why do venus and uranus rotate clockwise

Question posed by Jennie. If you were hovering above the north pole of the solar system, you'd see all of the planets* orbiting the sun in an anti-clockwise direction. If you took a closer look at the planets, you'd see most of them rotating in the same direction. There are two exceptions: Venus and Uranus. Uranus Uranus rotates on its side, with its poles almost in the plane of the solar system, almost like a barrel rolling around the sun, rather than spinning like a top, as the other planets do. Uranus's
orbits its equator, so that at certain times they look much like a target to us, with Uranus at the bull's eye. Venus Venus actually rotates backwards compared to the rest of the planets (except Uranus, of course). That is, if you resumed your vantage point over the solar system's north pole, you'd see that it spins in a clockwise direction, albeit very slowly. Why do most planets spin in the same direction?

The solar system was formed from a rotating cloud of gas. The direction of this rotation determined the direction of the orbits of the planets as well as their spin direction. Why are there exceptions? The early solar system was comparatively crowded, and in such a system collisions were frequent. Most of these collisions would have been between small bodies, or of smaller bodies onto larger ones. Some of them would have been violent enough to cause changes to more massive objects. Scientists currently believe that both Venus's and Uranus's odd spin properties were caused by two such collisions. The colliding objects must have been very massive and the collision violent to have caused changes to the two planets' direction of spin. Interesting info Most of the solar system's planets, moons and other objects bear the scars of early solar system bombardment. The most widely accepted argument for the involves a collision with a very massive object early in the solar system's formation.

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.

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