– If all motion is relative, how can we assign speeds to celestial objects?

Question

What are we comparing the speeds of far away objects to in order to come up with a speed? Is there a point at which we reach the single thing all motion is compared to? Is that thing light?

In: 1

Answer 1

It depends. It might be the earth (for stuff in orbit around the earth) or the sun (for stuff in orbit around the sun) or the center of our galaxy or the cosmic background radiation. Could also be some other star (for stuff in its orbit), or a host of other things.