From: Nature News Service
Posted: Tuesday, November 14, 2000
Kate Spence (available from 13th November)
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Jo Webber, Nature
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A new theory of how the ancient Egyptians oriented the pyramids according to the stars could put a date on the construction of the Great Pyramid at Giza that is accurate within 5 years. Builders probably got started between 2485 and 2475 BC, Kate Spence of the University of Cambridge, UK, proposes in this week's Nature. Previous estimates have been accurate only to within 100 years. Spence's insight could enable Egyptologists to refine their chronologies for the entire Old Kingdom.
Spence's theory of how the ancient Egyptians oriented the pyramids according to the stars is inspired by the subtle deviations in the alignment of their bases from true north. 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. Each star was about 10 degrees from the pole.
One is Kochab (beta-Ursae Minoris) in the bowl of the Little Dipper (Ursa Minor), the other Mizar (zeta-Ursae Majoris) in the middle of the handle of the Big Dipper (Ursa Major). 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 10deg above the invisible pole and the other 10 deg below it. The sight line to the horizon point directly below the plumb line would then point straight to north.
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 'accurate' period was effectively 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 corresponds to a period of several years.
"Spence has come up with an ingenious solution to a long-standing mystery," explains Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, in an accompanying News and Views article.
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