Bonapart's Expeditionary
Force Survey; 1798
Since the 19th century
many archeologists have developed the idea that
alignment of the Egyptian pyramids at Giza harks back to one of the first
moments in the history of civilization in which astronomy was pressed into the
service of the arts, in this case to monumental architecture. In particular, the
pyramids of the IV Dynasty Kings Cheops, Khephren and Mycerinus, are
orientated with extraordinary accuracy with the four cardinal points of the
modern compass.
The British Egyptologist,
K. Spence has suggested that the alignment of the
pyramids in relation to the cardinal points of the modern compass could have
been achieved via observations of simultaneous transits across the local
meridian (an imaginary semicircle that divides the sky into its eastern and
western halves) of two circumpolar stars, Kochab and Mizar, that were on
opposite sides of the north celestial pole at the time when the pyramids were
built. If this is true, the chronology of the Great Pyramid is shortened by almost
80 years. The article by the IAC's Juan Antonio Belmonte now critically
examines Spence's proposal and improves on this with a new hypothesis.
Responding to Spence's
ideas, J.A. Belmonte suggests that this orientation,
following the local meridian, could have been achieved through meridian transit
observations of the stars Phecda and Megrez, belonging to the Leg of the Bull,
one of the most imporatant Egyptian constellation (equivalent to our Plough in
the constellation Ursa Major). Extending the line joining these two stars leads us
to Thuban (the "pole star" at that time) in the same way that the two stars Merak
and Dubhe today act as pointers to the present Pole Star, Polaris.
According to this new
hypothesis, the greatest accuracy of alignment would
have been achieved around the year 2562 BC; consequently, the Great Pyramid
could have been orientated close to this date, at a moment falling between the
two dates proposed Spence and Belmonte, which would place it at the start of
the reign of Cheops (2589 - 2551 BC) and render unnecessary the shortening in
the chronology advocated by Spence.
Belmonte's hypothesis
has important chronological and historical - even
mythological - implications that could help towards a a better understanding of
how the Egyptians of the pharaonic era understood the cosmos and made use
of it, among other things, to align precisely their most important monuments.
The ancient Egyptians
were extremely interested in the night sky, articularly the
circumpolar stars. These stars circle around the North Pole, and as you can always
see them, the Egyptians always referred to them as 'The Indestructibles'. As a
result, they became closely associated with eternity and the king's afterlife. So that
after death, the king would hope to join the circumpolar stars - and that's why the
pyramids were laid out towards them.
The north-finding stars
were Kochab, in the bowl of the Little Dipper (Ursa Minor),
and Mizar, in the middle of the handle of The Plough or Big Dipper (Ursa Major).
An Egyptian astronomer
would have held up a plumb line and waited for the night sky
to slowly pivot around the unmarked pole as the Earth rotated.
When the plumb line
exactly intersected both stars, one about 10 degrees above the
invisible pole and the other 10 degrees below it, the sight line to the horizon would
aim directly north.
However, the Earth's
axis is unstable and wobbles like a gyroscope over a period of
26,000 years. Modern astronomers now know that the celestial north pole was
exactly aligned between Kochab and Mizar only in the year 2,467 BC.
Either side of this
date, the ancient astronomers trying to find true north would lose
some accuracy.
Spence shows that the
orientation errors of earlier and later pyramids faithfully track
the slow drift of Kochab and Mizar with respect to true north. And because the error
in the Kochab-Mizar alignment can be readily calculated for any date, the error in
each pyramid's orientation corresponds to a period of several years.