# How is distance in light-years converted to years back in time?

When I look at the Alnitak,the left most star of Orion's belt, it is 736 light years away. How could I convert this distance to an estimate of how long ago what I am seeing happened.

Would it be simply be 736 years ago?

Are there instances of when this isn't as straight forward?

• For short distances (e.g. within our galaxy, or even within our local cluster of galaxies), this is an accurate method. It becomes inaccurate once the scale exceeds the point at which gravitational attraction is outweighed by the expansion of the Universe. As an extreme example, the Universe is roughly 46 billion light years in radius but light has only travelled for 13.8 billion years. Hopefully someone with greater expertise can post an answer about exactly how far away metric expansion of the universe becomes a significant factor. – Chappo Hasn't Forgotten Monica Oct 30 '18 at 6:23

Yes, for stars in our galaxy, if a star is 736 light years away, the light took 736 years to reach here.

For very very distant objects, you need to account for the expansion of space. The light might have been travelling for 10 billion years, but in that time space has expanded and so the distance that you would find if you froze time and measured would be greater (to get the proper distance at the current time) it would be larger. This effect is not significant at distances less than about a billion light years.

• +1, but since the OP seems to be interested in calculating lookback time, I think this answer would be more helpful if you gave the general relation between distance and lookback time / age of the Universe (just my opinion :) ). – pela Oct 30 '18 at 9:36

Actually, the common standard is that whenever distances are reported in light years, they are actually lookback times and not distances at all. There is no difference in our galaxy, but at much larger distances where the metric expansion of the universe comes into play, it is almost always lookback times that are reported in the popular press, rarely comoving distances (even though "light year" is a distance, not a time). If they don't say "comoving distance," and they generally don't, they mean lookback time! Apparently a decision was made long ago that this distinction would only confuse people, so ingrained are we in the idea of a static universe. Here's a random example, at https://www.eso.org/public/usa/news/eso9304/ : " This corresponds to a look-back time of 83% of the age of the Universe, i.e. we see them as they were when the Universe was less than one-fifth as old as it is now. Assuming the age of the Universe to be 20,000 million years, the distance to this quasar would be about 16,000 million light-years." Don't ask me why they also decided to "dumb down" the numbers themselves! It's pretty awful what article writers think the public can understand.