Age of the pyramids may be in the stars

A remarkable insight into the way the ancient Eyptians used the stars to align their pyramids could overturn current estimates of when they were built.

A remarkable insight into the way the ancient Eyptians used the stars to align their pyramids could overturn current estimates of when they were built.

For many years the world has known that the pharaohs must have employed sophisticated methods of aligning their pyramids to due north with great accuracy. But with no compass and an absence of a pole star - Polaris was then in a different part of the sky - no one quite knew how they did it.

Now an Egyptologist from Cambridge University has devised a possible explanation by proposing that the pyramid builders used the alignment of two well-known stars. If true, it means that the dates of the pyramids could be estimated to within five years rather than 100 years as previously suggested.

Kate Spence, a researcher at Cambridge's faculty of Oriental studies, examined the slight errors made in aligning the bases of several pyramids to true north, which suggested that whatever principle the Egyptians had adopted, it became increasingly inaccurate.

Dr Spence concentrated on studying the pyramids at Giza, which have never been accurately dated although they are known to have been built about the middle of the third millennium BC. "The chronologies of this period have been reconstructed from surviving lists of kings and the lengths of their reigns, but the lists... contain known inconsistencies and errors," she said.

The west and east sides of the base of each pyramid are aligned due north-south because of the religious significance the Egyptians attached to celestial north, the point in the night sky around which all stars appear to rotate. Dr Spence said the circumpolar stars - those that closely circle the celestial north and do not fall below the horizon - did not disappear at night and so were called the indestructible stars. "Because of their apparent immortality, the Egyptian kings wanted to join them in the afterlife," she said.

The pyramid most accurately aligned to the north is that of Khufu (or Cheops), which deviates from true north by just three arc minutes - equivalent to one-tenth of the diameter of the Moon as seen from Earth (an arc minute is one-60th of a degree).

In a study published in the journal Nature, Dr Spence argues that this alignment could have been made by careful observations of two circumpolar stars, Mizar in the Ursa Major constellation, and Kochab in Ursa Minor.

In their anticlockwise journeys there is a point at which they can be exactly aligned with a vertical plumb line held by hand. If this is done today, the plumb line's intersection with the Earth's horizon will be well away from due north. But astronomers calculate that in the year 2467BC, the intersection would have fallen precisely on due north as the alignment of Mizar and Kochab would have exactly crossed the point of celestial north in the sky.

The reason for the difference between now and 4,500 years ago is due to the precession of the Earth's axis of rotation, which has a 26,000-year cycle. Using the perpendicular alignment of Mizar and Kochab as a measure of true north would be less accurate after and before the key year of 2467BC.

The slight misalignment to the west of the Khufu pyramid suggests it was built about 2478BC, more than 70 years earlier than previously thought.

Dr Spence says the later pyramid of Menkaure, the second pharaoh after Khufu, errs in the opposite way by 13 arc minutes to the east. The three earlier pyramids of Snofru are aligned more to the west as they get further from 2467BC.

What makes the idea so attractive to scientists is that Dr Spence can draw a graph of the suggested dates of each of these pyramids against their deviation from true north and produce a straight line - a clear indication that she may have found the method used to determine true north in ancient Egypt. Owen Gingerich, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said: "Spence has come up with aningenious solution to a long-standing mystery."