Stonehenge, one of the most famous archaeological sites in the world, has long been associated with astronomy. Situated on the chalk uplands of the Wessex region in southern England, in the county of Wiltshire, the monument is characterized by its huge sarsen stones, some of which stand over seven meters (twenty-two feet) tall. The tallest are the remains of five trilithons (two uprights with a horizontal lintel placed across their tops) arranged in the shape of a horseshoe, and these are surrounded by what was once a continuous ring of only slightly less tall uprights and lintels. Almost unimaginably, these huge stones, weighing up to forty tons apiece, were dragged here, down- and uphill, over a distance of some thirty kilometers (eighteen miles) from the Marlborough Downs to the north. Where the stones have collapsed it is possible to see some of the carefully carved mortice-and-tenon and tongue-and-groove joints (joints that would normally be associated with woodworking) that were used to hold them together. The horseshoe is oriented northeastwards. Looking out in this direction through a well-preserved section of the outer circle of sarsens, one sees a much rougher stone, leaning over and badly weathered. This is the famous Heelstone, which stands some sixty meters outside the outer ring.

The purpose and function of the great stone monument at Stonehenge has been a source of speculation for centuries. That it was a place of great power is self-evident. Regarding astronomy, it has been portrayed variously as a cosmic temple, a calendrical device, an observatory, and an eclipse calculator. Many of these ideas have attracted widespread public interest.

However, the majority of astronomical theories concerning Stonehenge have little hard evidence to support them. Many are based on the idea that, at one stage or another, the monument incorporated deliberate architectural alignments upon the rising and setting positions of celestial bodies, particularly the sun or moon. The problem is that every architectural alignment must point somewhere, and many factors might have influenced any particular orientation. Just because a pair of stones is aligned upon, say, the position of sunrise at one of the solstices, does not mean that this was deliberate or that it meant anything at all to the people who built and used the monument. We must be particularly cautious if we only find a modest number of potential astronomical alignments among a great many possibilities: and there are hundreds of ways of aligning different pairs of stones at Stonehenge.

Books Stonehenge Astronomy
The view out along the axis of the sarsen monument at Stonehenge, in the direction of midsummer sunrise. The Heel-stone is visible in the distance to the right of the axial alignment; another stone, probably similar, once stood symmetrically to the left. (Courtesy of Clive Ruggles)

One of the most famous astronomical speculations about Stonehenge is that it functioned as a calculating device for predicting eclipses. Put forward in the early 1960s in a popular book by Gerald Hawkins entitled Stonehenge Decoded, and later elaborated by the astronomer Fred Hoyle, the idea is basically that a small number of wooden posts were moved around the Aubrey Holes according to strictly defined rules (the two authors suggested different methods). When certain configurations were reached, then it was a "danger period" when eclipses could occur.

The Aubrey Holes are a ring of fifty-six pits surrounding the sarsen monument, named after the antiquarian John Aubrey who discovered them in the seventeenth century. Concrete markers mark their positions for the modern visitor. Those who challenged the theory were often asked "why were there fifty-six Aubrey Holes, then?" But to try to answer this question is to miss the point. There has to be some number of holes in the ring, even if it was created for some other purpose; and we may never know with any certitude what that purpose was. Some astronomers in the 1960s showed that the number fifty-six is not that special with regard to predicting eclipses: similar post-moving schemes can be devised with other numbers of holes in the circle. Archaeologists know of many similar pit circles in Britain, vary ing in number from less than twenty to over a hundred. The number fifty-six is not special, then, either archaeologically or astronomically. Finally, to further confound the Hawkins/Hoyle theory, the archaeological evidence shows that at least some of the Aubrey Holes at Stonehenge held permanent timber posts, albeit possibly after being left open for a time.

To be plausible, astronomical theories at Stonehenge must be consistent with what we know of the chronological development of the monument from the many archaeological investigations that have taken place there, particularly during the twentieth century. The great sarsen circle and trilithons, constructed in about 2400 B.C.E., were not the first construction that left its trace at this spot. The smaller bluestones had been brought there all the way from the Preseli mountains in South Wales, a distance of some 300 kilometers (200 miles), perhaps a century earlier. And some four or five centuries earlier still, an earthen enclosure, consisting of a circular ditch and inner bank, was built here, probably broadly contemporary with the Aubrey pits/posts. Yet, more remarkably, the postholes of five huge timber poles, erected several millennia earlier, in about 8000 B.C.E., were discovered during the construction of the Stonehenge parking lot in the 1970s.

During the first half of the third millennium B.C.E., the earthen enclosure seems to have been left to decay. The ditch silted up and the timber posts eventually rotted away, leaving only the Aubrey Holes. People did come, though, and carefully placed offerings such as articulated animal bones and later animal and human cremations, in the ditch and in the Aubrey Holes themselves. The distribution of these offerings around the site is far from haphazard and does seem to have been related to the motions of the sun and moon, perhaps reflecting the cosmological beliefs of the visitors to what may already have been a sacred ancestral site. This recent discovery is important, because it demonstrates that in order to understand better how and why people's activities related to the celestial bodies in the past we must try to look beyond the architecture of monuments. Archaeological excavations, which can reveal evidence of ritualistic activities that have no apparent "rational" explanation in our terms, may give us some important insights.

When stones were first brought to Stonehenge, its axis was changed. It was now aligned in the northeast upon midsummer sunrise. In other words, at dawn on the longest day of the year (and for some days before and after) the sun would have risen squarely between the Heelstone and a companion that has since disappeared, a beam of light shining along a corridor of stones and into the interior of the sarsen ring. The axis was also aligned, in the opposite direction, upon sunset on the shortest day. It is likely that this was the real focus of the monument, since the ceremonial approach was along an avenue from the northeast, so that people would have approached facing the southwest.

Modern druids participating in a ceremony at Stonehenge, England, on the morning of the summer solstice in 1976. (Courtesy of Clive Ruggles)

Among many other astronomical alignments claimed at Stonehenge, one of the most intriguing concerns the four station stones. Two of these remain in situ, just inside the ditch where the timber circle once stood. They appear to be broadly contemporary with the first appearance of other stones at the site (though this is unconfirmed archaeologically) and formed a rectangle. The lengths of its sides are in the ratio 5:12, a fact that has led some people to speculate that the builders had knowledge of Pythagorean geometry, since this ratio gives a diagonal whose length is a whole number (thirteen) of the same units. The shorter sides of the rectangle are aligned along the main axis of the monument, on midsummer sunrise and midwinter sunset, while its longer sides are aligned toward the southeast upon a point close to the most southerly rising point of the moon. This alignment has led to claims that the latitude of Stonehenge was carefully chosen to optimize this arrangement. However, such an argument is flawed on a number of grounds, most obviously because the earlier monument had already existed on this spot for several centuries. In any case, the optimal latitude would actually be well to the south, somewhere in the English Channel.

In comparison, there is little doubt that the solstitial alignment of the main axis of Stonehenge in its later phases was intentional. Its purpose may have been as a display of power, showing that this monument, erected with a huge input of human labor from exotic materials (stone brought from great distances), was in harmony with nature. Its power would reinforce the power of those who were seen to be in control of it. Certainly by the middle of the third millennium B.C.E., Wessex cheftains were powerful and commanded great long-distance exchange networks. During the ensuing millennium, dozens of rich and powerful chieftains were buried along with their personal treasures in round barrows within sight of Stonehenge, relating in some way to the power of this great ancestral monument.

See also:

Cosmology; Lunar Eclipses; Solar Eclipses; Solstitial Alignments.

Bush Barrow Gold Lozenge; Crucuno; Megalithic Monuments of Britain and Ireland.

Moon, Motions of; Solstices.

References and further reading

Burl, Aubrey. The Stone Circles of Britain, Ireland and Brittany, 349-375. New Haven: Yale University Press, 2000.

Cleal, Rosamund, Karen Walker, and R. Montague. Stonehenge in Its Landscape: Twentieth-Century Excavations. London: English Heritage, 1995.

Cunliffe, Barry, and Colin Renfrew, eds. Science and Stonehenge. London: British Academy/Oxford University Press, 1997.

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 35-41, 136-139. New Haven: Yale University Press, 1999.

Souden, David. Stonehenge: Mysteries of the Stones and Landscape. London: Collins and Brown/English Heritage, 1997.

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