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Sundial, calendar and Khmer temples
Astro-archaeology = Astroarchaeology
Determining celestial north

A method for determining cardinal directions - proposed by Dr. Kate Spence
าพระวิหาร

     Dr. Kate Spence, an Egyptologist at Cambridge University, UK, proposed in an article in Nature, 16 November 2000, that the ancient Egyptians might have used the circumpolar stars Kochab and Mirza for aligning the Great Pyramid at Giza in Egypt straight true north-south in the middle of the 3rd millennium BC.

     Nowadays Polaris is located close to celestial north, but due to the precession of the equinoxes it was not so 4500 years ago. Kate Spence's theory is that the master planners before digging out the foundation of the pyramid at night time waited until Kochab and Mirza were located vertically above one another because they knew that e vertical line connecting the two stars would pass celestial north.

''A measurement of alignment taken with a plumb line toward the stars at this time would be oriented exactly to true north.''
''The simultaneous transit method involves making an alignment toward two stars when they are vertically aligned. The alignment can be taken when either of the stars is in the upper position. Because the upper culminations of the two stars fall approximately twelve hours apart, with the time of culmination of each star moving slowly through 24 hours in the course of a year, for about half the year the vertical alignment that falls within the hours of darkness will have one of the stars always above, and for the other half of the year the second star will be in the position of upper culmination when the two stars are vertically aligned during the course of the night.
When the chord between these two stars passes precisely through the celestial pole, an alignment taken toward the stars when they are in simultaneous transit will be exact with either star at its upper culmination.''

Kate Spence: Ancient Egyptian chronology and the astronomical orientation of pyramids

Above: The stars around celestial north in August 2467 BC.

The magnitude of Polaris, the Northern Star, is 1.90, which is a little brighter than Kochab and Mirza (2.08 and 2.27)
 

     Since Spence's article in Nature her theory has been launched by popular TV-programs as the HISTORY Channel and Discovery as if it was a generally accepted method. Also scholars working in the field of Khmer archaeology have been considering whether the method was implemented by the ancient Khmers as well. The latter is the reason why this web-page is made.
     The author see no reason why the ancient Khmer master builders should have used a method as proposed by Spence. First of all because the Khmer constructions are of a much later date. Secondarily because they most likely used the Indian Vedic Circle method.

     A third reason (which might also exclude the ancient Egyptians) is that the method is appealing on paper or in a astronomical animation program on a desk top, but difficult to implement in real life.
     One obstacle is that the alignment has to be done at night time aligning two dim stars with a plumb line on a night with absolutely no wind.
     A second obstacle is that there must be two plumb lines (not only one as mentioned by Spence) in order to obtain a foresight and a back sight (two points are needed in order to construct a line). As the altitude of Kochab was app. 40.5 the distance between the plumb lines must be relatively short or the back sight plumb line rather long (app. equivalent to the distance between the plumbs): 1. A long plumb line demands an environment with no wind. 2. Long distance between foresight and back sight gives more accurate readings.

     Proposal (experimental archaeology): Chose two stars of similar magnitude and similar altitude and test the theory in real time in Egypt.



Above
: Illustration showing the proportions of sight lines (plumb lines) needed for constructing a true N-S line when using the two stars Kochab and Mizaras foresight.
 

Right: The moon at an altitude of 8 degrees.

 


Internet sources:

Kate Spence: Ancient Egyptian chronology and the astronomical orientation of pyramids
The American Scientist: Astronomy and the Great Pyramid
Robert G. Bauval: A Brief Evaluation of Kate Spence's Article in NATURE
Magli: On the astronomical orientation of the IV dynasty Egyptian pyramids
Shaltout and Belmonte: On the orientation of ancient Egyptian Temples I: upper Egypt and lower Nubia

 

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12 January 2007 Asger Mollerup

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