Ancient Egyptian astronomers aligned the pyramids due north by
using two stars that circle the celestial polar point.
Nearly 4,500 years ago, each star was about ten 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 2500 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
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 ten years of 2480 BC.
[This theory contradicts another, far more controversial one,
that dates the alignment of the pyramids to stars which were
correctly positioned in 10,500 BC.]
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.
She believes the ancients may have used a pair of fairly bright
stars, which in 2467 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 said.