Mystery of the pyramids solved with stars
A new theory on how ancient Egyptians used the stars to orient the great pyramids could pin down a construction date as well as tell us why they're aligned to the north.
Egyptologist, Kate Spence of the University of Cambridge, proposes in this week's Nature that the pyramid builders probably got started between 2485 and 2475 BC. She also says she can now explain how the great pyramids came to be aligned in a northerly direction.
"Spence has come up with an ingenious solution to a long-standing mystery," writes Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, in an accompanying commentary.
Spence's theory was inspired by the subtle deviations in the alignment of the pyramid bases from true north. The Khufu (Cheops) pyramid is most accurately aligned to the north, deviating just slightly to the west. The pyramid of Menkaure, the second pharaoh after Khufu, however, errs in the other direction, while the three pyramids of Khufu's predecessor are oriented more westward as they get older.
The pyramid builders, she suggests, used a pair of fairly bright stars, which in 2,467 BC lay precisely along a straight line including the celestial pole. One is Kochab in the bowl of the Little Dipper the other Mizar in the middle of the handle of the Big Dipper.
In 2,467 BC an Egyptian astronomer could wait while the heavens slowly pivoted around the unmarked pole until a plumb line exactly intersected both stars, one about 10° above the invisible pole and the other 10° below it. The sight line to the horizon directly below the plumb line would then point straight to north.
However, because of the Earth's precession - the revolving axis of the Earth is unstable and rotates like a gyroscope with a period of 26,000 years - the celestial north pole was exactly aligned between Kochab and Mizar only in the year 2,467 BC.
Given the precision of the measurements, the plumbline method was only accurate for a few years. The orientation errors of earlier and later pyramids faithfully track the slow drift of Kochab and Mizar with respect to true north.
Because the error in the Kochab-Mizar alignment can be readily calculated for any date, the error in each pyramid's orientation tells when the pyramid was built.
Previous estimates of the age of the pyramids have been based on reconstructing information on the reign of pharoahs and so have only been accurate to within 100 years. Spence's insight, which she argues gives a date accurate to within 5 years, could enable Egyptologists to refine their chronologies for the entire Old Kingdom.
Like Gingerich, the Anglo Australian Observatory's Dr Fred Watson, who is currently writing a book on astronomical history, is impressed with Spence's theory.
"It's a very good paper and does seem to hold water. What's nice about it is that it's self testing and is impressively consistent," he commented to ABC Science Online.