Delamoor t1_j3ud8i8 wrote
Reply to comment by curiousmind111 in TIL Pluto hasn't completed an orbit around the sun since its discovery. Pluto's orbit takes about 248 years, and Pluto was discovered in 1930. by irbinator
Long answer that I'll try to make short, but basically... Because tidal forces.
The tide on earth comes from the spinning, and the gravity from the moon interacting with the earth's spin. The ocean's water sloshes about relatively easily, but the tidal forces (the gravity) is actually affecting everything, all the crust and mantle. The entire planet actually flexes a tiny, tiny bit with each spin.
That flexing carries a cost; energy has to come from somewhere and it has to go somewhere. So the flexing turns into thermal energy; heat. Only very slight on earth, but still there.
Jupiter's moon Io is a giant pile of volcanoes because of this effect. It's super close to Jupiter and still spins pretty quickly and so flexes a huge amount, generates a lot of heat, so lots of magma flying about.
But that heat radiates into space slowly. Which over billions of years means the spinning planet loses energy. Which means it spins slower and slower.
Eventually it stops spinning and will always have the same face pointing towards the bigger object. Just like our moon does now. Eventually, far far in the future, the earth will also stop spinning (because of the tidal forces) and one side will always face the moon. That's why we believe the days were much shorter when the planet was still newly formed; it spun faster, but has lost roughly half it's spinning speed over the last 4.5 billion years. Because of the moon.
That has happened with a lot of moons in the solar system (because moons are small and don't have store much energy) and it can happen with planets too. Usually, the closer the little thing is to the big thing (e.g. moon near planet, or planet near star) the faster the energy comes out of it, the sooner it stops spinning.
And the thing that causes that to happen... Tidal forces. Same forces that create the tides on Earth. Thus; locked into place by tidal forces; tidally locked.
We're just fortunate that we have the oceans which slosh about so easily and obviously, otherwise it would have taken us a lot longer to figure it out.
curiousmind111 t1_j3vd1my wrote
Interesting.
But what if we had no moon, and we didn’t spin - from the start. We just always had the same side facing the sun. That would have nothing to do with moons or tides. That’s why I was surprised to hear “tidal” in the comment. But. Googled and that’s what they call it.
Even stranger when there’s no water on the planet.
Delamoor t1_j3vswa0 wrote
Interestingly, that wouldn't be physically possible.
The process of accretion in zero gee creates angular momentum by its nature: stuff slams into other stuff and that kinetic energy has to go somewhere, and it can't go any further towards or back away from the centre of gravity... so it goes sideways and turns into a spin.
That's why neutron stars generally rotate near the speed of light; all that matter came inwards with the force of a supernova, so supercharged the spin. Also why black holes spin and accretion disk form. You basically can't have a body of matter coalesce in space without it starting to spin at least a bit. Well, unless you're physically there to carefully place the matter bit by bit with near zero kinetic force, anyway.
If we had no moon then there would be no signficant drag (though there would still be a tiny, tiny, tiny bit from the sun and other bodies in the solar system, but barely noticeable even in astronomical timescales), and we would for all intent and purposes basically never stop or slow our spinning.
curiousmind111 t1_j3yhf9l wrote
Nteresting. Even if we had water but no moon? I’m imaging that water adding Damone drag as we rotate. thanks for the Excellent answer!
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