Idiom:
Let's call it a day (it is time to stop working on something)

Hit the book:
Why is looking out into space the same as looking back in time?
Why do people sometimes say that when we look at stars that are very far away that we are "looking back in time"?
This is because of the finite speed of light. When we look at objects that are very large distances away from us, the light that is hitting us now will have started from the object quite a long time ago, so in effect we aren't looking at what the object looks like now but what it looked like some time ago (when the light was emitted).