Precession of Earth's axis and alignment with stars

  • Thread starter Hornbein
  • Start date
  • #1
663
312

Main Question or Discussion Point

I picked up a book about the pyramids at Cheops. (Relax, it wasn't a crackpot work.) It said that the pyramid was aligned with the star Polaris, and that Sirus and Orion may be sighted through some of the ventilation shafts.

Now the pyramid is aligned with true north. It the time of the construction of the pyramid construction, Polaris was not located at true north. This is due to the precession of the Earth's axis, which is on a cycle of about 25,500 years in period. No big deal: it is easy to find true north from the apparent rotation of the stars.

However it seems to me that it is rather unlikely that Sirius would have aligned with that ventilation shaft at the time of the pyramids construction. Am I right, or is there some invariant that I'm missing? It isn't very satisfying to chalk the alignment up to "chance," but I can't come up with a better explanation.
 

Answers and Replies

  • #2
Bandersnatch
Science Advisor
2,911
1,815
However it seems to me that it is rather unlikely that Sirius would have aligned with that ventilation shaft at the time of the pyramids construction.
What do you mean? The whole 'star pointing shafts' hypothesis is based on the fact that they were constructed to match the stars' position.
 
  • #3
663
312
What do you mean? The whole 'star pointing shafts' hypothesis is based on the fact that they were constructed to match the stars' position.

What do you mean? The stars can be seen at some point in the night by sighting along the shaft. It's an observation to be explained, not a hypothesis.

Your post is insulting in its carelessness. If you can't be bothered to read what I wrote and give it some thought, maybe you had better not post.
 
  • #4
russ_watters
Mentor
19,447
5,637
Relax. The ancient Egyptians aren't around for us to ask, so by nature it will be highly speculative.

I would go further though to say that the idea of a star alignment seems pretty meaningless since the stars are only at the same place and time twice a year (same place once a day). So a shaft that points at Sirius would point at dozens of other stars at other times. Indeed, use of fixed shafts was an early way to accurately map the sky.
 
  • #5
Bandersnatch
Science Advisor
2,911
1,815
I'm not sure what you've found insulting about my post.
I acted surprised because the observation you speak of (of stars shining through, or rather aligned with, the shafts) is no longer there. The four shafts of the Khufu's pyramid are aligned with four stars culminating on the sky around the time of Khufu's reign, not now. At least that's what I've ever read or heard about it.

I'm hardly an egyptologist, but here's a quote from one (Miroslav Verner from his 'Pyramids'):
The fact that the shafts are "astronomically aligned" corresponds to the logic of the structure and to its builders' practical conceptions and religious ideas. The ancient Egyptians were probably well aware that the dominant wind was from the north, as they commonly made use of it in sailing on the Nile. Thus there was nothing unusual about the alignment of the shafts with a given star in the northern and southern skies; it was thoroughly practical. (Verner's note: A simulated reconstruction of the sky over Giza in 2,500 B.C.E. shows that the north shaft in the King's Chamber was aimed at Alpha Draconis (Thuban) and the south shaft at Zeta Orionis (Alnitak); in the Queen's Chamber the north shaft was aimed at Beta Ursae Minoris (Kochab) and the south shaft at Sirius (Alpha Canum Majoris)
Bolding mine.
Taken from this site:
http://www.kolumbus.fi/chris.tedder/OKAD/HistoryofShafts.htm
(apologies for the source not being up to PF's standards, but the field is alien and somewhat esoteric, and it's the best I could find; at least it does cite its sources)

I referred to it as 'hypothesis' because the purpose and astronomical relevance of the shafts doesn't appear to be unanimously agreed upon (apparently the Sun also shined through the southern shafts at certain days back then).

If you have on hand a source for the statement that the stars are still aligned, then please quote from it. I'll be happy to revise my admittedly rudimentary knowledge (though maybe I'll just calculate whether it's possible later).

In any case, precession should misalign the shafts and the particular stars over time, as you said.
 
  • #6
Bandersnatch
Science Advisor
2,911
1,815
Alright, so the southern shaft of the queen's chamber (which is by the way blocked, so nobody has ever observed any stars though them apart from maybe the Egyptian constructors) is sloped at about 38 degrees above the horizontal and points due south (angles listed here: http://www.legon.demon.co.uk/geomgp.htm or here: http://egypt.hitchins.net/the-pyramids/pyramid-myths/khufus-starshafts.html [Broken]). Sirius' declination as of today is close to - 17 degrees.
The elevation of the culmination point (the point where the star crosses the meridian - is at its highest point due south) is then 90-30-17=43 degrees (you take the declination and latitude of the observation point and deduct both from 90 degrees). This means Sirius could not be currently visible even if the shaft were unblocked.

So no, it is most definitely not an observation to be explained. You either have a dodgy source or you read that wrong.

However, due to precession, as can be calculated using this handy tool: http://www.bbastrodesigns.com/precession.html
Around 2350 B.C.E. the declination of Sirius was about - 22 degrees. This corresponds to it having culmination elevation above the horizon of 38 degrees - aligned with the shaft's direction.
 
Last edited by a moderator:

Related Threads on Precession of Earth's axis and alignment with stars

  • Last Post
Replies
3
Views
8K
  • Last Post
Replies
2
Views
7K
  • Last Post
Replies
1
Views
678
  • Last Post
Replies
4
Views
10K
  • Last Post
Replies
2
Views
4K
Replies
6
Views
22K
  • Last Post
Replies
2
Views
3K
Replies
5
Views
2K
Replies
4
Views
2K
  • Last Post
3
Replies
64
Views
18K
Top