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Thread: Question on the north facing of the Giza pyramid

  1. #1
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    Question on the north facing of the Giza pyramid

    At a website far far away a comment abou the Giza pyramid

    The following astronomical math has popped up and I'm not in the know about this so any comments on its validity would be appreciated!

    .....In my wanderings and studies I chanced upon studying the tilt of the Earth's axis and related topics and then I noted a line at Giza which I thought should go 45 degrees and then it hit me. Why do the pyramids face North ?

    That is to say why do they hit North of the present day ?

    They were built a minimum of 4500 years ago? ... Should they not hit North of that era ?

    Using calculations that translates to the shift:

    41000 = 1.5 degrees (24 - 22.5 agreed maximum and minimum tilt)

    This is a cycle so divide by 2

    1 yr = 1.5 / 20,500

    4500 yrs = 4500 x (1.5 / 20,500)

    4500 yrs = 0.08 degree = 28.8 minutes ( I think my math is correct)

    However apparently the pyramids are only 5 minutes off due North and even this Petrie writes is very noticeable.

    Why is the deviation not 28.8 minutes ?

    ............................

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    Quote Originally Posted by Hans View Post
    .....In my wanderings and studies I chanced upon studying the tilt of the Earth's axis and related topics and then I noted a line at Giza which I thought should go 45 degrees and then it hit me. Why do the pyramids face North ?

    That is to say why do they hit North of the present day ?

    They were built a minimum of 4500 years ago? ... Should they not hit North of that era ?
    If you define "North" as "pointing towards the spot where the axis of Earth's rotation touches the surface of the Earth", then the direction "North" will not change appreciably on timescales of a few thousand years. The axis of rotation doesn't move around relative to the crust of the Earth very much at all.

    Perhaps you have in mind some other means to define "North," such as "the location of a particular star in the sky," or "the projection of the Earth's axis onto the celestial sphere," or "the intersection of the Earth's magnetic field with the surface of the Earth." Those directions will change over millenia.

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    North 4500 years ago was the same north as today, because the rotation pole was at effectively the same point on the surface of the Earth 4500 years ago as it is today.
    The Earth's north pole points in a different direction relative to the stars, as you say, but that doesn't affect the geometrical relationship of points on the Earth's surface.

    Grant Hutchison

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    Quote Originally Posted by Hans View Post
    At a website far far away a comment abou the Giza pyramid

    The following astronomical math has popped up and I'm not in the know about this so any comments on its validity would be appreciated!
    I'll give it a shot.

    .....In my wanderings and studies I chanced upon studying the tilt of the Earth's axis and related topics and then I noted a line at Giza which I thought should go 45 degrees and then it hit me. Why do the pyramids face North ?

    That is to say why do they hit North of the present day ?

    They were built a minimum of 4500 years ago? ... Should they not hit North of that era ?
    The North of today and the North of 4500 years ago are the same. The Magnetic North Pole and the Celestial North Pole change with time, but the Geographical North Pole does not.

    Using calculations that translates to the shift:

    41000 = 1.5 degrees (24 - 22.5 agreed maximum and minimum tilt)
    I assume 41,000 is supposed to be the number of years in the cycle of the Earth's axial tilt. A quick look at Wikipedia suggests that the numbers should actually be 24.5 - 21.5, which gives a difference of 3, not 1.5.

    This is a cycle so divide by 2

    1 yr = 1.5 / 20,500

    4500 yrs = 4500 x (1.5 / 20,500)

    4500 yrs = 0.08 degree = 28.8 minutes ( I think my math is correct)
    Using 3 instead of 1.5, I get 39.5 minutes.

    However apparently the pyramids are only 5 minutes off due North and even this Petrie writes is very noticeable.

    Why is the deviation not 28.8 minutes ?

    ............................
    It's not 28.8, nor is it 39.5, because there's no reason for it to be. The real question is "Why is the deviation not 0?" It would have been easy enough for the Egyptians to find geographical north, and that doesn't change with time. If the pyramids are only off by 5 minutes, then that's probably due simply to error in measurement of where north is.

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    Quote Originally Posted by grant hutchison View Post
    North 4500 years ago was the same north as today, because the rotation pole was at effectively the same point on the surface of the Earth 4500 years ago as it is today.
    The Earth's north pole points in a different direction relative to the stars, as you say, but that doesn't affect the geometrical relationship of points on the Earth's surface.

    Grant Hutchison
    No, Grant. The Earth wobbles, and the periodicity is approximately 13,000 years. So it's no wonder that the pyramids are 5 degrees off today compared to when they were built.

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    Quote Originally Posted by mugaliens View Post
    No, Grant. The Earth wobbles, and the periodicity is approximately 13,000 years. So it's no wonder that the pyramids are 5 degrees off today compared to when they were built.
    You might just want to reconsider that one, thinking about the difference between the geographical north pole and the pole star.

    Grant Hutchison

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    Yeah, this is a subject of much confusion, it seems, since people bring it up way more often than they need to.

    We've defined 3 different "North Poles". The Geographical North Pole is at latitude +90 degrees. For all intents and purposes, it doesn't move. The Magnetic North Pole is where a compass points to, and it moves rather quickly. The Celestial North Pole is the projection of the Earth's rotational axis on the celestial sphere. It's the rotational axis that precesses (wobbles), so it's the Celestial pole that changes. The Geographic pole is always in the same place on the surface of the Earth, but it points at a different place on the celestial sphere at different times.

    And the quoted article, or blog post, or whatever it is claims the pyramids are off by 5 minutes, not 5 degrees. That's 0.08 degrees, and is easily small enough to have been simple error.

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    Quote Originally Posted by Kristophe View Post
    Yeah, this is a subject of much confusion, it seems, since people bring it up way more often than they need to.
    Laymans view...
    Picture a top (or gyroscope) that is spinning with a slight wobble.

    From any point on the top, the axle will always be in the same location no matter how it is spinning (or even stopped). That is geographical north.

    The wobble is defined by where the top's axle is pointing...Celestial North

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    Quote Originally Posted by Kristophe View Post
    Yeah, this is a subject of much confusion, it seems, since people bring it up way more often than they need to.

    We've defined 3 different "North Poles". The Geographical North Pole is at latitude +90 degrees. For all intents and purposes, it doesn't move. The Magnetic North Pole is where a compass points to, and it moves rather quickly. The Celestial North Pole is the projection of the Earth's rotational axis on the celestial sphere. It's the rotational axis that precesses (wobbles), so it's the Celestial pole that changes. The Geographic pole is always in the same place on the surface of the Earth, but it points at a different place on the celestial sphere at different times.
    The geographic north pole is not coincident with the rotational north pole. which moves about (geographically) with 12- and 14- month wobbles as well as secular wanderings.

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    Quote Originally Posted by hhEb09'1 View Post
    The geographic north pole is not coincident with the rotational north pole. which moves about (geographically) with 12- and 14- month wobbles as well as secular wanderings.
    The Chandler wobble is on the order of 10m (as I know you know; I'm just adding detail for any others who might not ), and the true polar wander looks to have been about the same distance in the last century. That'd give us about half a kilometre in the last 4,500 years, I suppose, unless the movement was faster in the past.

    The reported error in the pyramid alignment would correspond to a polar displacement of at least 8km, if we assume the Egyptians aligned perfectly on a displaced rotation pole, rather than merely got things very slightly wrong (or impressively right, I think, given the technology).

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    You might just want to reconsider that one, thinking about the difference between the geographical north pole and the pole star.

    Grant Hutchison
    Why thank you, Grant. I have thought about it, and I've decided that our fluid Earth does indeed wobble, both from external forces as well as internal forces (precession and nutation), and the geographical North pole actually changes location over time.

    The most famous movement of the geographic North pole is known as the Chandler wobble where the geographical north pole wanders in a rough, 3 m to 15 m circle about once every 433 days. This wobble is driven by both climactic changes between the Northern and Southern hemispheres, as well as by changing ocean currents. Other forces such as tidal braking cause additional instabilities which can cause a planet's geophysical North pole to change location over time.

    In addition to precession, there's also nutation, the largest component of which has a periodicity of 18.6 years. Both precession and nutation must be accounted for while conducting celestial navigation, and both factors are available in the almanacs used by navigators.

    However, more pronounced is the effect to tectonic shifts, where entire continents not only translate, but also rotate over time. I suspect that plays a far greater role in the 5 deg difference for the Giza pyramids than anything else.

    It would be interesting to measure how much various ancient observatories aligned to True North built at different times vary from the True North of today.

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    Wow, all this detail to wade through and it would seem that maybe the question was missed (???) Why are the pyramids only off of true north by 5 arcminutes despite all the movement around of the earth's polar direction?

    It's because except for construction errors and continental drift (and perhaps something to do with an actual shift of where the pole is located), the darned pyramids are stuck on north, just like the position of the pole itself. When the earth precesses around and wobbles all over the place - where ever it's pointing is north and unless the plates shuffle around, that pyramid is fixed to the surface. That's why it hasn't shifted around by degrees or substantial fractions of degrees as the earth precesses and wobbles around.

    LOL

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    Quote Originally Posted by grant hutchison View Post
    (or impressively right, I think, given the technology).
    I tend to agree with that point of view, but I'm open to demonstrations of incredible accuracy
    Quote Originally Posted by cbacba View Post
    Wow, all this detail to wade through and it would seem that maybe the question was missed (???) Why are the pyramids only off of true north by 5 arcminutes despite all the movement around of the earth's polar direction?
    ? I think the question of the OP was addressed (and answered) in each of the first three responses, in this thread.

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    Thank you gentlemen for the expert answers, much appreciated

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    Quote Originally Posted by hhEb09'1 View Post
    I tend to agree with that point of view, but I'm open to demonstrations of incredible accuracy.
    Me too.
    I'd also be interested in the confidence limits for the modern-day measurement. The original covering of the pyramids has been largely stripped off, so one wonders how, from the remaining structural blocks, we can measure the orientation of the original structure with arc-minute accuracy.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    I'd also be interested in the confidence limits for the modern-day measurement. The original covering of the pyramids has been largely stripped off, so one wonders how, from the remaining structural blocks, we can measure the orientation of the original structure with arc-minute accuracy.
    Just to illustrate what I'm worried about: here is a view of the base of the Khufu pyramid, with some people and camels to provide scale. The original facing would have covered all the visible blocks. I do wonder how it is possible to judge the original alignment of the pyramid with any great angular precision, considering what's left to measure from.

    Grant Hutchison

    Original context of photo

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    I would sight along many rows of blocks on all four faces and average the results. That should give a good overall result.

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    Quote Originally Posted by Hornblower View Post
    I would sight along many rows of blocks on all four faces and average the results. That should give a good overall result.
    Agreed, since we have samples of the coverings (they were confiscated and used for subsequent construction projects) and the samples indicate a relatively uniform thickness.

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    Quote Originally Posted by Hornblower View Post
    I would sight along many rows of blocks on all four faces and average the results. That should give a good overall result.
    Ah, neat.
    If anyone apart from me is interested, here is the original survey report from 1925. They excavated down to the surrounding pavement and found remnants of the casing blocks to give them a good line. The original corners (which are now much degraded at the surface) are also marked by deep foundations. So they didn't use the erratic line of the visible structural blocks at all.
    I feel happier now.

    Grant Hutchison

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