How far is a light-year?

04 December 2018

Question

Would aliens looking at the Earth from 100 million light years away, see the Earth as it was 100 million years ago?

Answer

Chris Smith put Emily's question to astronomer Carolin Crawford...

Carolin - Even though it sounds like it ought to be a measure of time, a light year is a measure of distance. So it's how far light travels in one year, and we all know that light has a finite speed: it’s the fastest thing there is. It travels at 300 thousand kilometers per second but that means in a year it travels nine and a half million million kilometers. And astronomers kind of get fed up saying millions and millions and millions! So we just stick to light years. So therefore a light year is telling you a distance that it takes light to travel in one year.

So getting back to your question, you've got your aliens or whatever a hundred million light years away. The light that they receive in their telescope that’s come from the earth, has taken 100 million years to get to them. So they are just seeing the light that left Earth a hundred million years ago, so they're seeing the Earth as it was a hundred million years ago. So you're exactly right about that. And this is such a neat thing for astronomers though, because it does mean that we can effectively look back in the past because we can compare a galaxy that's, you know, 100 million light years away to a galaxy that 6 billion light years away and we're seeing the universe at different epochs so it does work in our favour at times.

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