# Relativistic Mercury orbit around the Sun

My question is the following,would the scalar of precession of Mercury orbit be the same if Mercury orbited Sun in the oposit direction?

• There might be the smallest variation due to the rotation of the sun pulling space along with it. sciencedaily.com/releases/1998/03/980330073701.htm Too small to be observed perhaps, but there might be the tiniest of differences. Commented Aug 10, 2019 at 5:34

## 1 Answer

General relativity is only one cause of the precession of Mercury's orbit, and not the largest factor. Gravitational perturbation by the other planets and the non-spherical sun also cause precession. Newtonian gravity predicts precession of 532 arcseconds per century, but 575 arcseconds is observed. The discrepancy can be explained if one uses General Relativity as the model of gravity.

The extra precession of 43 arcseconds caused by GR would be the same, the perturbations due to the other planets would not be the same, unless they were also orbiting the sun in the opposite direction.

• While the answer is correct, I think there must be a missing decimal point in the precession rates. The Messenger results from 2017 show a measured total precession rate for Mercury of 575.31"/cy, with the largest contribuitons due to perturbations from Venus, Jupiter and Earth/Moon. The GR contribution is 43"/cy, as given in your answer. You should check your source for the precession data. Commented Aug 9, 2019 at 23:50
• This question and its answers likely explain the ~10X difference in values for perihelion precession of Mercury noted in my previous comment. Apparently some older sources added a contribution from precession of the Earth's axis, now called general precession in longitude. I don't know why change in Earth's axis was included in Mercury's orbital precession. These measurements were made from Earth, but corrections for general precession are well known, and unrelated to Mercury's orbit. Commented Aug 10, 2019 at 20:04
• I don't know much about it, but wouldn't GR frame dragging result in a tiny difference between prograde and retrograde precession?
– uhoh
Commented Aug 11, 2019 at 5:58
• Probably, but frame dragging is not the cause of the discrepancy Commented Aug 11, 2019 at 6:15
• Is space dragging the same as space curvature....it seems it is not?Dragging seems something like action-reaction Newton law. Commented Aug 11, 2019 at 12:23