Posted December 12, 2011
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1. As has been pointed out, we aren't "seeing" this light at it's source. Depending on the source of light, there are various ways of estimating it's distance. For stars that aren't too far away, we use parralax calculations. That means taking readings of the projected location from different sides of earth's orbit to "triangulate" (not actually the correct term in this case, but it'll do) the distance. For things many billions of light years, there are techniques that involve analysing how bright the light appears to us, compared with how bright it would be expected to be at source (the main way of judging this is the "cephid technique").
2. Also as has been pointed out, the light we are seeing was generated a long time before we see it. For all we know, the light source could no longer be there (got eaten by aliens or whatever).
3. As has also been pointed out, the universe is contantly expanding at what seems to be an expidential rate. That means (as a very simplistic example) it could have only been 5 lightyears away when it generated what we are seeing now, but we can work out that by this stage it would be 10 lightyears away.
If that doesn't answer it, then it seems I've misunderstood the question :(