THE mystery of how the ancient Egyptians aligned the Pyramids with astonishing accuracy four and a half thousand years ago has been solved by a Cambridge academic.
The key to their ability to position the vast structures - to within a fraction of a degree of true north - lies in the heavens, which the ancient Egyptians regarded as holding the key to immortality, according to Dr Kate Spence of the university's Faculty of Oriental Studies. If confirmed, her theory provides a way to date the Pyramids to within a few years, ending decades of debate among academics.
Many outlandish suggestions have been put forward to explain the skills used to build the Pyramids, ranging from helpful spacemen to the import from the Orkneys of Scottish building techniques. Dr Owen Gingerich of the Harvard-Smithsonian Centre for Astrophysics in America said that, at first sight, Dr Spence's idea seemed "as likely as sharpening razor blades by placing them under small pyramids".
But, after inspecting her theory, published today in the journal Nature, he declared that "Dr Spence has come up with an ingenious solution to a long-standing mystery". He is swayed because she explains why the ancients' ability to align pyramids peaked around 2467 BC and shows why, and by how much, those built before and after were increasingly misaligned.
All ancient people were fascinated by the pattern of stars, using them to track the seasons for their agriculture. Stars also aided navigation before the compass and before Polaris approximated to the pole position, as it does today.
Dr Spence believes that the Egyptians discovered that two stars could be used to identify true north. One is Kochab (b-Ursae Minoris) in the bowl of the Little Bear (Ursa Minor), the other, Mizar (z-Ursae Majoris) in the middle of the handle of the Great Bear (Ursa Major). They rotated around the position of the North Pole and were also of deep fascination for religious reasons: they never set, symbolising the afterlife. Pyramid texts refer to pharaohs wanting to join the circumpolar stars after death.
What is striking about Dr Spence's theory is that it provides an accurate date for the building of the Great Pyramid at Giza, also called the Khufu or Cheops pyramid: builders started in 2478 BC, plus or minus five years. Dr Spence shows that there was only one year - 2467 BC - when the two stars lay precisely along a vertical line that included the celestial pole.
That year, an Egyptian astronomer could wait while the heavens slowly pivoted around the unmarked pole until a plumbline hung from a wooden frame exactly intersected both stars, one above the invisible pole and the other below it, and a post in the distance. The sight line between the plumbline and post would then point north. The data suggests they used it to align the Pyramids' west faces.
This idea would be speculative, commented Dr Gingerich, were it not for the discovery that the same technique accounts for the orientation of other pyramids, which shifts over time, just as her theory predicts. Dr Spence found that pyramid building was influenced by the Earth's precession - its revolving axis is unstable and rotates like a gyroscope, turning once in 26,000 years.
The celestial North Pole was exactly aligned between the stars only in 2467 BC. The orientation errors of earlier and later pyramids faithfully track the slow drift of Kochab and Mizar with respect to true north.
The Khufu pyramid, being built around 2478 BC by her calculations, is the most accurately aligned, with a deviation of three arcminutes (three sixtieths of a degree) to the west. Three earlier pyramids deviate more westwards, the older they are, while a later pyramid (Menkaure) deviates by 13 arcminutes to the east.
Plot the date of construction against the deviation and you get a straight line. Dr Gingrich said: "It is not preposterous to believe that Dr Spence can calculate dates for pyramid construction to within five years or so, far better than the 100-year-error currently accepted for chronologies."
The Pyramids were designed to ensure that the pharaoh lived on after death: the tomb was to house the corpse and grave goods were stored inside to amuse them in the afterlife; the pyramid itself was aligned with the circumpolar stars to allow the king's spirit, in the form of a bird, to commute between the heavens and Earth.
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