The variation of the rising and
setting points of the Sun and the Moon on the horizon surprises many people.
The Sun doesn’t simply rise in the east and set in the west. In northern
latitudes like those of the British
Isles, the Sun rises quite far
north of east in summer, and sets well north of west. In winter, the Sun rises
south of east and sets south of west. This is a result of the tilt of the
Earth’s rotational axis: in summer, the northern hemisphere leans into the Sun;
in winter, it leans away from the Sun. The farther north an observer moves, the
greater the difference between the rising and setting points of the Sun and
Moon along the horizon.
Given this wide seasonal swing, it
would seem natural for ancient observers at high northern or southern latitudes
to have used ‘horizon astronomy’. Calendars and almanacs can be devised by
checking the motion of the Sun and Moon along the horizon rather than the
shadow length of a post in the ground or the height of the Sun. Instead of watching the Sun at or the Moon at , the Stonehenge
people watched the rising and setting points on the horizon.
They would soon have noticed that
the position at which the Sun rises on the eastern horizon oscillates back and
forth between a northern and southern extreme. These two extremes, known as the
‘standing still’ positions, are the points on the horizon where sunrise stops
its motion along the horizon, turns, and starts to cycle back to the other
extreme. These ‘standing still’ points for the Sun are marked at Stonehenge by
principal alignments of the standing stones.
The Sun’s motion is clearly
all-important for dividing up the year and preparing a calendar. But not so the Moon. Yet it seems that those who built Stonehenge
were also intensely interested in the Moon’s motion. Was it to further
sub-divide the year, or to predict eclipse? The full Moon’s extreme positions
on the horizon were also marked at Stonehenge. The variations of moonrise are even wider than those of
sunrise. The alignments through the standing stones at Stonehenge to
the extreme positions of the Sun and Moon are shown in Figure 4.2. The extreme
swings along the horizon of the rising and setting of the full Moon take
exactly 18.61 years. This includes the swings between the major ‘standstills’, a to a, and the minor ‘standstills’, b to b, within the
18.61 year cycle.
The realization that the principal
alignments in Stonehenge were with the extreme positions of the Sun and the Moon
came about in 1965, when the British-born astronomer Gerald Hawkins, a
professor of astronomy at BostonUniversity, published Stonehenge
Decoded. In this book Hawkins describes a computer analysis of the
principal alignments of the viewing directions between the stones. He found
that all the principal alignments pointed towards the extreme positions on the
horizon of the rising and setting of either the Sun or the full Moon. In other
words, Stonehenge was a megalithic astronomical observatory incorporating a
calendar based on the Sun and the Moon.
The book caused a sensation among the ancien régime of archaeologists – experts on the megalithic period who
could not accept the attribution of an advanced technology to such an early
However in what seemed to be a spectacular elaboration of
his computer work, Hawkins proposed that in addition to the principal
alignments of the external positions of the Sun and Moon, Stonehenge
was also used to predict eclipses. This was a radical idea which the British
cosmologist Fred Hoyle further developed several years later. Hoyle proposed
that Stonehenge was used to follow the positions of the lunar nodes around
the ecliptic. When full or new Moon occurs near a node, the ‘danger zone’, an
eclipse can occur.
Obviously, if the cycle of the nodes was known, the Moon
could be tracked very carefully to determine any relationship between the cycle
and eclipses that were observed at Stonehenge. It seems that the builders were convinced that by
accurately locating the extreme positions of the Moon, they could predict when
an eclipse would take place. There are problems with this. The Moon moves about
twelve times faster than the Sun through its extreme positions, and
consequently the positions are more difficult to mark. Critics have argued that
a much more accurate sighting system would be required than is available at Stonehenge.
Because of the rotation of the line of nodes, the Moon
swings back and forth between its extreme positions in the same period as the
nodal cycle. One result of this is the unusual movement of
the winter full Moon near the central axis of Stonehenge, aligned with the Marker know as the Heelstone, as shown in
Figure 4.3. If the rising of the winter full Moon is studied for many years,
and marked by sight lines, it will be found that its rising point swings back
and forth across the Heelstone with a cycle of 18.61
years. When the full Moon rises in the center of this swing, over the Heelstone, the point where the sun rises on the summer
solstice, it is eclipsed.
It seems that the Stonehenge people were at
least close to being able to predict eclipses. However, they have left no
written records that might persuade scholars that the alignments at the site
should be examined more closely. Unless the remains of an astronomer-priest are
found in a megalithic tomb on Salisbury Plain with artifacts from this
enigmatic structure, it is unlikely that the controversy over Stonehenge as
an eclipse predictor will ever be settled.