measuring its red shift. the gravity it exerts. contrasting it against nearby stars. observing its motion across the sky.
I gave my students this multiple choice question: The speed and general direction of motion of a galaxy relative to the Earth can be determined by

Only one answer is correct among these 4 choices:
- measuring its red shift.
- the gravity it exerts.
- contrasting it against nearby stars.
- observing its motion across the sky.

Could you tell me which one is correct?

measuring its red shift.