Then, as gravity made everything condense, the spin sped up, just like what happens when a spinning ice skater pulls in her arms. But planets spin at different speeds, for two reasons: First, the material joining each growing planet was moving in different ways and at different speeds. Second, each planet ended up with a different mass. Like bigger or smaller skaters, they all spin at different speeds.
Remember that flying debris coming in at all sorts of angles that got planets spinning in the first place? They rotate retrograde, east to west instead of west to east. The earth is spinning slower and slower. Tomorrow will be longer than yesterday. The solid surface of the earth, the crust, also goes up and down because of gravitational pull from other celestial objects.
That is called a solar day. Another way to measure a day is to count the amount of time it takes for a planet to completely spin around and make one full rotation. This is called a sidereal day. On Earth, a sidereal day is almost exactly 23 hours and 56 minutes.
For example, viewed from above the ecliptic an imaginary plane that corresponds to what was the protostellar disc for the solar system , Earth spins anticlockwise and this is the same direction that it orbits the sun. Venus will eventually become tidally locked to the sun, as the moon is to Earth. This means that its spin and orbital periods will become the same. Eventually Venus will show the same face to the sun at all times and a day on the planet will equal a Venusian year.
From the surface of a non-spinning planet, its sun would appear to move across the sky, but the other stars would be stationary. However, this would mean that Venus would still be spinning a tiny bit, with a period of something like million years, because our solar system orbits the centre of the Milky Way in that time.
Also, some rogue planets that have been knocked out of their solar systems and wander across the cosmos might have lost their spin due to chance interactions and collisions with other objects. The interesting part about this is how do you actually stop a planet from rotating? The way planets are actually formed, the only way planets can be formed, is in these very large spinning disks [around a developing star].
As young stars collapse [in on themselves as they form], they begin to to lose angular momentum and the dust clouds themselves are naturally spinning. It's much easy to make things spin than it is actually to have things stationary. So as the planet system forms, the disk spins faster and faster to lose angular momentum.
The material can then form and fall towards the star, and out of this collapsing disk you get these sort of lumps and bits and pieces, which become planets. These planets are intrinsically spinning from the angular momentum of the overall formation disk.
Then, as the planets evolve and as they begin to clear gaps and stuff, if they bump into other things these can either slow them down or in some cases even spin them up. It becomes a very complicated interaction.
Sort of like if you spin a whole bunch of marbles in a bucket and just let them go, they'll all start bumping into each other. That essentially is what happens in the early planetary systems. At the end of all these process, as more and more material gets swept away and the planets become more and more isolated, they're basically left with whatever spin they last had at their last interaction. That could be the intrinsic angular momentum of the disk or angular momentum that they actually gained or lost as they bumped and smashed into each other.
The moon is actually a very interesting case because this is what's called tidal locking. You can get to a certain point where, if you're spinning at a certain rate, the angular momentum of the Earth and the moon begin to interact and they can actually lock if they are the right distance apart and going at roughly at the right speed.
This is in fact what happened with the moon.
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