LONDON — It is a mystery that has perplexed the world's top scientists for more than a century because no one knows how they did it or exactly when.
But a British Egyptologist believes she may have solved the puzzle and figured out how the ancient Egyptians aligned the pyramids of Giza to true north and roughly when they did it.
The heavenly alignment of the pyramids, one of the seven wonders of the world, is precise enough that scientists were convinced the Egyptians had to have a good knowledge of astronomy even though there is no record of it in ancient texts.
The best estimate of the age of the royal tombs, roughly 4,500 years old, is based on chronologies of the period and the reign of kings and is only accurate to within 100 years.
Kate Spence, of the University of Cambridge, estimates the building of the pyramids began between 2,485 B.C.-2,375 B.C. and, crucially, that two stars helped the Egyptians to align them to true north, important to them for religious reasons.
"The correspondence between the archaeological data and the astronomical modeling is extremely compelling. It is very rare in archaeology that you get things that you can model that closely," she said in a telephone interview.
"This is a much more convincing argument than has been put forward in the past," she added.
In addition to solving a long-standing mystery, the findings reported in the science journal Nature add to the study and chronology of the ancient Egyptians and in the understanding of their technical ability.
Their building expertise is beyond doubt, but Spence said her findings show they were poor astronomers.
"This does show they did not have a sophisticated observation of astronomy," she said.
The Egyptians were trying to find true north but they did not have a star marking the pole. So they used two stars, Kochab in Ursa Minor or the Little Dipper, and Mizar in Ursa Major, or the Big Dipper to find the pole.
"It (the pole) is on a line between those two stars. You measure when the two stars are basically on top of one another and if you line them up with a plumb line that will give you true north," Spence said.
According to astronomical data, 2,467 BC is the year in which the line that goes between the two stars passes exactly the trajectory of the pole.
"If they had started building on that date we would have a pyramid which is absolutely aligned to north. But the fact is they seemed to have started work about 11 years before that, which means it is still a few minutes off north," Spence said.
In a commentary on the research in Nature, Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, described Spence's research as an ingenious solution to the long-standing mystery of how the great pyramids were so accurately aligned in relation to north.
"So it is not preposterous to believe that Spence can calculate dates for pyramid construction to within five years or so," he added.