Page 1 of 2 12 LastLast
Results 1 to 30 of 33

Thread: Tectonic plates, and alignment to the north

  1. #1
    Join Date
    Sep 2004
    Posts
    444

    Tectonic plates, and alignment to the north

    Okay guys a 'complicated' math question here.

    One of the pyramids presently has a three minute error of alignment with the north pole (the geographic one).

    I also understand that the Africa plate is moving to the NE at a rate of 2.15 centimeters a year. So that times a conservative estimate of 4,500 years of age means a movement of 9.6 meters to the NE.

    How much would that throw the alignment off?

    I remember reading that the this calculation was done, and I saw it many years ago, but I cannot find it on the web. Anyone know the answer to this?

    Added this: I believe the article I may have read was ‘Do the Pyramids Show Continental Drift’ in Science in 1973
    Last edited by Hans; 2008-May-26 at 09:00 PM. Reason: Added last line

  2. #2
    Join Date
    Jun 2005
    Posts
    439
    I don't think you can answer that unless you can quantify any amount of "twisting" of the Africa plate.

  3. #3
    Join Date
    May 2003
    Location
    of Greatest Eclipse, Aug. 21 2017 (Kentucky, USA)
    Posts
    4,556
    Quote Originally Posted by Hans View Post
    Okay guys a 'complicated' math question here.

    One of the pyramids presently has a three minute error of alignment with the north pole (the geographic one).

    I also understand that the Africa plate is moving to the NE at a rate of 2.15 centimeters a year. So that times a conservative estimate of 4,500 years of age means a movement of 9.6 meters to the NE.

    How much would that throw the alignment off?

    I remember reading that the this calculation was done, and I saw it many years ago, but I cannot find it on the web. Anyone know the answer to this?

    Added this: I believe the article I may have read was ‘Do the Pyramids Show Continental Drift’ in Science in 1973
    The distance from the equator to the pole is about 10,000 km (pi/2*R). That's 1 million times your 10 meter drift, so the angular shift is one-millionth of a radian. One arsecond is 1/360/60/60 = 1/130,000th of a full circle, and so about 1/40,000th of a radian. So this drift is something like 1/1000th of your 3' error.

    Edit: Terp has a point, and I'm not really taking any rotation into account. But with your drift rates, I wouldn't think that there'd by too much rotation, either.

  4. #4
    Join Date
    Oct 2006
    Posts
    3,283
    Quote Originally Posted by Tobin Dax View Post
    But with your drift rates, I wouldn't think that there'd by too much rotation, either.
    ...unless the entire crust together moved relative to the rotational axis, separate from the plates' relative motions.

  5. #5
    Join Date
    Jul 2002
    Posts
    334
    The ancient Egyptians would have aligned their north point with Polaris. Earth's north pole precesses around Polaris in a 26,000 year cycle. This may account for the drift.

  6. #6
    Join Date
    May 2003
    Location
    of Greatest Eclipse, Aug. 21 2017 (Kentucky, USA)
    Posts
    4,556
    Quote Originally Posted by Delvo View Post
    ...unless the entire crust together moved relative to the rotational axis, separate from the plates' relative motions.
    I stand corrected.

  7. #7
    Join Date
    Apr 2005
    Posts
    11,562
    Quote Originally Posted by Senor Molinero View Post
    The ancient Egyptians would have aligned their north point with Polaris. Earth's north pole precesses around Polaris in a 26,000 year cycle. This may account for the drift.
    Wouldn't it have been some other point, thousands of years ago? Why would they use Polaris, instead?

  8. #8
    Join Date
    Jul 2005
    Location
    Boulder, CO
    Posts
    3,480
    No, they wouldn't have aligned it with Polaris. Because of the precession, Polaris was not actually anywhere near the north pole at the time of the Pyramids IIRC.

  9. #9
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    4,930
    Quote Originally Posted by Senor Molinero View Post
    The ancient Egyptians would have aligned their north point with Polaris. Earth's north pole precesses around Polaris in a 26,000 year cycle. This may account for the drift.
    At that time Polaris was about 25 degrees from the celestial pole. If they had naively taken its bearing as true north at some moment when it actually was way off to one side or the other, the pyramid would be many degrees out of line.

    Finding true north by means of star sights is very straightforward. Any star near the pole will do nicely. Just measure its azimuth at maximum eastern and western excursions and find the midpoint. An error of no more than three arcminutes is indicative of excellent work with their unaided eyes.

  10. #10
    Join Date
    Mar 2004
    Posts
    131
    I'm not aware of a North orientation for the Pyramids.

    I don't recall the source, but I believe they are situated to reflect
    the "belt" in the constellation Orion.

    Comparing these two images, I can understand why someone would think so
    considering the pattern and size of the structures.

    http://media.skyandtelescope.com/images/Orion_l.jpg
    http://www.spaceimagingme.com/conten...s/pyramids.jpg

    A similar pattern can be seen at Abusir including what ever the nearby round structure is.

    I can't recall if any Egyptologist has ever confirmed it, however.

  11. #11
    Join Date
    Sep 2004
    Posts
    444
    related question

    3 arc minutes

    If I hold out my arm and stick up a finger (for measurement) how much bigger or smaller is 3 arc minutes in size?

  12. #12
    Join Date
    Jul 2005
    Location
    Boulder, CO
    Posts
    3,480
    Quote Originally Posted by David Mc View Post
    I'm not aware of a North orientation for the Pyramids.

    I don't recall the source, but I believe they are situated to reflect
    the "belt" in the constellation Orion.

    Comparing these two images, I can understand why someone would think so
    considering the pattern and size of the structures.

    http://media.skyandtelescope.com/images/Orion_l.jpg
    http://www.spaceimagingme.com/conten...s/pyramids.jpg

    A similar pattern can be seen at Abusir including what ever the nearby round structure is.

    I can't recall if any Egyptologist has ever confirmed it, however.
    They are oriented to the North - each pyramid is aligned such that the sides point to each of the cardinal directions. This isn't an alignment between several pyramids, but of each pyramid individually (you can check it on google earth - coordinates for the great pyramid are 29 degrees, 58' 30" N, 31 degrees, 7' 40" E.

  13. #13
    Join Date
    Aug 2007
    Posts
    137
    Quote Originally Posted by Hans
    If I hold out my arm and stick up a finger (for measurement) how much bigger or smaller is 3 arc minutes in size?
    About 1/20th the way across your index finger. 3' is a very small error, especially for unaided eyes, as Hornblower said.

  14. #14
    Join Date
    Jan 2007
    Posts
    1,166
    Um, maybe the original builders never intended for it to face due north or they just made an error.

    There's a 1 in 360 to which way they will align.

  15. #15
    Join Date
    Jul 2003
    Posts
    872
    Yes, Polaris should not have been the pole star or anywhere close. Even today it is not truly "on" the mark, but a smidgen to the side. About a year ago now I was reading a book discussing the pyramids, and the characteristic in question was brought up. The book was written to discuss the religous-Orion related hypothesis, and so on the topic of finding north it was brief but clear. The author suspected the ancient Egyptians would use one star from Draco, and one in...? Anyway, when the two would line up on a plumb line (i.e they were vertically aligned) that was north. I took it with a grain of salt, but it was an interesting concept.

    All things considered, they did a decent job. Three degrees, even with a compass, is a miniscule amount over the distance of the pyramid edges and can be difficult to determine without a laser site or rather long bit of string. I would as soon chalk it up to pilot error as anything else.

    They could, of course, have lined the pyramids up perfectly and any number of non-human reasons could have affected that in the ensuing time. The plates could have shifted, the covering stones could have been perfectly aligned and since removed (with the now exposed stones a rough sort of cut), the whole pyramid could have moved or shifted, the sand could have settled or been removed underneath it....

  16. #16
    Join Date
    Jun 2005
    Posts
    8,875
    Quote Originally Posted by man on the moon View Post
    All things considered, they did a decent job. Three degrees, even with a compass, is a miniscule amount over the distance of the pyramid edges and can be difficult to determine without a laser site or rather long bit of string. I would as soon chalk it up to pilot error as anything else.
    It's worse than that. An arc minute is not a degree. An arc minute is 1/60th of a degree. So the Egyptians were not three degrees off, they were 1/20th of one degree off.
    As above, so below

  17. #17
    Join Date
    Sep 2003
    Posts
    3,882
    Quote Originally Posted by hhEb09'1 View Post
    Wouldn't it have been some other point, thousands of years ago? Why would they use Polaris, instead?
    I've heard that Thuban in Draco was the ancient Egyptians' pole star...

  18. #18
    Join Date
    Oct 2006
    Posts
    3,283
    Did celestial alignment necessarily have to be for religious reasons, or could it have just been their way of ensuring that their right angles were "right"?

  19. #19
    Join Date
    Sep 2003
    Posts
    3,882
    Quote Originally Posted by Delvo View Post
    Did celestial alignment necessarily have to be for religious reasons, or could it have just been their way of ensuring that their right angles were "right"?
    The ancient Egyptians used right-angled triangles (Pythagoras' Theorem) to get their right angles right.

  20. #20
    Join Date
    Jul 2003
    Posts
    872
    Quote Originally Posted by Jens View Post
    It's worse than that. An arc minute is not a degree. An arc minute is 1/60th of a degree. So the Egyptians were not three degrees off, they were 1/20th of one degree off.
    yeah...I should go to bed before 3am. I should also be ashamed of myself for not reading that fully. :P

    Bottom line is they did a ridiculously good job with it.

    A loop of rope with knots spaced evenly, then staked out with sides, 3, 4, and 5--is that the correct hypothesized method for their finding right angles? I'm sure there must be something else too.
    Last edited by man on the moon; 2008-May-27 at 02:09 PM. Reason: spelling

  21. #21
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    4,930
    Quote Originally Posted by man on the moon View Post
    yeah...I should go to bed before 3am. I should also be ashamed of myself for not reading that fully. :P

    Bottom line is they did a ridiculously good job with it.

    A loop of rope with knots spaced evenly, then staked out with sides, 3, 4, and 5--is that the correct hypothesized method for their finding right angles? I'm sure there must be something else too.
    That is an excellent method of laying out a right angle on the ground, and it was known in ancient times. I used it myself many times, when our high school band needed football yardlines in a hurry for marching practice. Jute and sisal are good materials for this, because they do not stretch much.

    Let's add a final low tech touch to a pyramid project. When we have found true north, dig a trench around the planned perimeter and keep leveling the bottom until water will stand all around. Then we have a perfectly level surface from which we can put in elevation benchmarks.

  22. #22
    Join Date
    Apr 2008
    Posts
    50
    Don't claim to be a real expert on ancient Egypt - but I have watched Dr. Zahi Hiwass several times on the History Channel, and I have read Ken Follett's Pillars of the Earth - where Tom laid out the alignment of the cathedral at sunrise. (haven't stayed in a Holiday Inn in years though).

    Since the Egyptians were fixated on the Sun, maybe they used it for a true east - west alignment (or even a north south alignment - when the stick's shadow was minimum at noon - the shadow points true north). They must surely have seen Polaris was not fixed in the sky at that time.

  23. #23
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    4,930
    Quote Originally Posted by BlueTiger View Post
    Don't claim to be a real expert on ancient Egypt - but I have watched Dr. Zahi Hiwass several times on the History Channel, and I have read Ken Follett's Pillars of the Earth - where Tom laid out the alignment of the cathedral at sunrise. (haven't stayed in a Holiday Inn in years though).

    Since the Egyptians were fixated on the Sun, maybe they used it for a true east - west alignment (or even a north south alignment - when the stick's shadow was minimum at noon - the shadow points true north). They must surely have seen Polaris was not fixed in the sky at that time.
    To them, Polaris would have been just another bright star, no more special than anything else a similar distance from the pole. Kochab and Mizar were much closer to the pole, and they straddled it while Thuban was nearly on the pole. Those three made a striking asterism rotating around the pole, and they would have been my choices for finding true north by the method I described in my previous post.

    If we so wished we could do it entirely with the Sun's shadow. It would be more trouble because the tip of the shadow would be fuzzy, but with numerous observations we could average out the uncertainties and get a result just as precise as with star sights.

  24. #24
    Join Date
    Apr 2005
    Posts
    11,562
    Quote Originally Posted by Hans View Post
    Added this: I believe the article I may have read was ‘Do the Pyramids Show Continental Drift’ in Science in 1973
    This article: Do the Pyramids Show Continental Drift?, by Pawley and Abrahamsen, in Science 2 March 1973.
    The mystery of the orientation of the Great Pyramids of Giza has remained unexplained for many decades. The general alignment is 4 minutes west of north. It is argued that this is not a builders' error but is caused by movement over the centuries. Modern theories of continental drift do not predict quite such large movements, but other causes of polar wandering give even smaller shifts. Thus, continental drift is the most likely explanation, although somewhat implausible, especially as relevant measurements have been made over a 50-year period, whereas geophysical measurements of sea-floor spreading relate to million-year time scales.

  25. #25
    Join Date
    Sep 2004
    Posts
    444
    Thanks for all the information gentlemen

  26. #26
    Join Date
    Jan 2006
    Posts
    2,706
    How closely aligned are the three to each other?

  27. #27
    Join Date
    Dec 2005
    Posts
    14,315
    First, there are three forces at work here: precession, nutation, and tectonic plate shift.

    Precession is the simple wobble experienced by ordinary gyroscopic effects. When it comes to planets, however, it gets a bit more complex, as planets aren't resting on a point - they're suspended in essentially zero g. Since a gyroscope in zero g doesn't precess, why does the Earth precess?

    Astronomical precession, also referred to as precession of the equinoxes, occurs on the Earth in a cycle of 25,765 years. This is caused by the gravitational forces of the Sun and the Moon.

    On the other hand, nutation is a wobble within that precession. It, too, occurs due to the lunar and solar tidal forces, but it's period is much smaller, and it varies in more than one mode, the largest of which is 18.6 years, but the smallest of which can vary every lunary month.

    Both precession and nutation must be taken into account during celestial navigation, as both can throw off the celestial position by quite a bit.

    Plate tectonics, on the other hand, involves the movement of land masses on the Earth. As such, it doesn't effect the lat/lon of the celestial position, except by means of polar motion, largely cause be redistributions in the ice shelf and changes in the Earth's core. However, that effect is small enough and occurs over a large enough period that it's ignored, except for the inclusion of major updates in the celestial almanacs.

    It is, however, accounted for in the global positioning system, as it's effect is about 20 m since the year 1900.

  28. #28
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    4,930
    Correction: At 2.15 cm/year, the drift in 4,500 years should be 96 meters, not 9.6.

    I just did some rough and dirty trig/calculus on a hypothetical example, in which continental drift carries a pyramid 100 meters sideways from its original position, and leaves it parallel to the original true north line. That motion is about the same magnitude as that quoted in the OP, over a period of 4,500 years. As we all know the lines of longitude converge toward the pole, so the pyramid will be slightly off of true north now. I get about two arcseconds for this example.

    To change the azimuth by 3 arcminutes, we would need to rotate the plate in a manner that would have some parts of Africa drifting upwards of a kilometer over that period. That is vastly faster than any drift I have ever heard of, and such a rotation is contrary to sources such as the following animation.

    http://www.ucmp.berkeley.edu/geology/anim1.html

    It shows virtually no rotation of Africa in the past 100 million years.

  29. #29
    Join Date
    Jan 2008
    Posts
    160
    Quote Originally Posted by Hornblower View Post
    It shows virtually no rotation of Africa in the past 100 million years.

    Interesting; my understanding was that there was a clockwise rotation that was closing the straits of Gibralter while opening up the Red Sea, Great Rift Valley etc

  30. #30
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    4,930
    Quote Originally Posted by KLIK View Post
    Interesting; my understanding was that there was a clockwise rotation that was closing the straits of Gibralter while opening up the Red Sea, Great Rift Valley etc
    Here is a nice one which shows the continents at selected times.

    http://www.scotese.com/earth.htm


    Progressing from late Jurassic to the present, we can see Africa drifting slowly northward, in good agreement with the rate cited in the OP. There is a slight counterclockwise rotation over the period, a few degrees in 100 million years.

    To change the azimuth by 4 arcminutes in 4,500 years, we would need to have Africa spinning like a top by comparison, about one full rotation in about 24 million years. Clearly not the case here.

Similar Threads

  1. New Images Indicate Tectonic Activity on Rhea
    By Fraser in forum Universe Today
    Replies: 0
    Last Post: 2010-Dec-21, 06:00 PM
  2. Tectonic plate thought
    By ajkkja in forum Science and Technology
    Replies: 1
    Last Post: 2010-Mar-11, 08:01 PM
  3. Tectonic plates on Earth vs other planets
    By sourkraut88 in forum Space/Astronomy Questions and Answers
    Replies: 3
    Last Post: 2009-Jun-02, 04:44 PM
  4. Dione's Tectonic Faults
    By Fraser in forum Universe Today
    Replies: 0
    Last Post: 2006-Feb-01, 05:07 AM
  5. Bad planetary science with plate tectonic evidence
    By beskeptical in forum Astronomy
    Replies: 13
    Last Post: 2003-Mar-18, 03:14 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
here
The forum is sponsored in-part by: