# Relationship between Mars and Earth rotation

Is it by pure random chance that Mars and the Earth have nearly the same day duration (Mars day is barely 40 minutes longer, which is just 3% difference), or there is some causal relationship between the two?

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A good question and one that I deliberately avoided! :-) As far as I know there are no observations to tell how the length of the day on Mars has changed. However the main reason the day length changes on Earth is the tidal force from the Moon and the moons of Mars are too small to raise any significant tides. Tidal forces from the Sun will have some effect, but these are a lot lower on Mars, partly because they vary as $r^{-3}$ and partly because Mars has no oceans and much less energy is dissipated in tidal movements on land. So ... –  John Rennie Jan 25 '13 at 16:21