Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Whatever we see is basically based on the light that hits our eyes, right? When we look at the Moon we are looking at the Moon as it was couple of seconds ago, as the light takes some seconds (~2 seconds I believe) to reach us. In a way aren't we ALWAYS seeing the past as, no matter how close we are to something, we can never be at distance 0. Am I wrong in thinking about it like that?

share|improve this question
Looks like this question is related: physics.stackexchange.com/questions/29164/… –  Anixx Aug 29 '12 at 9:48

2 Answers 2

up vote 13 down vote accepted

That is technically correct. Since the speed of light is finite, it takes time for light from any object in the universe to reach us, even the Moon which is fairly close. For things on Earth, the distances involved are very small compared to the speed of light and so we effectively see thing "instantly".

The moon is about 1.2 light seconds away so we're seeing the surface of the moon as it was 1.2 seconds ago, still effectively instantly.

The sun in about 8.3 light minutes away and things get further away and more distant in time as you go out.

share|improve this answer
Though off-topic to astronomy, it's interesting to realize that our brains have evolved the ability to predict likely outcomes of what is currently happening based on the delayed image we are seeing, and that is likely what makes many optical illusions effective. –  jball Jun 20 '11 at 15:27
@jball: The light-speed delay of anything we can see that's not a celestial body is insignificant and has nothing to do with optical illusions. –  Keith Thompson Feb 21 '12 at 1:54
@Keith, I was a little unclear - I was referring to the neurological delay between seeing an event and our perception of it (I believe it's about 100ms) in my comment, the delays due to the speed of light referenced in the answer just reminded me of it. At least, that's how I interpret my 8 month old comment :) –  jball Feb 21 '12 at 20:03

You are correct. For every foot (~30 cm) away something is, you're looking back a nanosecond.

share|improve this answer
or in other words: the more far one looks into the universe, the more one looks into the past –  Andre Holzner Jun 21 '11 at 5:27
For terrestrial inputs processing lag in your brain dominates. In order to integrate everything your perception of the world lags visual input; IIRC by ~1/10 of a second. –  Dan Neely Feb 23 '12 at 14:08

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.