If our current best telescopes (Hubble, Keck, etc) were in space and looked at the night side of Earth, how far could it be and still be able to pick up our city lights?
What about a really huge telescope, say a mile wide objective, that an alien civilisation might build?
♪♫victor♫♪
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not too far. and the only reason for this would be because the sun wouldnt let the telescope catch anything. the further we get harder it will be the catch the earth and even harder when the sun is in the background.
http://en.wikipedia.org/wiki/Pale_Blue_Dot
This picture was taken from Voyager 1. This picture the sun was taken zoomed out because it would damage the spacecraft. So imagine a telescope trying to catch that little dot (even darker at night) with the sun in the background.