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Dating the pyramids

One of the Seven Wonders of the Ancient World was the group of Egyptian pyramids at Giza. They were built roughly 4500 years ago.

One of the Seven Wonders of the Ancient World was the group of Egyptian pyramids at Giza. They were built roughly 4500 years ago.

Note the word, 'roughly'. Until now, we dated the pyramids by working out how long each individual Egyptian Pharaoh held power, and then added all the years, and worked our way backwards.

But now Kate Spence, an Egyptologist from the Faculty of Oriental Studies from the University of Cambridge, has come up with another way to date the pyramids

The pyramids were built as tombs for the pharaohs. Even Pharaohs who reigned for only a few years started building their own tombs. So Dr. Spence assumed that by the second year of their reign, each pharaoh would have chosen a suitable location, and then started levelling and preparing the site.

The Ancient Egyptians managed to line up the sides of their pyramids to the points of the compass, with extraordinary accuracy. The most accurate is the Pyramid of Khufu, also called the Great Pyramid. The east and west sides miss true north by less than three minutes of arc (roughly one tenth the diameter of the full moon). With this kind of accuracy, it's no wonder they were one of the Seven Wonders of the World. It took over 4,000 years before the astronomer, Tycho Brahe, was able to take astronomical measurements to a greater accuracy.

And this led to a problem that has bothered Egyptologists for a long time - how did they manage to line up the east and west sides of the Great Pyramid so accurately with the North Pole?

Now we're not talking about Magnetic North. We're talking about Geographic North which is the pole about which the Earth seems to spin. The Magnetic North Pole is currently wandering at a few kilometres per year through the far north of Canada, while the Geographic North Pole is in the Arctic

One way to find Geographic North would be by looking at the Sun. You'd do this around the time of the solstice, when for a little while, the Sun would appear to have stopped marching higher or lower in the sky. Then you note the rising position of the Sun in East, and the setting position of the Sun in the West, and halve the angle between them - bingo, True North.

But there are a few problems with this method. First, when objects are near the horizon, the Earth's atmosphere causes a lot of interference. Secondly, you need a clear view to the horizon in both the East and the West, which was not available from the sites of the pyramids

The other way to find Geographic North involves the stars at night. The Celestial North Pole is a point in the sky about which the stars appear to rotate. If you took a time lapse photograph over a full night, you would see that all of the visible stars would appear to move in circles (some big and some small). The star Polaris would make an incredibly small circle because it's right next to the Celestial North Pole. So it's called the Pole Star

But there's a slight problem with this method as well. The North-South spin axis of the Earth is not fixed, but rotates slowly like a giant top. Imagine you have a big basketball and it's spinning. Now the spin axis is not straight up-and-down, but tilted by 23 degrees from the vertical. This how the Earth is currently spinning. Now start off with the North Pole of your spinning basketball slightly to the right. Gradually, the North Pole sweeps out a circle, so that it's next facing away from you, and then over to your left, and then coming around to face you, and then finally back over to the right again. The spin axis of the Earth sweeps through one complete rotation in 26,000 years

So the position of the Celestial North Pole changes with time, and this is what Kate Spence tried to use, to work out when the pyramids were built

For a few hundred dollars, you can get a computer program that tells you the positions of the stars, at any point in time. In the year 2467 BC (when building pyramids was all the rage), two stars appeared to rotate around the Celestial North Pole. These stars were Mizar (Eta-Ursae Majoris) in the Big Dipper and Kochab (Beta-Ursae Minoris) in the Little Dipper. Back in 2467 BC, it would be quite easy to find True North. All you'd have to do would be to build some scaffolding, and hang a string with a heavy weight. This would hang perfectly vertically, pointing to the centre of the Earth. You'd then wait until these two stars were vertically aligned exactly with your hanging string. Then a line from you, to the hanging string, would point due north to the horizon

But Kate Spence also realised that this method of finding True North would work only in that year - because the spin axis of the Earth is slowly sweeping out a great circle over some 26,000 years. If you use that method before 2467 BC, you'd be slightly to the west of True North and after that date you'd be to the east of True North. And when Dr. Spence looked at the Pyramids of Giza, she found exactly this relationship - the earlier ones were lined up slightly to the west, and the later ones slightly to the east

It could be that this is just a coincidence - after all, she was looking at only half a dozen pyramids. That's a very small sample size. But if her method is correct, it means that we can calculate the dates when the pyramids were built to within five years or so - which is much better than the currently accepted hundred-year error

Perhaps the only real error the Egyptian pyramid builders made was forgetting to install some doors.

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