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If the earth stopped rotating on its axis, would this influence its revolution motion? In particular, could it collapse on the sun? I ask this because on the one hand I thought the two degrees of freedom (rotation around the inner axis and revolution around the sun) were separated, on the other phenomena such as tidal locking seem to show a clear influence of one on the other.

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    $\begingroup$ Fun fact: Venus essentially doesn't rotate about its axis. Hasn't fallen into the Sun yet ;) $\endgroup$ – user10851 Mar 6 '16 at 23:19
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The coupling between rotation and revolution, that leads to tidal locking, is usually very weak so it takes a very long time. So in the short run, you could give the Earth any rotation you like, without affecting its orbit. However, in the long run, there would be orbital effects. Right now, the Earth is spinning more rapidly than its revolution rate, so angular momentum is being stolen very slowly from the Earth's spin, and put into its orbit. But it wouldn't matter much, there's not much angular momentum in the spin compared to the orbit.

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Simple answer, No. A planet stays in its orbit due to its orbital velocity and not due to its rotation. Tidal forces might play a part in determining the orbit of a planet but not nearly enough to make it collapse into the star.

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Rotating around itself is one thing and rotating around another object is something else. It is perfectly fine for the Earth to not rotate around itself while keep rotating around Sun. It is the "revolution" around Sun that keeps Earth not to fall into Sun. The answer in short: The condition mentioned would not influence the gravitational binding between Earth and Sun. Thanks,

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    $\begingroup$ hum, so your first words should be "No dear". Then what you say about the life conditions is more than contraversial : life most probably started in the depths of oceans, far from Sun light, atmosphere, and saisonnal influences. Beside there is a third possibility not mentionned by the OP: having an axe not parallel to the revolution axe, e.g., within the plane of ecliptic. $\endgroup$ – Fabrice NEYRET Mar 6 '16 at 21:58
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    $\begingroup$ The moon most assuredly does rotate on it's own axis; once every sidereal month. If it did not we would see all sides over the course of a month. $\endgroup$ – dmckee --- ex-moderator kitten Mar 7 '16 at 2:20
  • $\begingroup$ We always observe the same face of the moon because it is rotating with Earth rather than around itself. $\endgroup$ – Benjamin Mar 7 '16 at 2:40
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    $\begingroup$ @Benjamin - No, the moon has a spin (or rotation) axis about which the it turns as a rigid body. It happens to be experiencing tidal locking (or synchronous rotation) with Earth. I think you are confusing rotation with orbital motion or something... $\endgroup$ – honeste_vivere Mar 7 '16 at 17:37
  • $\begingroup$ Thanks dear honeste for the clarification. The analogy I had made was not a good one and I edited my answer to reflect the main concern of the question. $\endgroup$ – Benjamin Mar 7 '16 at 19:31

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