Precession of Equinoxes
Bug #1090215 reported by
Alexander Wolf
This bug affects 2 people
| Affects | Status | Importance | Assigned to | Milestone | |
|---|---|---|---|---|---|
| Stellarium |
Won't Fix
|
Medium
|
Unassigned | ||
Bug Description
The below message was sent to Fabien Chereau via his blog, regarding an error in Stellarium. Please help! Thanks.
Hello! I have found a discrepancy in Stellarium's representation of the Precession of Equinoxes. The Sun should be in the location in 25,722 years (or 25,920 if you go by older standards) from now. It is not -- it takes 26,500 years for that to manifest. Starry Night Pro has the same issue, although theirs is off by 4000+ (!!) years. My email address is <email address hidden>, and I am the head of the Heaven and Earth blogspot. Please get back to me through either of those channels if you can; I found your blog here through the Stellarium website and was in need of someone to contact. Thanks for your help.
| Changed in stellarium: | |
| importance: | Undecided → Medium |
Revision history for this message
| Ariane Stevens (sjstar) wrote | #1 |
| Changed in stellarium: | |
| status: | New → Won't Fix |
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There is no error here. Accuracy for most precession calculations is only between 4000BC and 8000AD, beyond that is uncertain.
The general formulae for precession of the equinoxes is subjected to long term errors as the behaviour of the motion is uncertain. One of the points often missed by most is that one cycle of 26,500 years does not return to the same place in the sky. Physically, this is caused by the torque (Q) of the Earth's rotation, and is not easy to determine its exact influence. (It is complicated to derive!)
Ultimately, this is because of the 'obliquity of the ecliptic' /epsilon/ as the Earth's axis is not fixed but can vary between 22 and 25 degrees. Its period is some 41,000 years. I.e. in 14000AD, the tilt is predicted as 22deg 38' 01.9". In 8000BC, it was 24 deg 13' 42,1". In 19466AD it will be exactly 23 degrees.
There should be roughly about 2 or 3 degrees difference in declination (+ve for the pole in northern hemisphere -ve in the pole south hemisphere) between the pole position 26000 years hence. Motion is actually a spiral not circular. (See attached figure) Worst this difference is a different amount each 26,000 years, cycling as a combination of both the 26,000 and 41,000 year. Biggest change expected about 210,000 year hence, smallest 830,000 years hence. (as Stellarium only moves 99,999 AD, these are not relevant. You only need to be right for about eight precessional cycles.)
The motion of the Earth is very complex group of physical processed. I.e. It included nutation, varying rotational velocity, change is Earth's eccentricity orbiting the sun, etc.
Recommend you read;
1) Laskar, J., "Accurate spin axes and solar system dynamics: Climatic variations for the Earth and Mars", A&A., 416, 799 (2004)
2) Simon et.al., "Precession Formulae and Mean Elements for Moon and Planets ", A&A., 282, 663 (1994)
Regards, Ari
Note: The statement by Fabien; "The Sun should be in the location in 25,722 years (or 25,920 if you go by older standards) from now. It is not -- it takes 26,500 years for that to manifest." is essentially wrong.