Archaeoastronomy and Stonehenge

Stonehenge in 2004

The prehistoric monument of Stonehenge has long been studied for its possible connections with ancient astronomy. Archaeoastronomers have claimed that Stonehenge represents an "ancient observatory," although the extent of its use for that purpose is in dispute. Many also believe that the site may have had astrological/spiritual significance attached to it as well.

The discovery of evidence for a neighbour to the Heel Stone has challenged the interpretation of it as a midsummer sunrise marker. The second stone may have instead been one side of a 'solar corridor' used to frame the sunrise.

Early work

Stonehenge features an opening in the henge earthwork facing northeast, and suggestions that particular significance was placed by its builders on the solstice and equinox points have followed. For example, the summer solstice sun rose close to the Heel Stone, and the sun's first rays shone into the centre of the monument between the horseshoe arrangement. While it is possible that such an alignment can be coincidental, this astronomical orientation had been acknowledged since William Stukeley drew the site and first identified its axis along the midsummer sunrise in 1740.

Stukeley noticed that the Heel Stone was not precisely aligned on the sunrise. Year to year, the movement of the sun across the sky appears regular. However, due to temporal changes in obliquity of the ecliptic, illumination declinations change with time. The purported Heel Stone alignment with summer solstice sunrise would have been less accurate four to five thousand years ago. The Heel Stone, in fact, is located at 1/7th of circumference from due North, as noted by archaeologist James Q. Jacobs.

Early efforts to date Stonehenge exploited tiny changes in astronomical alignments and led to efforts such as H Broome's 1864 theory that the monument was built in 977 BC, when the star Sirius would have risen over Stonehenge's Avenue. Sir Norman Lockyer proposed a date of 1680 BC based entirely on an incorrect sunrise azimuth for the Avenue, aligning it on a nearby Ordnance Survey trig point, a modern feature. Petrie preferred a later date of AD 730 although the necessary stones were leaning considerably during his survey and it was not considered accurate.

An archaeoastronomy debate was triggered by the 1963 publication of Stonehenge Decoded, by British born astronomer Gerald Hawkins. Hawkins claimed to see a large number of alignments, both lunar and solar, and argued that Stonehenge could have been used to predict eclipses. Hawkins' book received wide publicity, in part because he used a computer in his calculations, then a rarity. Archaeologists were suspicious in the face of further contributions to the debate coming from British astronomer C. A. 'Peter' Newham and Sir Fred Hoyle, the famous Cambridge Cosmologist, as well as by Alexander Thom, a retired professor of engineering, who had been studying stone circles for more than 20 years. Their theories have faced criticism in recent decades from Richard J. C. Atkinson and others who have suggested impracticalities in the 'Stone Age calculator' interpretative approach.

Newham and the Station Stones

Newham had found an alignment for the equinonxes by drawing a line between one of the Station Stones with a posthole next to the Heel Stone. Moving away from the sun, he also identified a lunar alignment; the long sides of the rectangle created by the four station stones matched the moon rise and moonset at the major standstill.

Two of the Station Stones are damaged and although their positions would create an approximate rectangle, their date and thus their relationship with the other features at the site is uncertain. Stonehenge's latitude is unusual in that only at this approximate latitude (within about 50 km) do the lunar and solar events above occur at right angles to one another. More than 50 km north or south of the latitude of Stonehenge, the station stones would have to be set out as a parallelogram.

Gerald Hawkins' work

Gerald Hawkins' work on Stonehenge was first published in Nature in 1963 following analyses he had carried out using the Harvard-Smithsonian IBM computer. Hawkins found not one or two alignments but dozens. He had studied 165 significant features at the monument and used the computer to check every alignment between them against every rising and setting point for the sun, moon, planets, and bright stars in the positions they would have been in 1500 BC. Thirteen solar and eleven lunar correlations were very precise against the early features at the site with precision falling during the megalithic stages. Hawkins also proposed a method for using the Aubrey holes to predict lunar eclipses by moving markers from hole to hole. In 1965 Hawkins wrote (with J. B. White) Stonehenge Decoded, which detailed his findings and proposed that the monument was a 'Neolithic computer'.

Atkinson replied with his article Moonshine on Stonehenge in Antiquity in 1966 pointing out that some of the pits Hawkins had used for his sight lines were more likely to have been natural depressions and that he had allowed a margin of error of up to 2 degrees in his alignments. Atkinson found that the probablity of so many alignments being visible from 165 points to be close to 0.5 (or rather 50:50) rather that the 'one in a million' possibility that Hawkins had claimed. That the Station Stones stood on top of the earlier Aubrey Holes meant that many of Hawkins' alignments between the two features were illusory. The same article by Atkinson contains further criticisms of the Aubrey Holes' interpretation as astronomical markers, and of Fred Hoyle's work.

A question also exists over whether the English climate would have permitted accurate observation of astronomical events. The modern researchers were looking for alignments with phenomena they already knew existed, the prehistoric users of the site did not have this advantage.

Alexander Thom's work

Alexander Thom had been examining stone circles since the 1950s in search of astronomical alignments and the megalithic yard but it was not until 1973 that he turned his attention to Stonehenge. Thom chose to ignore alignments between features within the monument, considering them to be too close together to be reliable and instead looked for landscape features that could have marked lunar and solar events. One of Thom's key sites, Peter's Mound turned out to be a twentieth century rubbish dump however and time has not been kind to his approach.

M. W. Postins' work

M.W. Postins published a short book entitled Stonehenge: Sun, Moon, Wandering Stars, where he postulated that the five trilithons represented the five planets visible to the naked eye. He believed that the two smallest trilithons represented Mercury and Venus, because they followed most closely the solar orbit, and also lined up with the solar axis in Stonehenge. The two intermediate trilithons represented Mars and Jupiter, which followed more closely the lunar trajectory, and lined up with Stonehenge's lunar axis. The great trilithon represented Saturn, because it moved on a unique trajectory, much more slowly than the other planets. Postins was unable to differentiate between the trilithons for Mercury and Venus, however, or for Mars and Jupiter, and suggested that when they were built they might have had names inscribed in the stone.

Later theories

Despite as many as 20,000 people visiting Stonehenge during the 2005 summer solstice, growing evidence indicates that ancestors did not visit at all in the summer, but rather during the winter solstice. The only megalithic monument in the British Isles to contain a clear, compelling solar alignment is Newgrange which famously faces the winter solstice sunrise. The most recent such evidence includes bones and teeth from pigs that were slaughtered at nearby Durrington Walls, their age at death indicating that they were slaughtered either in December or January every year. Mike Parker Pearson of the University of Sheffield has said "We have no evidence that anyone was in the landscape in summer."

Today, the consensus is that some of the astronomical case, although not all, was overstated.

See also

• Pseudoscientific metrology#Stonehenge

External links

• Temporal Epoch Calculations, An introduction to research considerations regarding temporal variations in archaeoastronomical and archaeogeodetic variables.