MARGINALIA
Astronomy and the Great Pyramid
J. Donald Fernie
An Answer Written in the Stars
That an astronomical method was used to orient the pyramids received
strong, if unexpected, support in the 1980s when historians
discovered that among most of the Giza pyramids, the departure of a
pyramid's eastern edge from a true north–south line correlated
strongly with the accession date of the king for whom each was
constructed. Which is to say that the direction of north as
determined by the Egyptian method varied systematically as the
centuries went by. The ready explanation for this is once again
precession of the equinoxes: The early Egyptians must have applied
some method of using the stars to find the north celestial pole
without realizing that the pole is not fixed, but rather drifts
slowly through the heavens.
In November 2000, Kate Spence, an Egyptologist at the University of
Cambridge, published a seminal paper in Nature in which she
suggested a method by which the pyramid builders determined what
they thought was north. She also showed that the resulting
orientation errors varied as a function of time—just as
predicted by precession. Moreover, by fitting the time–linked
precession errors to the slight deviations of each pyramid, she
revised their building dates. Instead of 2554 B.C., her data suggest
the Great Pyramid was constructed between 2485 and 2475 B.C.
The method proposed by Spence involved two stars on opposite sides
of the celestial pole. She had to choose them by trial and error,
since the pole drifts into different star fields as millennia pass.
For the period of interest, Spence found that the stars named Mizar
(Zeta Ursa Majoris) and Kochab (Beta Ursa Minoris) would have
appeared to revolve around the pole on almost (but not exactly)
opposite sides, so that a line joining them would always pass very
nearly through the pole. When these two were aligned vertically, the
pyramid builders might have hoisted a long plumb line and fixed it
at the moment when the two stars both lay on the line. The point
where the vertical line touched the ground would indicate north.
One idiosyncrasy of this method was that because these two stars
were circumpolar (they never set), they could be seen from Egypt
year–round. Thus, at some date during the year Kochab would
have appeared above Mizar at meridian transit (when they would have
been vertically aligned), but six months later Mizar would have
topped Kochab. Early in the pyramid era, the pole was really
slightly west (or east, depending on which star was uppermost at the
time) of the line. Because of precession, the opposite was true late
in the era. Support for Spence's theory came from two pyramids whose
deviation from true north was of the expected magnitude but opposite
sign. The explanation was that all the pyramids except these two had
been set during the time of year when Kochab was above
Mizar—these two must have been set six months later (or
earlier), when Mizar surmounted Kochab.
Like many groundbreaking papers, this one quickly became the center
of arguments and proposed improvements. Spence accepted a small but
significant correction by extending the pole displacement to an
azimuthal displacement, but she seems not to have been enthused by
other proposals to use different stars in a different way. The
method still has some practical problems. For one, the plumb line
would have to be very long to reach high enough to be seen against
the upper star, especially because the observer would need to be far
away from the line to achieve sufficient accuracy. And it would have
been difficult to see the line at all against a dark sky.
Nevertheless, the explanation for the two pyramids with errors of
reversed sign supports the basic idea. As centuries went by and the
errors grew, later builders may have realized the problem and
abandoned the method or used different stars. Thus, the failure of
Spence's scheme among later pyramids is not necessarily a valid
critique. My own inexpert view is that whether she is proved right
or wrong, Spence's basic idea marks a major breakthrough in dating
these pyramids.
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