Image: after Nature

For some time, scholars have known the heavens must have guided the ancient Egyptians in constructing Giza's remarkable pyramids. The Khufu (Cheops) pyramid in particular is so accurately aligned--its western and eastern sides deviate from true north by an average of three scant arc-minutes--that only astronomical points of reference could have allowed such precision. That said, scientists have long debated just what those references were. A paper in today's issue of Nature, though, seems to settle the matter once and for all. Kate Spence, an Egyptologist at the University of Cambridge, shows how the motions of a pair of bright stars can explain the layout of all eight pyramids near Cairo. What's more, the theory makes is possible to say within five years exactly when these tombs were erected and thus to refine existing timelines for when their occupants reigned.

Previous studies indicated that the pyramids were built sometime in the middle of the third millennium B.C., give or take 100 years. But scientists were unable to make a better estimate based on evidence from the pyramids themselves. Spence's work began with an analysis of subtle differences in how the eight pyramids are aligned. The earliest ones point slightly east of north, whereas the later ones point slightly west. These angles, she found, could be justified had the pyramid builders relied on Kochab (b-Ursae Minoris) in the Little Dipper and Mizar (z-Ursae Majoris) in the Big Dipper to orient the structures' square bases: in 2467 B.C. these stars, together with the celestial north pole, fell in a straight line (see illustration).

"An Egyptian astronomer could wait while the heavens slowly pivoted around the unmarked pole until a plumb line exactly intersected both stars," explained Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics. The sight line to the horizon below the plumb line would then point due north. And sure enough, the slight errors in how the earliest and latest pyramids are placed reflect the relative positions of Kochab and Mizar as they drifted across the sky with respect to true north over the years.