How did the ancient Egyptians manage to get the Great Pyramid precisely aligned to due north?
Owen Gingerich: Nature magazine never told me they were going to put it on the front cover and they didn't tell me they were sending out a press release that just happened to include my name. Now you have to understand, this whole idea was not mine but suddenly I was bombarded by radio interviews, by reporters literally from all over the world, and the pyramids apparently have a certain mystique that just captures the attention of people.
Robyn Williams: How do you account for that then?
Owen Gingerich: Well, you have to realise that the pyramids were as old to Heroditus and Heroditus is old to us. So that these really go far back and to have such a huge construction project, how in the world did they do it? Of course, there have been many attempts to rebuild small pyramids just to show that it could be done with a lot of brute force and a lot of piling up of ramps going around to get the stones in place. But still, it's awesome and very impressive, and then when you discover that the Great Pyramid is lined up within three or four minutes of arc to true North it becomes especially astounding.
Now, just to give you an idea of what that is; the very limit of naked eye accuracy is something around 1 minute of arc and you can only hit it that accurately if you have repeated observations. So this is quite incredible for something that is being done in the 3rd Millennium BC.
Robyn Williams: So how did they do it.? Owen Gingerich is Professor of Astronomy and the History of Science at Harvard.
Owen Gingerich: To have it lined that precisely I think, stretches the imagination to suppose that's just accidental.
Robyn Williams: So they must have planned to do it. Why would they plan to do it that way?
Owen Gingerich: That is a very interesting question. What would the significance of true north be? It most be an appreciation of a fixed pivot point in the heavens, because of course the only way to get north is by astronomical measurements at that time. So there must have been something about the mythological law that put a particular premium on that fixed north point in the sky that made it very important to get the buildings lined up that way. Perhaps because that's how the soul of the Pharaoh had to be transported.
Robyn Williams: Well of course, there were previous theories as to the way in which they did it before the discovery on which you are commenting was published. What were the theories?
Owen Gingerich: Well, if you're going to do it astronomically you have to think either of stars or of the sun. Now as the sun makes its course across the sky it rises directly in the east only at the time of the equinox and even that is a little bit tricky because refraction, this bending of the light and changing the position slightly, means that even at the time of the equinox the sun won't rise directly east. And furthermore, if it is the Spring equinox, which in our calendar is around March 21st, then the sun is on its way northward and if it would come up directly to the east, by the time it goes down in the west, it would already be a little bit to the north of west, if you're talking about this very great precision. So, it becomes tough to use the sun as this kind of a measuring device. Although I suppose if you used it repeatedly and particularly watched the shadows at the time of the solstice, when the sun is at its farthest north point and therefore not moving appreciably north and south in the course of the day, or when it's at its extreme south point, you might be able to do something like that but still, it's never been terribly convincing to me that solar shadows would have been the secret.
Robyn Williams: OK, so the research was done and Spence wrote her article. What was her conclusion:
Owen Gingerich: Kate Spence is an archaeologist and Egyptologist at Cambridge University in England, and she noticed a very interesting phenomenon in the northern sky. Namely, around the time the Great Pyramid was being built there were two fairly bright stars that were some distance from the Pole - one on each side. The Pole you have to understand, is invisible, that is to say, there's no star that's precisely marking the place. At the moment there is Polaris, the North Pole star, but even that isn't quite good enough if you're trying to get the alignment to one or two minutes or three minutes of arc.
Robyn Williams: What was her theory?
Owen Gingerich: She noticed that the star Kocab, which is the star at the opposite end of the Little Dipper from Polaris, the North Pole star and the star Mizar in the middle of the handle of the Big Dipper. Now, you've got to watch out. At that time Polaris was not marking the North Pole star. We have a phenomenon known as precession in which the Pole or the axis of the earth, sweeps out a very large circle in the sky and it takes about 26,000 years to sweep out this path. So, just at the moment Polaris is quite close to the Pole and this is the best Pole star that we ever get in the 26,000 year cycle. But now, let's spin time backwards about four and a half millennia and then you don't get a decent star right at that Pole point. But what you do get are these two reasonably bright stars that flank this invisible Pole. Now, here is Spence's neat idea. If you get those stars so that one is directly above the other in the north direction, and you can tell that by hanging a string with a weight on it, a plumb line down to cut through them both, that line will actually pass through the Pole in the sky - but not for a very long time. It's going to be at its closest point I believe somewhere around 2400 and something BC. And it just happens that the Great Pyramid was built at the time when that was a maximum alignment. And she proposes that that was how it was done, by hanging a plumb line and waiting till those two stars got one above the other as the sky slowly rotates around.
Now this is a little bit tricky, because to get that sufficient accuracy you've only got about one minute to make your observation. However, you can tell as it's approaching this up and down alignment, that it's coming there and you can get yourself pretty well set and you're place where you're observing from, so you know how it's going to come out. Then all you have to do is verify it, which is not so difficult to do in the one minute.
Well, OK it's an idea, it could maybe work. Now, here is where she really became clever. She realised that the Pole is moving in the sky with respect to those two stars, so that fix will not continue to be so accurate, and if you go some few centuries earlier, in fact it won't line up well at all. And what she found was that pyramids built earlier in fact had a slight rotation pointing to this sort of wrong Pole. When you get those two stars one above the other it would mark a direction that isn't true north but only slightly deviating from it. And when she plots the orientation of the pyramids, that is to say, their deviation from the north point, she could see this gradual rotation over the centuries as the pyramids line up on this false Pole, so to say, and by chance, when the Great Pyramid is built it's the best possible alignment and then it goes back out again.
Robyn Williams: Isn't it fantastic. I have this picture of these Egyptians, something like 5000 years ago, making these measurements and people standing by, presumably to design it, because you can't just put up pyramids just like that, waiting the get the nod to do this immense amount of construction. Is that how you pictured it?
Owen Gingerich: Well, there is evidence of some kind of a ceremony stretching the line, somehow establishing where the foundation should be and this must be a very serious kind of ceremony to do it, but I can't imagine that this would be the first time the astronomer architect brought his string out. I would figure that he knew pretty well from quite a series of observations where this was going to be, but you would do one final, sacred alignment to get it all set at some point. So I've put it to Kate Spence, can you get any evidence at all in the ancient hieroglyphic records that there is such a ceremony. And in fact she says there are hints of it in the texts. It isn't terribly explicit, it doesn't say OK, we're going to use a string hanging down and those two stars, but there are hints that something like that was going on. So it seems to me that, here is a rather convincing possibility, linked up kind of beautifully and, of course, once you do it this way you know the date when you had that particular error in the alignment of the Pole, and it allows you to date the times of the pyramids and this is really quite extraordinary.
Robyn Williams: What date do you get?
Owen Gingerich: The Great Pyramid is coming somewhere around 2467BC. And some of the other pyramids of course are earlier and some of them are later. And then eventually this whole system just fails because it moves too far away. But of course also, pyramid building basically stops after a while and you get other kinds of monuments, but this happens to correspond to the great heyday of pyramid building.
Robyn Williams: An amazing piece of detective work by Kate Spence of Cambridge, whose story was told by the man who wrote it up in the journal Nature, Professor Owen Gingerich of Harvard.
- Professor Owen Gingerich
- Research Professor of Astronomy and History of Science
Center for Astrophysics
60 Garden Street
Cambridge MA 02138
- Robyn Williams
- Polly Rickard