Thursday, January 19, 2012

Do astronomers take motion into account when they date celestial bodies?

When they look at a galaxy and say it's 4 billion ly away, do they mean that 4 billion years ago it was 4 billion ly away, or it is now? I mean do they take into account the speed it's moving away? If they do take this into account, how do they know that it is still moving at that speed? And how do they measure it's acceleration? your thoughts?

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