Pyramids lined up with the stars

The method was accurate for only a few years

By BBC News Online science editor Dr David Whitehouse

Ancient Egyptian astronomers aligned the pyramids due north by using two stars that circle the celestial polar point.

Spence has come up with an ingenious solution to a long-standing mystery

Owen Gingerich, Harvard-Smithsonian Center for Astrophysics

Nearly 4,500 years ago, each star was about 10 degrees from the celestial pole which lay directly between them. When one star was exactly above the other in the sky, astronomers could find a line that pointed due north.

But the alignment was only true for a few years around 2,500 BC. Before and after that time, the stars deviated from the north-south line and anyone using the stars to plot a direction would have made errors.

And it is these mistakes that a British Egyptologist now believes can be used to estimate very accurately when the pyramids were built. Her theory suggests that the Great Pyramid at Giza was constructed within 10 years of 2,480 BC.

'Indestructible' stars

Kate Spence is from the University of Cambridge. She developed her theory while trying to explain the deviations in the alignment of the bases of many pyramids from true north.

Alignment errors provide the clue

She believes the ancients may have used a pair of fairly bright stars, which in 2,467 BC lay precisely along a straight line that included the celestial pole.

"We know that the ancient Egyptians were extremely interested in the night sky, particularly the circumpolar stars," she told the BBC.

"These 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."

Ancient astronomy

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).

The Giza Pyramid is made of two million blocks of stone

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.

Writing in the journal Nature, Kate 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.

Owen Gingerich, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, said: "Spence has come up with an ingenious solution to a long-standing mystery."