Joel Primack Quotes
You cannot measure the absolute speeds of the stars, but you can measure their relative speeds along the line of sight, because if a star is moving toward us its light is shifted to shorter wave lengths, and if it is moving away from us its light is shifted to longer wave lengths. 
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You cannot measure the absolute speeds of the stars, but you can measure their relative speeds along the line of sight, because if a star is moving toward us its light is shifted to shorter wave lengths, and if it is moving away from us its light is shifted to longer wave lengths.
- Joel Primack
