Eli5: how do we know how fast the earth is moving in space?

Question

How can we measure how fast the earth moves around the sun, or how fast the earth is spinning, without having some stationary point outside of the movement/rotation from which to observe and measure it?

In: Planetary Science

Answer 1

We can’t. Any speed would have to be relative to something, be it around the sun, or around the center of the galaxy, etc.