If there’s no external force being applied to the earth, shouldn’t it be impossible for it to change its own orbit, similar to how you can’t pull yourself on a sled?

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furtherdimensions t1_jdtlxff wrote on March 27, 2023 at 2:18 AM

So earthquakes generally don't and can't change the planet's orbit (the path it takes around the sun) for basically the reason you said. But an earthquake can change it's rotation (how it spins around on its axis).

Very very simply, a shift in tectonic plates, while it doesn't change the mass of the Earth can change the distribution of mass. How fast the Earth rotates, and the degree to which it "wobbles" as it does so, much like a top, is based not just on its mass, but the distribution of that mass.

Change the distribution, change the rotational axis.

lurk876 t1_jdtv40c wrote on March 27, 2023 at 3:39 AM

As the other commenters have said, it does not change the Earth's orbit, but it can change its rotation. Think of it like a figure skater bringing her arms in to spin faster. The same thing can happen if an earthquake brings mass closer to the center of the Earth (lower elevation)

druidniam t1_jdtm2rr wrote on March 27, 2023 at 2:20 AM

It doesn't affect earths orbit, rather it affects the speed at which the earth spins by either slowing or speeding it up by minute amounts. This can affect perception of linear time as the day/night cycle changes. Normally however, these changes are small and outside of nuclear clocks, most people won't ever notice.

M8asonmiller t1_jduslo8 wrote on March 27, 2023 at 10:52 AM

Earthquakes, and tectonic activity in general, redistribute mass in the Earth's crust. This doesn't affect the Earth's orbit but it does effect the Earth's rotation (https://youtu.be/M6PuutIm5h4). The change is small enough to be irrelevant in day-to-day life, even if it is ultimately changing the length of a day.

d4rkh0rs t1_jdu4o4t wrote on March 27, 2023 at 5:21 AM

And you can move a sled, not far but if you kick out your legs or suddenly sit up or lay down you should get several inches movement.

Si_shadeofblue t1_jdudoze wrote on March 27, 2023 at 7:19 AM

But you need friction for that

d4rkh0rs t1_jdv50z2 wrote on March 27, 2023 at 12:57 PM

you need friction to stay on a sled, yes. ???

Si_shadeofblue t1_jdv8402 wrote on March 27, 2023 at 1:23 PM

What I meant is that to move the sled by kicking out your legs you need to have friction. So you are essentially pushing of the ground which means that this example is not comparable to an earthquake changing the earth's orbit.

Thats what I thought anyway. Might be wrong.

d4rkh0rs t1_jdvho9w wrote on March 27, 2023 at 2:34 PM

I thuoght the orbit thing was covered and was responding to the sled.

But they are comparable in the sense that if the rider or the earth change their shape the center of mass doesn't change but from points of view not on the center of mass things appear to change.