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PoppersOfCorn t1_iy6fu6s wrote

Reply to comment by Any_Palpitation_3110 in what would be different if we had two moons by Any_Palpitation_3110

The length of our day, depending on mass and its orbit

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Any_Palpitation_3110 OP t1_iy6nln8 wrote

So would more mass from our moon and/or moons inherently make earth spin slower or faster? Or is it way more complicated than that

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PoppersOfCorn t1_iy6ql60 wrote

The moon caused a lot of drag on our rotation and massively slowed us. It is reckoned the Earth used to have a ln 8/10 hour that gradually slowed.

So if we had another moon and depending on what type of resonance it had as well as its mass, yes, it could have changed our day either nearly nullifying the drag from the other moon, or even causing more "drag" and slowing our day even futher.

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Radiant_Nothing_9940 t1_iy77zjj wrote

How does a moon produce “drag” I’m curious as I haven’t heard this before. I thought our earth slowed for other reasons…

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PoppersOfCorn t1_iy79h20 wrote

So basically, from the gravitational interaction between earth and the moon. The moon creates a bulge on the earth surface(tides) but this bulge isn't directly underneath the moon, so it causes torque, and the difference slows the earths rotation and also results in the moon continually moving away from earth

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Ripper209 t1_iy7dm16 wrote

How does general relativity explain this?

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JapariParkRanger t1_iy7e26z wrote

There are no major relativistic effects needed to explain tidal forces.

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Ripper209 t1_iy7ekfy wrote

Can you please elaborate?

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JapariParkRanger t1_iy7f6e1 wrote

General Relativity has little to do with tidal forces.

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Ripper209 t1_iy7g210 wrote

Ngl im kinda stupid, how does the moon affect the tides then

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TwoUglyFeet t1_iy7lii5 wrote

Gravity. The moon pulls on the earth like two people holding a rope. This force is called tidal force and causes to water (and the earth) to move to side closest to the moon. These are what tides are.

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Ripper209 t1_iy7m41w wrote

In terms of general relativity, is this "rope" nonexistent? Are they just moving along curved paths caused by the mass of the moon and earth?

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TwoUglyFeet t1_iy7nlu4 wrote

The moon is in earth's gravity well. This bending is the reason we see the forces of the Earth and moon acting on each other.

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Ripper209 t1_iy7srji wrote

Thank you very much this was really helpful

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strange--alien t1_iy88lp4 wrote

If you imagine this scenario, having a tennis ball on a string and holding it above your head spinning it like a helicopter rotor. Generally speaking, the tennis hall represents a moon, rope is the gravitational tether between the two, and (with a bit of imagination) your hand are represents the earth. So, when spinning the tennis ball around, the mass of the tennis ball sets a limit on the speed of spinning. One rotation is an earth day. If you had a heavier ball like a basket ball (larger moon) the speed of rotation would be slower because the earth has to sling a heavier moon. Spinning a lighter ball like a ping pong ball (tiny moon) would be so easy that the tiny moon wouldn't have any affect on spinning speed basically. Smaller you get and the moon wouldn't be able to hold itself in the earth's gravity and would fling off in to space. To summarise, larger moon puts a brake on our earth's rotation, Smaller moon would ease pull back on our earth. Two moon's would be wack! Double the werewolves, double the astrology readings. Spooky

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Ripper209 t1_iy9mz4n wrote

This is exactly what I'm looking for thanks!

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strange--alien t1_iya1txe wrote

I could hear your unspoken question very clearly. Btw I messed up one piece. One tennis ball rotation is one moon orbit (27 days), not one day. Still same physical concept though.

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robit_lover t1_iy7rayi wrote

To add, the earth and moon produced drag on each other, and since earth is so much larger it slowed the moon down enough that it doesn't rotate relative to earth anymore.

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