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This question already has an answer here:

I just wanted to know if what we can only see out-side of our planet is just the past of other objects?

If 1 Light Year is equals to 365.25 Days, then I saw an object outside our planet that is 1 Light Year away from us, does it means I just saw its past 365.25 days ago?

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marked as duplicate by pela, Sir Cumference, Donald.McLean Mar 16 '17 at 15:18

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Yes, you are correct. Since Alpha Centauri is four light years away, you see it as it was four years ago. For example, if the sun was to suddenly disappear it would take eight minutes for us to see it disappear. The reason is that light takes eight minutes to travel from the sun to us. Also, the Moon is nearly one light second away, so were are looking at as it was one second ago.

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Light year is a measure of distance. Light in vacuum travels at a (large) finite value, $3 \cdot 10^5$ kilometers per second to be almost exact. We can exploit this speed as a function of time. If an object is 5 light years away, it means light must travel at that large speed for 5 years to reach Earth from where it left.

The Sun is about 8 light minutes away. It takes light 8 minutes to reach Earth from the Sun. The sunlight you see is from 8 minutes ago, as it was when it left its source — the Sun.

If there were a telescope powerful enough to clearly see objects hundreds of millions of light years away, and if some hypothetical advanced alien race used such a telescope right now from hundreds of millions of light years away, they would see dinosaurs on Earth since that light had enough time to reach its destination. We aren't visible to them yet because our light is still traveling on its way to destination.

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