Quote:
Originally Posted by gregbradley
Interesting. So when we say a galaxy is say 100 million light years away it is incorrect to say the light we recorded is 100 million years old. Because the galaxy has most likely moved further away during part of that time so it may have been closer when it was emitted and the galaxy is now further away. But then we use received light to work out how far away something is so that distance as in the above example of 100 million light years would be the elapsed time and the galaxy may in fact in present time be much further away.
Greg.
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Hi Greg,
When we observe a galaxy 100 million light years away, we are observing the galaxy as it existed 100 million years ago which also includes its distance at that time.
Its current distance is 100 million light years plus how far it has moved from us in the last 100 million years.
How far the galaxy has moved is a complicated affair and depends on factors such as the geometry of the Universe and the gravitational effects of any nearby galaxies.
Galaxies in our own Local Group exhibit what is known as "peculiar motion". M31 in fact is moving towards us.
Regards
Steven