Quote:
Originally Posted by dworth
Not all planets spin at the same rate and they slow down gradually over time. Essentially, the day can be as long or as short as you want it to be for the purposes of the story.
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The most significant factor in slowing down rotation is if, like the Earth, they have a significant sized satellite like the Moon. The tides raised by the Moon are very gradually transferring angular momentum from the Earth to the Moon. Without a large satellite, rotation could only be slowed by solar tides which, other than for planets very close to their Sun, are very weak.