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The axial tilt of Mars varies greatly over time, with a tilt varying between 15 and 25° over the last million years, and varying even more over the last 5 million years. The Earth's tilt on the other hand is stabilized by the presence of a large Moon, so its tilt doesn't vary so much.

Venus and Mercury don't have any moons at all, so does their axial tilt also vary a lot over time? If not, why not? Do we have enough data to know how much their axial tilt has varied over time?

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    $\begingroup$ Interesting question! The theory or conclusion that Mars' axis changes over millions of years comes from an interpretation of detailed observations of the planet's surface from close range combined with climate models. Neither of these are available (or relevant) for Mercury and Venus, so this may be difficult to answer. $\endgroup$ – uhoh Jul 21 at 23:32
  • $\begingroup$ I'm tempted to say that the Sun's tidal force stabilizes Venus & Mercury, but it's only a guess. Maybe someone here can show the math behind that. That doesn't mean that they can't wobble, but it might mean that their wobble is considerably smaller, even without moons. $\endgroup$ – userLTK Jul 23 at 23:13
  • $\begingroup$ @userLTK Wikipedia says "most likely". $\endgroup$ – Keith McClary Sep 11 at 22:39
  • $\begingroup$ Mercury and Venus both have 'days' that are longer than their 'years'. Since they are barely spinning ... I'm wondering how that effects axial tilt changes for those planets. $\endgroup$ – Tim Campbell Sep 13 at 0:34
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There is almost no data available on the internet as such.

Though, the theory that they (Mercury and Venus) are stabilised by the Sun's Tidal Force seems very likely as they are much closer to the sun than any other planet. The Sun very easily dwarfs these planets in both size and gravitational and tidal attraction.

I found this Wikipedia page that I think might be useful : https://en.wikipedia.org/wiki/Axial_tilt

It says that it 'might' be that they are stabilised due to the Sun.

But this article I found changed my mind : http://hosting.astro.cornell.edu/~jlm/publications/2005I09374.pdf

It clearly says that Mercury's obliquity is not consistent with time. It also contains other info that makes it must-read if you are interested in this topic.

And although I couldn't find anything convincing on the subject of the obliquity of Venus, I found this one article that seemed quite interesting :

http://articles.adsabs.harvard.edu/cgi-bin/t2png?bg=%23FFFFFF&/seri/AJ.../0075/600/0000273.000&db_key=AST&bits=4&res=100&filetype=.gif

Venus' obliquity is 177.4 degrees, to be exact, according to this : http://solarviews.com/cap/misc/obliquity.htm

It also displays the approximated obliquity of other planets in the Solar System.

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    $\begingroup$ "We find that the tidal torque acting on Venus is tending to decrease the planet's obliquity to less than 90 degrees." - the paper then goes on to discuss mechanisms that can stabilise the tilt near 180°. So the paper does not support the conclusion you seem to think it does. $\endgroup$ – antispinwards Sep 12 at 13:10
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    $\begingroup$ @antispinwards, Sorry, that was a mistake I made in understanding the paper. Thank You for pointing it out. I have edited the answer. $\endgroup$ – AyushBhatt Sep 12 at 15:27

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