Mercury rotates three times for every two revolutions around the Sun, apparently due to a gravitational resonance with the Sun.

Venus takes about 225 days to rotate, and rotates in the opposite direction of any of the inner planets. Maybe because its extreme nature makes it ornery.

Earth rotates once every 24 hours, a condition caused by the tidal interaction between Earth and its Moon. It's believed that the Earth was rotating about once every 5 hours before the theorized collision with a Mars sized coorbiting object referred to as Theia.

Mars shows no signs of a similar collision. Its two moons appear to be asteroids that were captured from the asteroid belt. So how did Mars come to have a day so close to the length of an Earth day?

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    $\begingroup$ You can't rule out pure coincidence here. In fact, that's my guess as to the reason. $\endgroup$
    – HDE 226868
    Oct 25, 2015 at 21:51
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    $\begingroup$ Mars does have signs of a huge impact, Borealis basin. But not clear that it has anything to do with the rotation period. I think it is comforting that some planet has something in common with Earth. The giant planets aren't far off either. $\endgroup$
    – LocalFluff
    Oct 26, 2015 at 0:27
  • $\begingroup$ Interesting question. A good answer would talk to the issue of what we know of how Mars day has changed over time, and the known or supposed causes behind any such changes. If it has not changed much, do we have any understanding as to why it might have formed with a day of around this length? $\endgroup$
    – Keith
    Oct 26, 2015 at 2:58
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    $\begingroup$ @Keith , it takes an enormous amount of energy to change a planet's rotation by any measurable amount or it takes a consistant tidal force to change it very slowly over time. Venus has a very thick atmosphere and a fairly strong tidal drag due to it's closeness to the sun. The Earth has oceans and is slowly being slowed down by the moon (mentioned in answer below). Mars with no oceans, almost no atmosphere, no moons of any size and it's fairly far from the sun has minimal tidal effects on rotation. The only method to significantly change Mars' rotation is large impact. $\endgroup$
    – userLTK
    Oct 26, 2015 at 3:39
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    $\begingroup$ @Michael: Switft's speculation may not have been all that uncanny. en.wikipedia.org/wiki/Moons_of_Mars#Early_speculation $\endgroup$ Oct 26, 2015 at 19:53

1 Answer 1


"It's believed that the Earth was rotating about once every 5 hours before the theorized collision with a Mars sized coorbiting object referred to as Theia."

Almost. Theia did not have to be co-orbiting, just an intersecting orbit. We have no idea what the Earth's spin was before the collision, but it is theorized that the Earth rotation had a 5 hour period after the collision with Theia, at the time of the Moon's formation from the debris.

The fact that Mars and Earth have such a similar period is a coincidence, perhaps you are asking why Mars is spinning so fast? Well actually Mars is not the odd man out, Mercury and Venus are. Most planets spin fast. exactly which spin orientation is somewhat arbitrarily determined by the vagaries of the ways the planetesimals collided to form them. The fact that Venus and Uranus have unusual spin orientations is just the way things turned out.

Both Mercury and Venus used to spin much faster. Mercury's spin was tidally slowed down by the Sun and Mercury's orbit was (and still is being) driven further away by the Sun (just like the Moon and Earth: Why is the Moon receding from the Earth due to tides? Is this typical for other moons?). Eventually Mercury was held in that 2:3 resonance. Which, by the way had a certain amount of luck involved (see: Mercury’s capture into the 3:2 spin-orbit resonance as a result of its chaotic dynamics ). Venus, we are not so sure of.

The tidal force from the sun is much much less for Venus than for Mercury, but much more than for Earth. However Venus has a dense hot massive atmosphere, which can be forced into both gravitational bi-modal (two peaks) tides and thermal uni-modal (one peak) tides. The bulge lags behind the tidal forcing peak, which creates a torque by the sun to slow it down. This is fiendishly complex (See: Long term evolution of the spin of Venus - I. Theory )

P.S. Actually Phobos, and probably Deimos, are thought to be constructed fairly recently (millions of years) from debris from a collision of Mars with a large asteroid. There is no way to capture a whole asteroid into orbits that close.

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    $\begingroup$ It's the "fiendish complexity" that makes a site like Astronomy Beta valuable. $\endgroup$ Oct 25, 2015 at 22:24
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    $\begingroup$ How is the sun driving Mercury away? Wouldn't it be drawing Mercury closer due to gravity? Or do you simply mean it is forcing Mercury into a more elliptical orbit? $\endgroup$
    – TylerH
    Oct 26, 2015 at 15:36
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    $\begingroup$ @TylerH it is the same type of tidal driving that is forcing the moon further out from the Earth. see astronomy.stackexchange.com/questions/420/… $\endgroup$
    – Eubie Drew
    Oct 26, 2015 at 16:53
  • $\begingroup$ @Aabaakawad I think your answer answers my questions as best as it can be answered. Thank you. $\endgroup$ Oct 26, 2015 at 20:08

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