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Dr_Wreck t1_j26rb7b wrote

Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

> An observer in that frame finds the CMB to be equally hot in every direction. We do not, so we infer that the Sun is moving at about 370 km/s (1/800 the speed of light) with respect to the CMB rest frame.

If it isn't that way for us, how do we know it should be that way?

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Aseyhe t1_j26tg17 wrote

The temperature variation in the CMB corresponds extremely precisely to what is expected if the CMB is isotropic (the same in all directions) in some frame and we are moving with respect to that frame. If you've seen the usual pictures of CMB temperature variations, the "dipole" temperature variation due to our motion (example) is about 10 times more extreme than those temperature variations, and those nice pictures are only obtained after subtracting it off.

Put another way, we can say purely by analyzing the CMB that there is a reference frame in which it is about the same in all directions, and we are moving at 370 km/s with respect to that frame.

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