Astronomers find that light emitted by a particular element at one
edge of the Sun has a slightly higher frequency than light from that
element at the opposite edge.  What do these measurements tell us
about the Sun's motion?

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Perhaps the side that has a lower frequency is moving away from us,
and the side that has a higher frequency is moving away from us, and
the frequency difference is due to doppler effect. However, that may
not mean that the entire sun is spinning, because the sun has a fluid
surface. If it is really due to doppler effect, then we could only
tell that the surface is spinning.

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