Astronomers will be able to pin the date of the construction of the Great Pyramid at Giza within five years. The theory assumes that the Egyptians used two bright stars -- one in the Ursa Minor and one in the Ursa Major -- to orient the pyramids. These stars in 2467 BC lay in a precise straight line including the celestial North Pole. Using a plumb line and waiting for the Earth to turn to bring the plumb line to intersect the line joining the stars, the builders could have determined the true north at the site.

The Earth, apart from spinning on its axis also precesses (the axis gyrates and rotates with a period of 26000 years), and thus the celestial North Pole was exactly aligned between the two stars only in 2467 BC. Some of the pyramids are not exactly aligned to the north direction. This may be because they were built later, or earlier, and the error in the alignment of the two stars could tell us the construction date of these other pyramids also (Nature, Vol 408, p320).

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