Already in the 18th century the British antiquarian William Stukeley had noticed
that the horseshoe of great trilithons and the horseshoe of 19 bluestones at
Stonehenge opened up in the direction of the midsummer sunrise. It was quickly
surmised that the monument must have been deliberately oriented and planned
so that on midsummer's morning the sun rose directly over the "Heel Stone"
and the first rays shone into the centre of the monument between the open arms
of the horseshoe arrangement. p6x21xk
This discovery has had tremendous impact on how Stonehenge has been interpreted.
For Stukeley in the 18th century and Sir Norman Lockyer in the first years of
the 20th century, this alignment implied a ritualistic connection with sun worship
and it was generally concluded that Stonehenge was constructed as a temple to
the sun. More recently, though, the astronomer Gerald Hawkins has argued that
Stonehenge is not merely aligned with solar and lunar astronomical events, but
can be used to predict other events such as eclipses. In other words, Stonehenge
was more than a temple, it was an astronomical calculator. The alignment also
made it clear that whoever built Stonehenge had precise astronomical knowledge
of the path of the sun and, moreover, must have known before construction began
precisely where the sun rose at dawn on midsummer's morning while standing on
the future site of the monument. This point needs to be made because, as I suspect,
with Stonehenge and many other such monuments, it was the site, a particular
place within the landscape, that was important; only later were these sites
marked in some more permanent manner by the digging of ditches and banks and
(or instead) the erection of wood or stone structures.
For reasons we shall never know, this particular spot in the landscape was so
important that not only were ditches and banks dug and, later, stone circles
and horseshoe arrangements constructed to mark it, but that some of the stones
were deliberately transported there with considerable effort from a great distance
away.Contrary to expectations, the great stone circles and horseshoe arrangements
for which Stonehenge is famous are later additions to the monument (mostly Stonehenge
III) and are not essential to the lunar and solar calculations.
Inside the bank were dug 56 holes -- discovered by John Aubrey, and known as
the "Aubrey Holes" -- placed at precisely regular intervals around
a concentric circle of about 285 feet in diameter. Archaeological investigations
have shown that these holes were not dug to hold upright stones or wooden posts.
Besides the "Aubrey Holes", of crucial importance are the four "Station
Stones" marked at positions 91, 92, 93, and 94, to form a rectangle that
stands in a precise relationship with the centre of the monument and with the
"Heel Stone". Only two of the "Station Stones" survive,
and one of those may not be original.
For the archaeoastronomists, the "Aubrey Holes" served as fixed reference
points along a circle, and their number was essential to astronomical calculations.
The cycle of the moon, for example, which takes 27.3 days, can be tracked by
moving a marker by two holes each day to complete a circuit in 28 days.
A much longer calculation is to move the marker by three holes per year to complete
a full circuit in 18.67 years. In this way, it is argued, it would be possible
to keep track of the "nodes", points where the paths of the sun and
the moon apparently intersect to produce an eclipse. Because the moon slews
around in its path, the two "nodes" move along the path of the sun,
a complete circuit of which takes 18.61 years. By means of the markers in the
"Aubrey Holes" and keeping track of the directions of the sun and
the moon, the astronomer at Stonehenge could calculate nodal points ahead of
time and thus predict both lunar and solar eclipses. Whether this was in fact
the intended use of the "Aubrey Holes" is highly debatable.