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


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?

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4 Answers

Anonymous 0 Comments

When we measure the speed of far away objects, it’s in relation to ourselves.

The speed of light is constant, regardless of how fast you’re moving relative to some other object. If I was moving at half the speed of light relative to you, and shined a light at you, both of us would still see the light moving at C, but its color would change.

Anonymous 0 Comments

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.

Anonymous 0 Comments

All measurement in determined by deciding something to be standing still. 80mph is compared to the road being still, but the earth is rotating compared to the sun, so 80mph in a car may be -17,000 mph compare to the space station. the sun moves through space at several millions’ mph.

The special about light speed is that no matter if you decide it is you who are standing still, and you measure the speed of light, or you think f the sun moving at millions of miles, the speed of light will remain 100% exactly the same.

Anonymous 0 Comments

You can pick any object that does not accelerate as a reference point. The speeds will be different, depending which one you pick, but the physics will work out anyway.

Picking light is actually a bad idea, as for light all distances and all time periods are zero, so all objects have indeterminate speed.