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I have a fairly basic question about the alignment of the great pyramid. The pyramids at Giza are aligned to "true north". This means north as in the axis of rotation of the earth, as opposed to magnetic north, correct? Then another question is, how would one go about aligning it. Using a sun clock, and measure when the shadow is longest, then you know you have either north or south, depending on what hemisphere you are in?
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Watch the night sky. Polaris might not be exactly due north, but it's close, and careful observation can let you split the difference.
Fred
"For shame, gentlemen, pack your evidence a little better against another time."
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Order of Kilopi
To doublecheck shortest shadow length, I think I'd note east and west sunrise and sunset points and bisect that angle. If they then couldn't bisect, an estimate would make a good check. Patient or full of foresight, they could have used an equinox line and measured 90 degrees (3-4-5 triangle) from that.
Edit: BBC History: Building the Great Pyramid
I like that one.How did this astronomically based surveying work in practice? The British Egyptologist IES Edwards argued that true north was probably found by measuring the place where a particular star rose and fell in the west and east, then bisecting the angle between these two points. More recently, however, Kate Spence, an Egyptologist at the University of Cambridge, has put forward a convincing theory that the architects of the Great Pyramid sighted on two stars (b-Ursae Minoris and z-Ursae Majoris), rotating around the position of the north pole, which would have been in perfect alignment in around 2467 BC, the precise date when Khufu's pyramid is thought to have been constructed. This hypothesis is bolstered by the fact that inaccuracies in the orientations of earlier and later pyramids can be closely correlated with the degree to which the alignment of the two aforementioned stars deviates from true north.
Order of Kilopi
I should think that the Egyptians used the night sky, a few wooden stakes set in the ground, a platform from which to suspend a plumb bob, and some stout string. The southern stake would have a plumb bob set above it at a sufficient height to respect the sag in the line from point to point north, and the altitude of Polaris ( fairly low on the night horizon north ) , with a sighting through those two points, from which the northern plumb bob would depend, and define the northern point. The triangle of 3, 4 , 5 solves the square. Simple tools andOriginally Posted by Jens
the passing on of knowledge , generation to generation.
The night sky in the desert is something to behold.
Best regards, Dan
Order of Kilopi
If the architects did it that way and did not take the sort of simple precaution that would have caught the precession-induced error before and after 2467, it had to be one of the great surveying blunders of all time. Suppose you sight on a plumb line with star b on top, and then repeat it with z on top. If the line between the stars is offset from the pole by a visible amount you will see the change in direction right away. Splitting the difference will give you true north or very nearly so. It will be most accurate if the two stars have the same declination.How did this astronomically based surveying work in practice? The British Egyptologist IES Edwards argued that true north was probably found by measuring the place where a particular star rose and fell in the west and east, then bisecting the angle between these two points. More recently, however, Kate Spence, an Egyptologist at the University of Cambridge, has put forward a convincing theory that the architects of the Great Pyramid sighted on two stars (b-Ursae Minoris and z-Ursae Majoris), rotating around the position of the north pole, which would have been in perfect alignment in around 2467 BC, the precise date when Khufu's pyramid is thought to have been constructed. This hypothesis is bolstered by the fact that inaccuracies in the orientations of earlier and later pyramids can be closely correlated with the degree to which the alignment of the two aforementioned stars deviates from true north.
You might have to wait weeks or months to do this, depending on the timing of the respective culminations with respect to darkness. Perhaps they were in a hurry and took shortcuts, not recognizing the possibility of significant error.
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I'm not entirely certain, but at the time the Pyramids were built, I think that because of precession, Polaris was a bit further from true north than it is today.
Today it's very close (within a minute). And the alignment of the pyramid of Cheops is a bit over 2 minutes off. But I think than in 2000 BC, or whenever it was that the pyramid was built, it Polaris would have been further away than that.
As above, so below
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Yes, But if it was off, it's deviation would have been measured, ie the minute circle the star describes would have been well known to those astronomers of the time. In effect, they would split the difference. Dead on North.
What sometimes appears complex, upon reflection, has a simple answer.
Best regards, Dan
Order of Kilopi
If wonder if they would have done it that way, because I can't figure out how you'd know where to split. What I mean is, if you can see the star at sundown and sunrise, you have two measurement times that are I think equidistant from midnight, so you could just find the middle. But unless you have a clock, you can't decide when it's say 1 hour before midnight and 1 hour after midnight.
I would think it would make sense to use sunrise and sunset and split the difference. I think you could do it fairly accurately by standing in one spot, and have a person drag a string to a place say 100 paces away where the sun rises. And similarly, have another person do the same for the sunset. Then they start walking in a straight line toward one another, measuring paces, and where they meet is the middle. You could actually do it very accurately by having them pull a string between them, then fold the string in two, and have one of them walk along a marked line the length of the half string.
As above, so below
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AFAIK, they used Kochab and Mizar, which they called "the indestructables" which rotated around true North back then. Could have lined both up on a plumb line and cross checked by bisecting the distance between them when they formed a line perpendicular to the plumb.
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I saw one method which involved setting up an artificial horizon - make it a board with a groove where you can pour water to adjust it to true horizontal. Then use a pole to sight from, and mark where along the board a certain star rises and sets. Midway between those positions will be true north, as seen from the sighting pole.
A little too involved for everyday use, but hey, if you're about to place a few million tons of rock, I think taking a day to mark the positioning isn't all that much trouble.
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Polaris was nowhere near the celestial pole at the time, so it wasn't useful.
yuzuha is correct. In fact, the Egyptians didn't realize that the north had moved between the building of the pyramids, so the pyramids are not exactly aligned.
For more, see Astronomy and the Great Pyramid.In November 2000, Kate Spence, an Egyptologist at the University of Cambridge, published a seminal paper in Nature in which she suggested a method by which the pyramid builders determined what they thought was north. She also showed that the resulting orientation errors varied as a function of time—just as predicted by precession. Moreover, by fitting the time–linked precession errors to the slight deviations of each pyramid, she revised their building dates. Instead of 2554 B.C., her data suggest the Great Pyramid was constructed between 2485 and 2475 B.C.
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They were looking for the center of the great circle descibed by the stars. Polaris doesn't have to be "Precisely" at the center, but it would have been close enough. The point is that their goal was to align the pyramid with
the axis of Earth's rotation. This would have been known to them with their methods of observation at the time. The method I described for laying out
the control line is valid, and,I suspect, authentic.
Best regards, Dan
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No, it was not close enough. Polaris was as much as 25° away from the celestial pole at the time. The nearest bright star was Draco's Thuban (see this image).
And why would they have used Polaris? Kochab (Beta UMi) has almost the same brightness and it was much closer to the pole.
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This reminds me... has there been a geological response (especially a counter!) to the guy who said that erosion indicates that the Giza construction was done much longer ago than archæoogists had thought? I know I've seen him dealing with archæologists who say it contradicts all of their evidence, but I don't know whether other geologists agree with his geology or say it's bad geology.
(His geological claims appear sound, at least in the absence of on-topic rebuttal, but his ideas on prehistorical anthropology are quacky enough to make it seem suspicious that his geological ideas just happen to have anthropological implications...)
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If he's arguing that erosion of the pyramids indicates they are older than currently thought, he's probably forgetting about the effects of nearby people using them as sources for building materials.
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Well, if trying it that way, you'd also have to account for material that was ADDED to the surface that we can't see anymore... but anyway, actually he's talking about the cliff where the stones were apparently removed. He calculated, based on the different layers' erodibility and how far "back" they were compared to each other (the less erodible layers sitcking out above and below the more erodible ones), that the layers line up into a straight vertical cut face something like 10-12 millennia ago instead of 4 or 5, and that they've been eroded by water, which means they had to be exposed when Egpyt was rainy, which was also at least a few thousand years longer ago than archælogists say the Giza stuff was done.
There was more to it (something about lining up with stars that were only in the right places much longer ago, like maybe the statue facing Leo or something like that, and a temple higher upstream someplace that's supposedly buried too deep in sediment for current archæological theories)... but that stuff is stuff he found when he was challenged on the basic idea's anthropological viability and went looking for the backup he wanted. The thing about the cliff's erosion rates is the core idea that the rest was just dug up to support. (He was told there needed to be a culture there to do it but there was no sign of one, so he needed evidence of a culture of the time that could possibly have done it.)
But the other big major idea he proposes, which this Egyptian business might just be a small part of, is that even before the established ancient civilizations, people around the world were already much more advanced and had contact with each other over great impossible-seeming distances of both land and sea.But still, just doubting the guy isn't enough, and I don't know of a geological counter to his geological claims, which are indeed anthropologically revealing if they're accurate.
I just wish I could remember his name; then we could go to his website and/or look up his books, shows he's been on, debates he's been in, and such...
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Order of Kilopi
The stars may change, but the method remains.
Order of Kilopi
I think you could, if you put up some sighting stakes. When the star is up or down, half-way between the sunrise and sunset stakes, that’s midnight.
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It has been suggested that Egyptians got their idea to build pyramids and the Sphinx from natural rock formations which greatly resemble them.
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I don't see how that can be done accurately enough. Besides, there were no good stars nearby. It is very rare to have a star as bright as Polaris so close to a celestial pole.
Order of Kilopi
Hi, You simply observe the night sky, and set pointers to the different positions of the star nearest the center of rotation. Once you have defined those points,
mechanically dividing those will give you the accurate result. An advantage would be in a truly dark sky and a dry desert atmosphere. Nice conditions to observe.....(I see many people nodding their head).
Best regards, Dan
Order of Kilopi
The stars do not need to be super close to the pole to be useful. If I were doing it today with equipment available to the ancient Egyptians, I would sight on each star at its maximum eastern and western excursions from the azimuth of the pole. This could be done by hanging a plumb line from a scaffold, laying out a level rail in a circular arc around the plumb line, and marking the positions of each sighting on the rail. Mizar and Kochab are excellent for this since they are bright and were only a few degrees from the pole at the time. (Of course Polaris would be my choice for doing it today.) Splitting the difference between the sighting points would give an accurate north-south line between them and the plumb line. If I had trouble seeing the string in the dark I could have a helper with a lantern assisting me.
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Hi Hornblower, Well said.
Best regards, Dan
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The post you're responding to used the term "the construction at Giza" somewhat vaguely, but IIRC, the geologist was talking about the Sphinx in particular, not the pyramids.
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