If you were 30 light-years from earth looking at earth it would look like 1990s. Light year is how far light travels in a year. Since everything we see is lightwaves being interpreted by our brain, light hits earth in 1992, reflects an image off, travels for 30 years and you see it today, if you were there to receive the image.

Or a mirror 15 light years away we could see the earth 30 years ago

If you could travel some distance from the earth, say one light year (the distance light travels in a vacuum in one earth year), and you had a powerful enough telescope, you could look at the earth and see the light that was just now arriving. The light would be one year old and thus you'd be looking one year in the past. The further you travel, the longer the light took to reach you, and the further you can look into the past.

It already works this way from the opposite perspective. When we look at stars, exoplanets, galaxies, etc. we are seeing them as they were hundreds/thousands/millions or more years ago, depending on how far away they are. They might be totally different now, but we won't know until new light arrives.

Shorter distances work too. At closest approach, Mars is 182 lightseconds from Earth. Standing on Mars and looking through a telescope at the Earth, you'd see Earth as it was about 3 minutes ago.

Imagine how crazy it would be to be traveling at light speed towards earth and see everything happening super quickly

Anything you see right now is a past event. It is just that for most human day to day activity, light travels so fast that this "past" is a very very small bit of time.

If someone stands 10 meters away from you and throws a ball, you would only see (from your perspective) the ball leaving their hand 1/30000000 of a second after they (from the thrower's perspective) throw the ball.

The further the distance, the longer the time difference between when you see it and when the event happens.

I don't think this is accurate. To see the light from the past, you'd have to travel faster than light to get ahead of the current wave front. No traveler starting on the Earth's surface could ever see it at any time previous to the date they left unless I'm very much mistaken.

It's accurate in the sense that is how it would work if you could be a light year away, but you're right, there's presently no way to get that far away. You could still go to Mars and look a couple minutes in the past. :)

The Voyagers are about a light day away, so they could see "yesterday" if they had the imaging capabilities onboard.

Not disagreeing, just clarifying. You could look arbitrarily into the earth's past from our reference frame on the planet by viewing it from any distance as long as you did not travel from the earth to get there. When I saw the word travel, that is what I was referring to. If you left from the surface, you could never see anything that happened before you left, no matter how far nor how fast you traveled. That was my point. Which, in retrospect, was not the point you were making.

It might be easier if you think in terms of hearing instead of seeing. If someone is far away from you, you can shout and it might take a second for the sound to reach them. So at this moment, they are hearing what you said one second ago.

Light is the same way, but it travels a lot faster. So if someone is very far from Earth, they will see the light that left Earth some time ago. If they are 32 light years away, they will see light that was on Earth 32 years ago in 1990.

It's important to note that even with the most powerful telescopes we could make right now, the image would be so blurry that you couldn't see the shape of the continents.

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