The ancient Egyptians used two bright stars in the Big Dipper and Little Dipper constellations to align their pyramids in a north-south direction, a British Egyptologist says. If she's right, we can now pin down the ages of the pyramids far more accurately than before.
It's an "ingenious solution to a long-standing mystery", says astronomer and science historian Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "It seems quite convincing to me. This could pin down the dates of the Old Kingdom of Egypt - the oldest fixed dates in history."
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The Egyptian pyramids at Giza were built during the third millennium BC as tombs for kings. Their relative ages are known from patchy records of the lengths of the kings' reigns. But previous estimates of their actual ages have been accurate only to the nearest century.
The tombs are aligned north-south with an accuracy of up to 0.05 degrees. How the Egyptians did this has been unclear. Today, you could align a building north-south by pointing the sides towards the pole star, which sits roughly at true north. However, a wobble in the Earth's axis of rotation (called precession), means that the positions of the stars changes gradually over time.
In the third millennium BC, no star sat at the north pole. Instead, all the stars in the north sky rotated around an imaginary point marking the north pole.
Experts thought the ancient Egyptians might have watched a single star circle this imaginary point and aligned their pyramid with the circle's centre.
Alternatively, they might have measured north by tracking the path of the Sun. Either way, the accuracy with which they mastered this should have been about the same during the entire pyramid-building period.
But the accuracy wasn't constant, Kate Spence of Cambridge University points out. Instead, the alignment of successive pyramids first steadily improved up to the building of the Great Pyramid, then later deteriorated. This makes perfect sense, she says, if the architects were measuring the alignment using not one star, but two: Mizar in Ursa Major (the Big Dipper) and Kochab in Ursa Minor (the Little Dipper).
Rewinding the astronomical clock using modern computers shows that the two stars rotated around the pole opposite each other in the Old Kingdom sky. In other words, an imaginary line joining the stars passed through the north pole. When the two stars lay vertically above each other, both would mark the position of true north for the pyramid builders.
However, due to precession, the line joining Mizar and Kochab only drifted into precise alignment with the north pole in 2467 BC, then wandered away (see graphic). The orientation of successive pyramids faithfully tracks this drift, Spence says, explaining the rise and fall of north-south alignment precision.
If Spence is right, it is now possible to use astronomical records to date the pyramids to the nearest five years. For example, construction of the Great Pyramid at Giza began between 2485 and 2475 BC.
"I'm as convinced as I think one can be in archaeology," Spence says.
More at: Nature (vol 408, p 320)
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