The Essential Stonehenge

Mark Littmann holds an endowed fellowship in science writing at the University of Tennessee.  Fred Espenak is an astrophysicist at NASA’s Goddard Space Flight Center.  Ken Wilcox was a research chemist and an adjunct professor of physics and astronomy at Bartlesville Wesleyan College.  Together they wrote, Totality: Eclipses of the Sun, which takes us to eclipses past, present, and future, and explains why people travel to the ends of the Earth to observe them.  In the excerpt below we learn about the connection between Stonehenge and eclipses.

Stonehenge was begun about 2800 B.C. by a people who had no written language, no wheeled vehicles, no draft animals, and no metal tools. To dig holes in the ground, they used the antlers of deer.

The initial Stonehenge consisted of a circular embankment 350 feet (107 meters) in diameter, four marker stones set in a rectangle, some postholes, and the Heel Stone. The Heel Stone was apparently the first of the great boulders brought to this site as construction commenced. But it may not have stood alone. A similar huge stone stood just to its left as seen from the center of Stonehenge. In that ancient time, the Sun at the beginning of summer probably rose between the famed Heel Stone and its now-vanished companion, and the alignment with sunrise at the summer solstice was probably exact.

For someone standing at the center of Stonehenge, the embankment served to level the horizon of rolling hills. Within the embankment, four stones- the Station stones- outlined a rectangle offering interesting lines of sight. The short side of the rectangle pointed toward the same spot on the horizon that the two Heel Stones framed, the position where the Sun rose farthest north of east, marking the commencement of summer. Facing in the opposite direction along the short side of the rectangle, an observer would see the place where the Sun set farthest south of west, signaling the beginning of winter.

In contrast, the long sides of the rectangle provided alignments for crucial rising and setting positions of the Moon. Looking southeast along the length of the rectangle, an observer was facing the point on the horizon where the summer full moon would rise farthest south. In the opposite direction, looking northwest, this early astronomer’s gaze was led to the spot on the horizon where the winter full moon would set farthest north. These positions marked the north and south limits of the Moon’s motion.

The structure of Stonehenge offers additional testimony to its builders’ efforts to understand the motion of the Moon. Evidence of small holes near the remaining Heel Stone strongly suggests that the users of Stonehenge observed and marked the excursion of the Moon as much as 5° north and south of the Sun’s limit.  This motion above and below this Sun’s position is caused by the tilt of the Moon’s orbit to the Earth’s path around the Sun.  Because of this tilt, the Moon does not pass directly in front of the Sun (a solar eclipse) or directly into the Earth’s shadow (a lunar eclipse) each month.

Because the builders of Stonehenge had discovered and accurately recorded the range in the rising and setting positions of the Sun and Moon and had built a monument that marked these positions with precision, they may have been able to recognize when the Moon was on course to intercept the position of the Sun, to cause a solar eclipse.  Perhaps they could tell when the Moon was headed for a position directly opposite the Sun, which would carry it into the shadow of the Earth for a lunar eclipse.  They almost certainly could not predict where or what kind of solar eclipse would be seen, but they might have been able to warn that on a particular day or night, an eclipse of the Sun or Moon was possible.

In the last phase of building at Stonehenge, two concentric circles of holes were dug just outside the Sarsen Circle – one with 30 holes and the other with 29.  These circles reinforce the evidence that astronomers at Stonehenge were counting off the 29 1/2 day cycle of lunar phases, from new moon to full moon and back to new moon again.  Eclipses of the Sun can only take place at new moon; lunar eclipses can only occur at full moon.  If indeed the lunar phasing cycle was watched carefully, perhaps some ancient genius noticed a periodicity in eclipses as well.  With a knowledge of that period, that early astronomer could have converted a mere warning of a possible eclipse into a prediction of a likely eclipse, especially for lunar eclipses, which are visible over half the Earth.

The builders of Stonehenge left no written records of their objectives or results, so we must judge from the monument and its alignments what they knew.  Whatever that was, the thought it so worth celebrating that the rulers and apparently the common people were willing to devote vast amounts of time, physical effort, and ingenuity to raising a lasting monument of great size, precision, and beauty.