Ancient Egyptians built pyramids with stars in mind

Astronomers working nearly 4,500 years ago used two stars circling the celestial polar point to align the Egyptian pyramids due north, according to a study in Nature.

The stars were Kochab in the bowl of the Little Dipper, and Mizar in the middle of the handle of the Big Dipper.

Dr. Kate Spence, of the University of Cambridge, theorises that an Egyptian astronomer probably held up a vertical line, and waited for the night sky to slowly rotate around the unmarked pole.

When the line exactly intersected both stars, one about 10 degrees above the pole, and the other about 10 degrees below it, the sight line to the horizon aimed directly north - and the pyramids were built along these lines.

But Spence says these measurements were only accurate for a few years around 2,500 BC because the Earth's axis is unstable and wobbles slightly.

It's because of this window of accuracy that Spence thinks the Great Pyramid of Giza was constructed within 10 years of 2,480 BC. Its base aligns properly with the stars as they would have been in those years.

Before and after that time, the stars deviated from the north-south line and anyone using them to plot direction would have made mistakes.

Spence shows in her study that the orientation errors of earlier and later pyramids faithfully track the slow drift of Kochab and Mizar with respect to true north.

Star light, star bright

Those who study the ancient Egyptians know they were fascinated by astronomy, particularly by the circumpolar stars. They circle around the North Pole and you can always see them. The Egyptians referred to them as 'The Indestructibles."

Because of this, they became closely associated with eternity and the afterlife. Pharaohs would hope to join the circumpolar stars after death, so the pyramids were laid out towards them.