# Thread: Looking into the past with a telescope

1. I know this will sound like a very childish question but ill go with it anyway.

Forgetting the logistics of it all for a moment, if there was a massive mirror on the moon for example and you looked at it with a telescope, since light takes 1.3 seconds to get to the moon and the same back, would you see 2.5 seconds into the past?

And if so, when you amplify the effect, would you see yourself 16 minutes ago in the case of the sun?

2.

3. Yes, this is basically like a light echo.

4. Originally Posted by niall
Forgetting the logistics of it all for a moment, if there was a massive mirror on the moon for example and you looked at it with a telescope, since light takes 1.3 seconds to get to the moon and the same back, would you see 2.5 seconds into the past?
1. Imagine two giant TV screens, one of them showing a clock.
2. Now, using a telescope, look via the mirror on the Moon, at the giant TV screen showing the clock.
3. Let's project the image from the telescope on the other giant TV screen.

The first giant TV shows the current time, eg. 13H 27M 22.0s
The second giant TV shows the clock, but 2.5 second behind the first clock, eg. 13H 27M 19.5s

Hence the image of the Moon we see is 1.25 second in the past. Likewise, the Sun is as we see it 8 minutes ago. If the Sun was "stolen" 7 minutes ago, we still wouldn't find out for another minute.

5. Thanks for the reply, i find this fascinating.

How far could the effect go though, if someone was on pluto with an extremely powerfull telescope , would they be seeing the 1980's?

6. Originally Posted by niall
How far could the effect go though, if someone was on pluto with an extremely powerfull telescope , would they be seeing the 1980's?
Well, we can go back a long way. We do not have to rely on light taking a round trip between a TV screen on Earth, and a mirror on a planet. We could just use natural light emitted by an object, such as a star. The only difference is that light that takes a round trip, takes twice as long to reach us, as light from a star.

The all we have to know, is how far away the star is. We've already mentioned that out Sun is eight light minutes away, so the light we from it is 8 minutes old. Our nearest star is proxima centauri, which is four light years away, which means the light we see from it, is four years old. If proxima centauri blew up 3 years ago, it would still be another year before we see the light from the explosion, an event which will then appear to have happened four years ago.

7. There are even exciting scientific applications for this. Imagine the light of supernovae being reflected by interstellar clouds. The light travel time to us is longer with this detour than the direct light path from the supernova. In fact, scientists investigate these light reflections to characterise the nature of long passed supernova events to produce spectra, almost as if the supernova would just be seen exploding. This has been done with the famous Tycho supernova (SN 1572 - Wikipedia, the free encyclopedia) and Cassiopeia A (Cassiopeia A - Wikipedia, the free encyclopedia).

8. Originally Posted by niall
Thanks for the reply, i find this fascinating.

How far could the effect go though, if someone was on pluto with an extremely powerfull telescope , would they be seeing the 1980's?
On Pluto "right now", they would be seeing the Earth as it was something over 4 hours ago.

9. Yep, absolutely.

And it goes on forever. If we discovered a mirror perfectly angled back at us in the Virgo Cluster.....and had a high enough resoloution telescope, we could take a peek at real life Dinosaurs

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