Question of the Month Submitted by Michael Dole, Covina, Calif., and answered by Peter Goldreich, Lee A. DuBridge Professor of Astrophysics and Planetary Physics at Caltech.
You're undoubtedly thinking of Venus as the planet that spins east to west. In other words, if you arrived on Venus in the morning, the sun would be in the west and would set in the east. The only thing is that it would set about four Earth-months later! That's because a day on Venus lasts for 243 of our Earth-days.
Actually, you should probably add Uranus to your list of planets in retrograde (or "backward") rotation, because it is tipped more than 90 degrees. The day would be a short one, because Uranus completes a rotation on its axis every 17 hours, which is a pretty typical time for all the gas giants. The Uranian year is 84 Earth years. Over that time there are large seasonal variations at the poles as they alternately point toward and away from the sun.
As a rule, the inner planets (the solid ones) have much longer spin periods. Mercury completes three rotations every time it goes around the sun once because it is in a tidal lock with the sun, in a manner similar to the tidal lock that causes the moon to always face Earth. A day there lasts about 30 Earth-days.
Mars has the same spin period as Earth, but the angle between its spin axis and the axis of its orbital angular momentum is predicted to vary chaotically between about 11 and 44 degrees on a time scale of millions of years. This is due to the gravity of the sun and other planets. So if you go to Mars now, the sun would rise in the east southeast if you landed at a Southern California latitude during the summer. But if you wait a few million years, the planet might be so tilted that the sun would come up a few degrees north of east each morning while you were at that same latitude at the same time of year.
To get back to your question, nobody knows why the planets have the spins they have. It's plausible that the spin rates date back to the formation stage of the solar system, which began about 4.6 billion years ago and lasted about half a billion years. Because fairly big bodies were being gobbled up by the planets that we observe today, the inclinations of the axes as well as the spin rates are probably relics of these collisions.
Probably, both Venus and Uranus originally rotated from west to east, just like the other seven planets. Perhaps the collisions of other bodies with these two planets flipped them over permanently. In the case of Venus, the tidal effect of the sun's gravity also undoubtedly had a profound effect.
