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
While there is no preferred reference frame for motion, rotation is straightforward. A reference frame that experiences no pseudo forces such as centrifugal force is not rotating. A better practical reference is to simply look at distant stars, for all purposes they appear stationary.
So because you have a handle on rotations, its very straightforward to count that it takes a day for Earth to do a trip around it’s own axis of rotation and a year for Earth to do a full trip around the Sun. Knowing the orbital radius of Earth is 1AU, it’s easy to calculate that Earth’s linear speed around Sun is pi*2 AU/year or about 30km/s
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