be followed on Twitter @prehistory and @neandertalwatch.

When time permits, I hope to use this blog for posting photographs and for writing about non-prehistory related topics.

The Celestial Clockwork

Ask most people what they associate with astronomy and they will mention planets, stars, galaxies, etc. Ask them what they associate with astronomers, however, and the answer will probably be telescopes – but it is less than four hundred years since the invention of the telescope. By contrast, human awareness of the heavens goes back millennia; almost certainly deep into prehistoric times. Until comparatively recently, astronomers have studied the heavens with nothing more sophisticated than the naked eye. From earliest times, they would have tried to make sense of the great complexity of what can be seen in the skies.

Even the most casual observer will be aware the skies present a differing appearance from hour to hour, as the Sun, Moon and stars march steadily across the heavens. They will realise, however, that the stars are fixed in relation to one another, often making distinctive patterns in the sky. They will also realise that the Moon changes its appearance from night to night, sometimes waxing sometimes waning. But why does the Sun rise and set in different places at different times of the year? Why is the Moon sometimes visible in broad daylight? Why do some stars remain close to the same point in the sky and never set? And what is to be made of the bright star-like objects that do not remain in a fixed position in relation to their neighbours, but move at differing speeds across the starry background, always keeping to the roughly same plane as the Moon in its wanderings.

Today, we know that the Moon goes round the Earth and the Earth and other planets go round the Sun. However, this does not even begin to tell the full picture and the achievements of people such as the Maya and the ancient Babylonians – who were not even armed with these basic facts – cannot be overstated. The workings of the celestial clockwork make the achievements of the finest Swiss watchmaker pale into insignificance, yet from a modern perspective they are not difficult to understand; the object of this short (9,000 word) work is to give the reader just such an understanding.  

The Celestial Sphere

Astronomers often use a model known as a celestial sphere to illustrate the movements of the Sun, Moon, planets and stars as seen from Earth. This can be thought of as being similar to a geographer’s globe, except it surrounds the Earth and we look at it from the inside. Another way of thinking of it is as a grid system projected onto the heavens to help us find our way around. To demonstrate such a grid would at one time have required the facilities of a planetarium, but there are now many ‘augmented reality’ smartphone apps available that can achieve almost the same effect.

Let’s first consider the finer points of the celestial sphere itself. Like the geographer’s globe, it will have two poles, an equator and lines of latitude and longitude. The celestial North and South Poles and the celestial equator (usually referred to simply as the “equator”) are projections of their terrestrial counterparts onto the grid system. The coordinate system used to define a point on the grid system differs slightly to that used by geographers. The position of an object to the north or south of the equator is given by declination (Dec.). Like latitude, it is measured in degrees, but instead of suffixing the declination with an N or an S, northern declinations are given positive values and southern declinations negative values. The equivalent of longitude is right ascension (R.A.), which has a zero line similar to the Greenwich meridian that passes through a point known as the vernal point (to be discussed shortly) or first point of Ares. Right ascension is measured eastwards from the first point of Ares. It can be measured in degrees but is usually measured in hours, minutes and seconds. An hour corresponds to 15 degrees, so 24 hours is equivalent to 360 degrees.

From anywhere in the world, our field of view will be bounded by the horizon, which divides the celestial sphere exactly into two. The point directly above us is known as the zenith. Running through both poles and the zenith is a great circle known as the meridian. The angle of elevation of any object above the horizon is known as the altitude; the angular distance around the horizon, measured clockwise from due North is known as the azimuth.

Let us take an imaginary or smartphone augmented reality trip to the (geographical) North Pole. The celestial North Pole is directly overhead, at the zenith, and the equator lies exactly on the horizon. Between the equator and the Pole are a series of concentric circles, growing ever smaller the nearer they are to the Pole. These represent differing declinations. A series of lines extend upwards from the equator, converging at the Pole. These are the lines of right ascension. Note that because the declination circles are parallel to the horizon, we can see them in their entirety. However, we cannot see anything south of the equator at all.

If we observe the sky for a few hours, the grid and stars will appears to revolve in a clockwise around the celestial North Pole. This is happening because the Earth is rotating on its axis, in a west to east direction, or anticlockwise as viewed from “above” the North Pole. This motion is known as diurnal motion. During the course of its diurnal motion, a star will reach its highest point in the sky when it is on the meridian. When a star reaches the meridian, it is said to culminate.

If we look close to the Pole, we will see a bright star. This is Polaris, the Pole Star. It appears to be almost fixed while everything else wheels around it. The further a star is from the Pole, the larger the circle in which it moves, with those near the equator moving in the largest circle of all. Note that no star ever rises or sets; we can see half of the stars the whole of the time.

Before we move on, how long do you think it takes the stars to make a complete circuit of the skies? The answer is of course the same length of time it takes the Earth to make a complete turn on its axis – but this is not 24 hours. The Earth makes a complete turn on its axis once every 23 hours 56 minutes and four seconds. This period of time is known as the sidereal day. However, we reckon time by the Sun rather than the stars, and as we shall see, the solar day is slightly longer.

Now we will travel to London, which is at latitude 51.5 degrees N. The first thing we notice is that the grid now appears tipped over towards one side. The celestial North Pole is no longer at the zenith; in fact its altitude will be equal to the geographical latitude. The declination circles are now no longer parallel to the horizon, so only those close to the Pole will be visible in their entirety and the others will be visible only as ever-decreasing arcs. However, we can now partially at least see declination circles that are south of the equator. Exactly half of the equator is visible, and it cuts the horizon at points due east and due west. The celestial North Pole lies due north.

There is a declination circle whose southernmost point just touches the northern horizon, and only the stars within this circle now remain permanently above the horizon or, as Homer put it in The Odyssey, “never bathe in Ocean’s stream”. Such stars are said to be circumpolar from that latitude (at either Pole, as we have seen, all the stars are circumpolar). Stars further south do spend increasing amounts of time below the horizon, though those North of the equator are still up for more than half of a sidereal day. Stars lying directly on the equator are visible for exactly half of a sidereal day. Note that these stars rise due east and set due west. Stars located still further south are visible for less than half of a sidereal day. Finally, on the southern horizon, is the northernmost point of a declination circle that lies entirely below the horizon. Stars lying within this circle are permanently out of view and include those making up the Southern Cross and our nearest stellar neighbour, Alpha Centauri.

Next let us go down to the equator. The grid will now appear to be completely tipped onto its side, and the declination circles will now lie at right-angles to the horizon. Exactly half of each circle will be visible, but our observer can now see all of them. The celestial North Pole lies exactly on the northern horizon and, 180 degrees away, the celestial South Pole has come into view. Every single star will be above the horizon for half of the sidereal day.

It will now be clear that were we to continue south, the celestial North Pole would dip below the horizon and the process we have just witnessed would occur in reverse until upon reaching the geographical South Pole, we would see the celestial South Pole at the zenith.

The Sun and Seasons

Now let us observe the Sun and stars as a sidereal day passes. At the end of one sidereal day, all the stars will be back where they started – but the Sun will be lagging behind. In fact, it will take approximately four minutes for the Sun to catch up. This is because while the Earth has been spinning on its axis, it has also been moving in its orbit around the Sun, completing a 1/365.24th of a circuit. Like the axial rotation, the orbital motion is west to east, or anticlockwise. (Astronomers refer to such motion as direct; clockwise motion (which is rare in the Solar System) is said to be retrograde.) The Sun, as viewed from Earth, appears to have changed its position slightly with respect to the stars. A solar day is defined as the time between successive crossings of the meridian by the Sun, but because the Earth’s orbital speed varies slightly over the course of a year, the solar day is not constant in length. It is only over the course of a year that it averages out to the familiar 24 hours.

The result of the solar day being about four minutes longer than the sidereal day is that any given star will rise four minutes earlier each (solar) day. This is why the constellations visible at a given time vary over the course of the year. After a year has passed, the solar and sidereal days come back into step. Our star will rise at the same time that it did on the corresponding day a year ago.

If we follow the Sun’s apparent movement over the course of a year, we will see that it will make a complete circuit of the celestial sphere. The path it traces out is known as the ecliptic. The ecliptic represents the plane of the Earth’s orbit around the Sun and it is inclined to the equator at an angle of 23.5 degrees. The reason for this is that the Earth’s axis of rotation is not perpendicular to the plane of its orbit, but inclined at an angle of 23.5 degrees. This inclination is known as the obliquity of the ecliptic.

Constellations straddling the ecliptic are said to be zodiacal. These include the familiar twelve signs of the Zodiac, but to make matters confusing there is actually a thirteenth zodiacal constellation, Ophiuchus the Serpent Bearer, that has been ignored by astrologers and is not considered to be part of the Zodiac. The two points at which the ecliptic and the equator intersect are known as the vernal point (which we have already encountered) and the autumnal point.

What effect will the Sun’s movement along the ecliptic have over the course of a year? The ecliptic is inclined to the equator, so the Sun will spend half of the year north of the equator and the other half of the year south of it. Recall that for the Northern Hemisphere, if a star is north of the equator it will be up for more than half of the sidereal day, but if it is south of the equator it will be up for less than half of the sidereal day. The same applies to anything on the celestial sphere, and this includes the Sun.

Thus for half of the year, day will be longer than night and for the other half it will be shorter. On the two days of the year known as equinoxes, day and night will be of equal length; this will occur when the Sun is at the vernal or autumnal point. The vernal point is known as the Sun’s ascending node because this is where it crosses into the northern hemisphere from the south. Similarly, the autumnal point is known as the descending node. Mid-way between the equinox points, the Sun will reach its most northerly and most southerly positions; these points are known as the solstices. In the northern hemisphere, the summer solstice occurs when the Sun is at the most northerly point on the celestial sphere and the winter solstice when it is at the most southerly point. This is the explanation for the seasons.

In lower latitudes, the Sun attains greater elevations above the horizon. When it is at either equinox, the Sun will be directly overhead at midday along the equator. At the summer solstice, it will be directly overhead at midday in the latitude defined by the Tropic of Cancer and at the winter solstice it will be directly overhead at midday in the latitude defined by Tropic of Capricorn. This is why it gets rather hot in these parts of the world. Conversely, the Sun is circumpolar during the summer months within the Arctic Circle, but never rises at all during the winter months. This is the explanation for the famous ‘Midnight Sun’. The situation is reversed for the Antarctic Circle.

Most people think of the Sun rising in the East and setting in the West. But the rising point of the Sun is actually only due East on two days of the year, those when it is at one or other of the equinox points. In the summer months, the azimuth of rising point moves north, reaching its maximum extent at the summer solstice, before returning south. In the winter months, the azimuth of the rising point moves south reaching its maximum extent at the winter solstice, before returning north. Around the solstices, the rising appears to stand still for a few days; the word ‘solstice’ is derived from this phenomenon. The setting points move in the same manner, with the winter sunset limit lying opposite the summer sunrise limit, and the summer sunset limit lying opposite the winter sunrise limit. These seasonal variations in the rising and setting points vary with latitude, being more pronounced in higher latitudes.

The Earth’s Orbit 

Let us now consider the Earth’s orbit around the Sun in a little more detail. The orbit is not circular but elliptical, meaning that distance between the Earth and the Sun isn’t constant but varies over the course of the year, ranging from 147 million km (when Earth is said to be at perihelion) to 152 million km (aphelion). The Earth’s orbital speed is at its greatest at perihelion and at its least at aphelion. This is because its movements are governed by Kepler’s Laws of Planetary Motion, which we will examine in more detail presently. The mean distance is 149.6 million km (93 million miles) and this distance is referred to as the astronomical unit (AU). The departure of the orbit from a perfect circle is known as the orbital eccentricity. Perihelion does not occur in the same place each year, but advances by 11.64 seconds of an arc on each orbit. This is due to gravitational effects of other planets in the Solar System.

Precession, Nutation and other cycles

There are in addition a number of more gradual motions which are only significant over a long term. The most important of these is precession. In addition to rotating on its axis, the Earth also oscillates like a spinning top, each oscillation taking about 25,800 years and causing the Earth’s spatial orientation to gradually change. This motion is due chiefly to the pull of the Sun and the Moon, though the planets make a small contribution.

The observable effect is to make the nodes of the ecliptic gradually move westwards at about 50 seconds of an arc per year. The stars will remain fixed in relation to the ecliptic, but as our celestial sphere grid system uses the Earth as its frame of reference, they will appear to move very slightly against it. This means that star maps have to be calibrated for a particular epoch which since 1984 has been Epoch 2000.0, the start of the year 2000.

The effect, though small, is cumulative and just about visible to the naked eye over a lifetime (a good amateur telescope on a suitable mount could show it in a matter of weeks if not days). More significant effects are experienced over longer periods – in antiquity, for example, the Southern Cross could be seen from Greece and the ancient Greeks included it in their star charts. 14,000 years from now it will be visible all over Britain. The precessional motion is not smooth but slightly wavy. This irregularity is known as nutation and it is the result of a slight nodding of the Earth due to variations in the distances and positions of the Sun and the Moon.

In addition to these effects, the obliquity of the ecliptic varies with time, chiefly due to nutation, though the gravitational effects of other planets also play a part. Finally the eccentricity also varies, albeit very gradually. Again, this is due to the gravitational effects of other planets. The precessional cycle and the cyclical changes in the obliquity and orbital eccentricity are now known as the Milanković cycles. They are named for the Serbian mathematician Milutin Milanković, who proposed a link between them and cyclical changes in Earth’s climate while interned in Budapest during World War I. Climatologists now accept that Milanković was right, but his views attracted little interest in his lifetime.  

Four types of year

Up until now, we have used the term ‘year’ rather loosely. Most people think of a ‘year’ as being the time it takes the Earth to go once round the Sun. That is certainly a type of year, but not the only type. The Earth in fact has four different types of year. The first is the sidereal year (365.256 days), and this is indeed the time it takes the Earth to go once round the Sun – but this is not the ‘year’ we base our calendar on.

The Gregorian calendar, which is now used throughout the Christian world, is actually based on the tropical year (365.243 days), which is defined as the time between the Sun making two passages through the vernal point. The vernal point is moving slowly in the opposite direction to the Sun along the ecliptic as a result of precession, so the Sun ‘arrives’ there about twenty minutes before it completes its circuit of the celestial sphere. Hence the tropical year is slightly shorter than the sidereal year. However, the progression of seasons is dictated by the former, so the calendar is based upon it.

The third type of year is the anomalistic year (365.260 days), which is defined as the time between the Earth making two returns to perihelion. As we have seen, the perihelion advances with each circuit, the Earth requires a bit longer to ‘catch up’; hence the anomalistic year is fractionally longer than the sidereal year.

Finally there is the eclipse year (346.620 days), which we shall encounter presently.

The Moon and its Orbit

Most people will think nothing if they happen to see the Moon in the night sky, but are often surprised to see it in broad daylight. In fact, the Moon spends on average as much of its time above the horizon in day time as it does in night time. The most singular feature of the Moon is, in fact, something which most of us take completely for granted. This is that it appears almost exactly the same size as the Sun. The explanation is simple – the Sun is about four hundred times larger in diameter than the Moon, but it is also about four hundred times further away. The odds against this happening – if not quite astronomical – are pretty low.

The Moon’s orbit around the Earth is inclined at an angle of 5 degrees to the ecliptic. The Moon’s apparent path around the celestial sphere intersects the ecliptic at two points again known as nodes; as with the intersections between the ecliptic and equator there is an ascending node and a descending node. These nodes do not remain fixed, but move westwards on the celestial sphere at 19 degrees per year due to perturbation by the Sun, taking 18.61 years to complete a nodal cycle. This phenomenon is known as the regression of nodes.

The orbit itself is rather more elliptical than that of the Earth around the Sun. Distance from Earth (centre to centre) varies from between 356,410 kilometres (minimum distance, or perigee) to 406,697 kilometres (maximum distance, or apogee). The Moon’s orbital speed increases at perigee, and it decreases at apogee. Like the Earth, this is due to Kepler’s Laws of Planetary Motion, which apply to all orbiting bodies. The eccentricity of the orbit is quite pronounced, so the effect is quite noticeable in terms of nightly movement on the celestial sphere, and this has been known since ancient times. In a manner similar to the Earth’s perihelion, the Moon’s perigee advances with each orbit, taking 8.85 years to complete a cycle.

Phases of the Moon

The most noticeable feature of the Moon is that its appearance changes from night to night. These phases are due to differing portions of its day-lit side being presented to us as it moves around the Earth. At the start of the cycle it cannot be seen because it lies in the same direction as the Sun and its illuminated side faces away from us. A few days later it will have moved eastwards away from the Sun and will be seen as a slim crescent in the evening sky. As the days pass, the Moon is seen ever higher in the evening sky as it continues to grow or wax. After seven to eight days the Moon will be 90 degrees east of the Sun in the sky; this point is known as the first quarter and the right half of the Earth-facing side is illuminated. At around fifteen days, the Moon’s distance from the Sun reaches 180 degrees. At this point the Moon rises at sunset. The entire Earth-facing side is now illuminated and we see a full Moon. Thereafter, the Moon begins to wane, going through its phases in reverse as its angular distance from the Sun begins to decrease once more. After about 22 days the Moon is 90 degrees west of the Sun; this point is known as the third quarter and the left half of the Earth-facing side is illuminated. Subsequently the Moon becomes an increasingly slim crescent, moving ever closer to the Sun and appearing only just before sunrise. Finally, after 29.531 days on average, it disappears from view and the cycle begins again.

Five types of month

Most people think of this cycle of 29.531 days as being a month, but they also think of the Moon going round the Earth once a month. In fact, the Moon takes only 27.321 days to go round the Earth. So which ‘month’ is right? Well actually both are. It all depends on what is meant by a month. As we have seen, the Earth has four types of year; the Moon, not to be outdone, has five types of month.

The most obvious, perhaps, is the time the moon takes to go once round the Earth. This is known as the sidereal month and as we have seen, it is 27.321 days. But because the Earth is moving round the Sun at the same time the Moon is moving round the Earth, it takes the Moon a bit longer than a sidereal month to return to the same position with respect to the Sun and the Earth. As it is this which governs the phases, it takes more than a sidereal month to go through a complete cycle or lunation. The time for a lunation is known as the synodic month. The Earth’s orbital speed varies slightly over the course of a year, so the synodic month is not fixed. 29.531 days is only the average figure.

The tropical month is slightly shorter than the sidereal month. It is defined as the time from one lunar equinox to another. The lunar equinox occurs when the Moon crosses the equator; this takes slightly less than a sidereal month due to the effects of precession (c.f. tropical year).

Next is the anomalistic month, the time taken for the Moon to go from perigee to perigee. The perigee advances, so this is longer than the sidereal month and is 27.554 days.

Finally we have the draconic month of 27.212 days. This is the time between successive passages by the Moon through the same node. The nodes are moving westwards and the Moon is moving eastwards along the celestial sphere, so it takes less than a complete orbit for the Moon to return to the node, and thus the draconic month is shorter than the sidereal month. The word ‘draconic’ refers to a mythical dragon thought to devour the Sun and the Moon during solar and lunar eclipses; the eclipse year is also sometimes referred to as the draconic year for this reason.

The interrelationship of various types of month and year are of great importance when it comes to predicting eclipses, and these cycles may have been understood as far back as prehistoric times.

Lunar and Solar Calendars

In the Western world, we have long been accustomed to a year of 365 days, with a leap day inserted into February every fourth year. The Gregorian calendar, now the most widely used civil calendar in the world, does have exceptions to this leap year every fourth year rule, but the last such ‘non-leap year’ was in 1900 and the next will not occur until 2100. Most of us will live our lives without ever having been troubled by such details, but they are important.

The Gregorian calendar is an example of a solar calendar, or one based on the tropical year. Since this is not an exact number of days, a leap day must be intercalated (inserted) at intervals, and the Gregorian calendar provides for an extra day in February if the year is divisible by four. An exception to the rule is made if the year is divisible by 100 but not by 400, as is the case for 1900 and 2100, but not 2000. The Gregorian calendar was introduced in 1582 in the time of Pope Gregory XIII. It was a refinement to the earlier Julian calendar, which inserted the leap day every fourth year without exception. This gave a year of 365.25 days, which is slightly longer than the tropical year of 365.243 days. The Julian calendar was introduced by Julius Caesar in 46 BC and the error, though small, had amounted to several days by the sixteenth century. The Gregorian was not immediately adopted everywhere, due to resistance in Protestant countries to a Catholic innovation. In Britain, the changeover did not occur until 1752, by which time the correction amounted to eleven days and so Wednesday, 2 September was followed by Thursday, 14 September. The story that this led to riots by people demanding the return of their eleven days is probably apocryphal. In Russia the new system was not adopted until early in the communist era, by which time thirteen days had to be dropped from the calendar. An ironic consequence was that the date of the Great October Socialist Revolution was shifted into November.

Solar calendars follow the seasons, but the months do not follow the phases of the Moon because there are not an exact number of synodic months in a tropical year. A lunar calendar is one based on the phases of the Moon and examples include the Islamic calendar, which comprises twelve synodic months and therefore lags the solar calendar by 11 to 12 days each (tropical) year. The Islamic calendar is the official calendar of Saudi Arabia, but elsewhere in the Islamic world it is used mainly for religious purposes.

To get round the problem of a lunar calendar fairly rapidly drifting out of synch with the tropical year, some calendrical systems insert an intercalary month every so often, though various calendars use different systems for determining how and when these occur. Such systems are known as lunisolar; examples include the Hebrew and Chinese calendars.

Lunar and solar calendars generally come into line every 19 years. This is because 19 tropical years are almost exactly 235 synodic months; thus every 19 years the Moon will have the same phase on the same day of the year. This 19-year cycle is known as the Metonic cycle after the Greek philosopher Meton of Athens (ca 440 BC) who noticed it, though it was undoubtedly known earlier. The Metonic cycle formed the basis of the Greek calendar until 46 BC, when the Julian calendar was adopted.

Moonrise, Moonset and lunar movements

Like the Sun, the Moon does not rise and set in exactly the same place every day. The azimuth of the rising and setting points varies cyclically over the course of a sidereal month between northern and southern limits and, as with the Sun, these variations are more pronounced in higher latitudes. Note that the ‘month’ in question here is the sidereal rather than synodic month hence the Moon will not be at the same phase at two successive risings or settings at a particular point. Another way of looking at this is to consider only the azimuth of rising and setting of the full Moon, which will vary between the same limits over the course of a year.

However, these limits themselves open out and close up over the course of the 18.61 year nodal cycle. In the Northern hemisphere, the variation reaches a maximum when the ascending node is co-incident with the summer solstice; these are the major standstill points. When the descending node reaches this point, the variation is at a minimum; these are the minor standstill points. Between these limits, the standstill points gradually close up and then re-open. The situation is reversed in the Southern Hemisphere.

In simpler terms, at the major standstill the Moon’s 5 degree orbital inclination is added to the effect of the Earth’s axial tilt; at the minor standstill it is subtracted. Thus the variation exceeds that of the Sun at major standstill, but is less than it at the minor standstill.

As we have seen, the cycle is driven by the sidereal and not the synodic month, so different phases of the Moon will be best observed at different times of the year. The full Moon, for example, rides majestically high in the winter skies, but in summer its performance is decidedly lacklustre. It struggles into the sky, staggers wearily along the southern horizon for a few hours before giving up and disappearing again. The explanation is straightforward enough: when full the Moon is in the opposite part of the sky to the Sun, so in winter it behaves as the Sun in summer, and vice-versa. In spring, the waxing first quarter Moon is most favourably presented for observation, and in autumn it is the turn of the waning last quarter Moon. The waxing crescent is best seen in mid-spring; the waning crescent in mid-summer. These rules hold in both hemispheres, because the seasons are reversed in the Southern Hemisphere. As with the standstill points, these effects are accentuated and diminished over the course of the 18.61 year nodal cycle.

The Dark side of the Moon

When people refer to ‘the dark side of the Moon’ they really mean the side that cannot be seen from here on Earth. As is correctly pointed out in the eponymous Pink Floyd album, there is no dark side of the Moon and both sides experience equal portions of day and night. It is, however, true that the Moon’s sidereal day is exactly one sidereal month, so in the main one side permanently faces the Earth. However, it is not strictly speaking true to say that we can only see one side from Earth.

The orbital speed is not constant, so the orbit and rotation get slightly out of step at times, which causes a slightly different face to be presented. This effect is known as libration in longitude. In addition, because the Moon’s axis is inclined by 6.5 degrees to its orbit, it appears to ‘nod’ back and forth over the course of a month – this is libration in latitude. Finally, parallax effects result in slightly different faces being presented to the observer at different times of the day; in total 59 percent of the Moon’s surface may be seen from Earth (though of course no more than 50 percent at any one time).

The Wanderers

The word ‘planet’ comes from the Greek word planetes, meaning ‘wanderer’. Long before the time of the Classical Greek civilisation, man would have been aware of bright star-like objects that did not did not remain fixed in relation to the stars but moved in roughly the same narrow band to which the Sun and Moon are constrained. Five planets (excluding the Earth) have been known since prehistoric times – Mercury, Venus, Mars, Jupiter and Saturn. They fall into two groups, the inferior planets, whose orbits lie close to the sun that of the Earth (Mercury and Venus) and the superior planets whose orbits whose orbits lie further away from the Sun (all the other planets, excluding Earth). The distance of each planet from the Sun is often given in astronomical units. Incidentally, the terms ‘superior’ and ‘inferior’ do not mean that the superior planets are ‘better’ planets.

The motion of each planet around the Sun is governed by Kepler’s Laws of Planetary Motion, which were formulated by the German mathematician Johannes Kepler between 1609 and 1618 and they apply not just to planets but all orbiting bodies, such as the Moon, satellites of other planets and even artificial Earth satellites.

The First Law states that the orbit of any planet around the Sun will be an ellipse, with the Sun at one focus. (If you add the distances of any point on an ellipse from each of the two foci you will always get the same result. By comparison, if you measure the distance of any point on a circle from the centre of that circle, you will always get the same result. In fact these properties define circles and ellipses, which are both examples of what are termed conic sections by mathematicians.

The Second Law states that the movement of any planet in its orbit is such that its radius vector (an imaginary line joining the planet to the Sun) sweeps out equal areas in equal times. This explains why the Earth and other planets move faster when they are close to perihelion and why the Moon moves faster when it is close to perigee. The sector swept out in, say, 24 hours, is shorter at these times, but because the Earth (or Moon) is moving faster, it is also ‘fatter’ and these two effects exactly cancel out.

The Third Law states that the square of a planet’s orbital period in years is equal to the cube of its mean distance from the Sun in astronomical units. More generally, the square of the orbital period of any orbiting body is proportional to its mean distance from the body it orbits.

These laws arise naturally from Newton’s Law of Universal Gravitation, which states that between any two objects, there exists an attractive force that is proportional to their masses multiplied together and divided by the square of their distance apart. Objects under consideration can be stars, planets, satellites or even the apocryphal apple that is said to have given Newton the idea in the first place.

Aspects of the planets

As seen from the Earth, certain positions of the planets relative to the Sun are known as aspects. For superior planets the two principal aspects are opposition and conjunction.

At opposition, a planet is opposite to the Sun in the sky, i.e. they are 180 degrees apart. It will be visible throughout the night and will reach the meridian it midnight. Opposition is the best time to observe a superior planet, because it is at its closest to the Earth. At conjunction, a superior planet is on the opposite side of the Sun to the Earth. It will not be visible from earth at this time, being lost in the Sun’s glare.

When a planet is at either opposition or conjunction (i.e. it, the Earth and the Sun are in a straight line) it is said to be at syzgy. The Moon is at syzgy when it is both new and full. When a planet is at an angle of 90 degrees from the Sun as seen from Earth, it is said to be at quadrature. We see a half-Moon when it is at quadrature.

Inferior planets cannot reach opposition or quadrature, but have two types of conjunction, inferior conjunction, when they lie between the Earth and the Sun and superior conjunction, when they are on the far side of the Sun. When an inferior planet is at its greatest angular separation from the Sun it is at greatest elongation. At its greatest elongation west it will appear in the morning sky; at greatest elongation east it will appear in the evening sky. An inferior planet can never be seen throughout the night.

The inferior planets display phases like the Moon but when best seen (i.e. at elongation) they are crescent. They will be at full phase at superior conjunction and “new” at inferior conjunction, but cannot be seen at these times. The superior planets show very little phase effect; only Mars shows a pronounced gibbous phase when it is at quadrature.

Movements of the planets

As seen from the Earth, the planets normally appear to move from west to east. However, around opposition a superior planet can appear to halt and then move briefly in an east to west direction before resuming its normal progress. This retrograde motion, so beloved of astrologers, occurs because the Earth, which is moving more rapidly, catches up and overtakes the planet in question. The points where the planet halts before changing direction are known as stationary points.

The planets all keep fairly close to the ecliptic, but all have orbits that are slightly inclined to it. Orbits are defined in terms of six elements or quantities. These are the semi-major axis (a) or mean distance from the Sun; the eccentricity (e); the inclination to the ecliptic (i); the longitude of the ascending node (Ω); the argument of perihelion (ω) which is angular displacement from Ω; and the time of perihelion passage (T).

A planet’s ‘year’ is known as its sidereal period, corresponding to the Earth’s sidereal year. The time taken for a planet to return to a particular aspect (such as opposition) as seen from Earth is known as the synodic period (c.f. the Moon’s synodic month).

Eclipses

There is little doubt that a total eclipse of the Sun is one of the most awesome spectacles of Nature available anywhere in the Solar System. On no other planet is there such an exact match between the apparent size of the Sun and the apparent size of a satellite – despite some planets having upwards on fifty of the latter to choose from, while we on Earth have to make do with just the one. Not quite as spectacular, perhaps, but still noteworthy is the sight of the Moon turning a deep blood-red as it enters the Earth’s shadow during a lunar eclipse.

The phenomena are related, but strictly speaking a solar eclipse is an occultation or hiding of a self-luminous body (in this case the Sun) by the Moon. In principle there is no difference between this and the occultation of stars that occur throughout the month as the Moon pursues its course around the Earth. By contrast, a lunar eclipse entails the Moon being cut off from the source of its illumination as it enters the Earth’s shadow.

Unlike point sources (such as a distant searchlight), extended luminous objects such as the Sun do not cast sharp shadows. A shadow will of course be cast when an object is interposed between the observer and the light source, but it will have two regions: the umbra in which the light source is wholly obscured and the penumbra in which it is only partially obscured.

For a disc such as the Moon, the Earth as seen from the Moon’s surface or a hot-air balloon drifting in front of the Sun as seen by an observer on the ground, the umbra will be cone-shaped, converging to a point; the umbra will be fan-shaped and diverging.

Types of Solar eclipse

The Moon’s umbra under favourable conditions will just reach the Earth. It does not remain stationary but races across the Earth’s surface as the Moon moves in its orbit. The path it follows is known as the track. Observers inside the umbra will see a total solar eclipse; those outside it but still within the penumbra will see a partial solar eclipse; those completely outside the Moon’s shadow will see nothing.

The degree of obscuration of the Sun by the Moon or magnitude will increase the closer an observer is to the zone of totality. Magnitude ranges for 0 (no obscuration) to 1 (totality) and it refers to the solar diameter covered, not area. A 0.5 magnitude eclipse is one in which half the Sun’s diameter is covered, but a little geometry will show that only 40 percent of the Sun’s area will actually be hidden by such an eclipse.

The actual duration of totality for any eclipse varies and is dictated by three factors: the distance of the Earth from the Sun when the eclipse occurs; the distance of Moon from the Earth when the eclipse occurs; and the latitude at which the eclipse occurs.

If the Earth is at its maximum distance from the Sun its apparent diameter will be diminished and if the Moon is at its minimum distance from Earth its apparent diameter will be increased; these factors favour long eclipses.

The Earth is rotating in the same direction as the Moon’s shadow is moving, and this has the effect of prolonging the time the latter will linger over a particular region. The speed the Earth’s surface is moving depends on latitude – at 40 degrees north or south of the equator, the west to east motion is 1,270 kilometres per hour but at the equator it is 1,670 kilometres per hour. The relative speed of the Moon’s shadow is thus lower at lower latitudes and thus eclipses that take place in tropical latitudes tend to be of greater duration than those occurring in temperate latitudes.

If the Moon is at or close to its maximum distance from Earth, even if it passes directly in front of the Sun the umbra will not quite reach Earth and a ring of sunlight is left showing. Such eclipses are said to be annular. Total and annular eclipses are referred to as central eclipses, and annular eclipses are the slightly more frequent of the two types.

Occasionally, an eclipse is just total at mid-track, but due to the curvature of the Earth the umbra doesn’t touch the end-points. The result is a hybrid total/annular eclipse with observers at mid-track experiencing a total eclipse but those at either end-point viewing only an annular eclipse.

Finally in about one third of all solar eclipses only the penumbra reaches the Earth with the umbra missing it altogether. Such eclipses are partial only; nowhere on Earth is a total eclipse seen.

Stages of a Solar eclipse

The key events in a solar eclipse as viewed from a particular site are known as contacts. First Contact occurs when the Moon’s western edge begins to slide across the Sun and is the point at which the penumbra first begins to move across the site. It is abbreviated to P1, for first penumbral contact. Second Contact occurs when the Moon’s eastern edge touches the Sun’s eastern edge. The marks the onset of totality or annularity, and for a total eclipse is the point at which the umbra begins to move across the site. It is abbreviated to U1 for first umbral contact (though strictly speaking this term is only appropriate for a total eclipse). Third Contact occurs when the Moon’s western edge leaves the Sun’s western edge. This marks the end of totality or annularity and is the point at which the umbra leaves the site. It is abbreviated to U1. Finally Fourth Contact, abbreviated to P2, marks the departure of the penumbra from the site and the end of the eclipse. In a partial eclipse, only P1 and P2 occur.

Types of Lunar eclipse

Whereas the Moon’s umbra will affect only a small portion of the Earth, the Earth’s umbra is large enough to fully immerse the Moon. During a lunar eclipse, the Moon never entirely disappears from view but appears reddish. This is due to refraction or bending of sunlight by the Earth atmosphere into the umbra; red light is more easily refracted. There are three types of lunar eclipse; total, when the whole of the Moon enters the umbra; partial when only a portion does; and penumbral when the Moon just grazes the penumbra. The latter type generally results in only a slight dimming of a portion of the Moon and is often undetectable to the naked eye. Unlike a solar eclipse, a lunar eclipse may be viewed anywhere on Earth where the Moon is above the horizon.

Stages of a Lunar eclipse

As with solar eclipses, the key stages of a lunar eclipse are referred to as contacts though unlike a solar eclipse these are the same from any point on Earth. P1 occurs when the Moon begins to enter the Earth’s penumbra. U1 is the point at which the Moon begins to enter the umbra; U2 is the point at which it is fully inside the umbra, marking the onset of totality. U3 is the point at which the Moon begins to leave the umbra, marking the end of totality; U4 is the point at which the Moon leaves the umbra altogether. P2 is the point at which the Moon leaves the penumbra and the eclipse ends. U2 and U3 do not occur in a partial eclipse. In a penumbral eclipse, U1 and U4 do not occur either.

When do solar eclipses occur?

As you might have inferred, a solar eclipse can only occur at new Moon – but why don’t they occur at every new Moon, i.e. once every lunation? Recall that the Moon’s orbit is inclined at about 5 degrees to the ecliptic. So the Moon usually ‘misses’ the Sun. Recall that there are two nodes where the Moon’s path crosses the ecliptic. Only when the Sun is close to a node at new Moon can an eclipse occur, although it does not have to be exactly at a node for an eclipse to occur; for the two discs to touch in a ‘grazing’ encounter will at minimum produce a partial eclipse.

The region the Sun has to occupy at new Moon to produce an eclipse is known as the eclipse limit. This varies, depending on the distance of the Moon from Earth at the time the new Moon occurs, and that of the Earth from the Sun. It ranges from between 30.70 degrees to 37.02 degrees in total, or from 15.35 to 18.51 degrees each side of the node. For a central eclipse to occur, the limit is less, ranging from 9.92 to 11.83 degrees each side of the node.

With the Sun moving along the celestial sphere at just under one degree per day, it will be apparent that it will take it more than a synodic month of 29.53 days to traverse even the minimum distance. In other words, the Sun will never be able to get through one of these ‘danger zones’ without the Moon catching up with it at some stage and causing an eclipse. Furthermore, if the Sun has only just entered the eclipse limit when the Moon comes around, the latter will have time to cause a second eclipse before the Sun can get out of the way.

The time period during which the Sun is within the eclipse limit is known as an eclipse season. Eclipses can only occur during an eclipse season and as we have just seen, at least one must occur. How many eclipses will occur in a calendar year, given at least one must occur whenever the Sun approaches a node?

Recall the nodes are moving along the celestial sphere in the opposite direction to the Sun, completing a complete cycle every 18.61 years. It will therefore take the Sun slightly less than a year to make successive passages through the same node. This is the ‘fourth kind of year’, the eclipse year mentioned earlier, of 346.62 days. There will be two eclipse seasons in each eclipse year and a minimum of two solar eclipses and a maximum of four.

The calendar year is longer than an eclipse year and so the eclipse year will end at different times of the calendar year. Normally there will only be two eclipse seasons (and hence a minimum of two eclipses) in a calendar year, but if an eclipse year ends in December, a portion of a third eclipse season can be squeezed into that calendar year, meaning that a maximum of five solar eclipses could occur. Unfortunately, you will have to wait until 2206 before this next happens.

When do lunar eclipses occur?

Just as solar eclipse can only occur at new Moon, so a lunar eclipse can only occur when the Moon is full. The condition for a lunar eclipse is for the Moon to pass through the opposite node to the one through which the Sun is passing during an eclipse season. As with a solar eclipse, this must happen at least once during an eclipse season, and can happen twice.

The maximum number of both types combined in an eclipse season is only three, because it would take 1 ½ lunations to produce two solar and two lunar eclipses, which is longer than the maximum length of an eclipse season. However, there will always be at least one of each. This rule does include penumbral lunar eclipses, which many authorities omit from eclipse statistics.

The Saros

The word ‘saros’ is taken from an ancient Babylonian word meaning ‘repetitive’ and was adopted by Sir Edmund Halley to describe an 18-year cycle of eclipses first recorded by the Babylonians in 400 BC, though it may well have been known much earlier.

The saros results from a series of coincidences of nature: 223 synodic months (6585.32 days) is almost exactly the same length of time as 19 eclipse years (6585.78 days) and also coincides with 239 anomalistic months (6585.54). The net effect is that at the conclusion of 223 synodic months from the time of an eclipse, not only are the Sun and Moon in the same places in the sky (thus giving rise to another eclipse) but the Moon will be at the same distance from the Earth as for the previous eclipse and the eclipse limit will thus be the same. This latter factor is equally important because were the Moon to be at a greater distance from Earth than previously, the eclipse limit would be smaller and an eclipse might not occur at all.

However, there is one important difference. 223 synodic months does not contain a whole number of days. The odd 0.32 of a day means the second eclipse will occur at a longitude of 0.32 times 360 degrees, i.e. approximately 115 degrees west of the first eclipse due to the Earth’s additional rotation.

Eclipses occur more frequently than every 18 years, so there are a number of saros cycles in operation at any one time, and each one is given a number. Saros cycles involving the Moon’s descending node receive even numbers and those involving the ascending node receive odd numbers. Each saros cycle evolves and has a finite life. For a saros cycle involving the Moon’s descending node, the series begins with an eclipse at the South Pole. Each successive eclipse then has a track more northerly than the last, until a final eclipse at the North Pole concludes the cycle. For a saros cycle involving the Moon’s ascending node, the reverse happens, with the series beginning at the North Pole and concluding at the South Pole.

At any one time there will be 43 saros cycles in operation and as soon as one concludes at one Pole another one will begin at the other Pole. The length of a cycle ranges from between 1,206 to 1,442 years.

This all happens because 19 eclipse years are actually 0.46 days longer than 223 synodic months, and the Sun will not be in exactly the same place for successive eclipses. Given the Sun moves approximately one degree per day along the celestial sphere, each eclipse will occur about 0.46 degrees west of its predecessor. The Moon of course will also be 0.46 degrees further west than before. For the descending node, this will additionally put the Moon slightly further north than before; for the ascending node, the Moon is slightly further south than before.