easily fix the date, for, owing to the precession of the equinoxes, the
point of the midsummer rising is continually altering, and the position
for any year being known the date of that year can be found
astronomically. But how was the precise direction of this very irregular
avenue to be fixed? The line from the altar stone to the Friar's Heel,
which is popularly supposed to point to the midsummer rising, has
certainly never done so in the last ten thousand years, and therefore
could not be used as the direction of the avenue. Eventually Sir Norman
decided to use a line from the centre of the circle to a modern
benchmark on Sidbury Hill, eight miles north-east of Stonehenge. On this
line the sun rose in 1680 B.C. with a possible error of two hundred
years each way: this Sir Norman takes to be the date of Stonehenge.

Sir Norman's reasoning has been severely handled by his
fellow-astronomer Mr. Hinks, who points out that the direction chosen
for the avenue is purely arbitrary, since Sidbury Hill has no connection
with Stonehenge at all. Moreover, Sir Norman determines sunrise for
Stonehenge as being the instant when the edge of the sun's disk first
appears, while in his attempts to date the Egyptian temple of Karnak he
defined it as the moment when the sun's centre reached the horizon. We
cannot say which alternative the builders would have chosen, and
therefore we cannot determine the date of building.

Sir Norman Lockyer has since modified his views. He now argues that the
trilithons and outer circle are later additions to an earlier temple to
which the blue-stones belong. This earlier temple was made to observe
"primarily but not exclusively the May year," while the later temple
"represented a change of cult, and was dedicated primarily to the
solstitial year." This view seems to be disproved by the excavations of
1901, which made it clear that the trilithons were erected before and