Concepts explained
Why Is the Night Sky Dark?
If stars were endless, every line of sight would end on one and the night sky would blaze. It is black instead — and that darkness is evidence that the universe has an age.
But bring the stars in and it turns strange. If stars were endless in number, the universe endless in extent and endless in age, then whichever way you turned your eyes there would be a star at the end of that line. Point anywhere and you must eventually hit one.
In that case the night sky should be blinding with starlight. Brighter than day.
It is black instead. People puzzled over that mismatch for a long time.
The other half is that the universe does not sit still. It is expanding, so the most distant light is stretched on the way until it is no longer light we can see. That comes in the tenth piece.
A dark night sky does not merely mean the Sun is elsewhere. It is evidence that the universe is not infinitely old. We look straight at that evidence every night and think only that it is dark.
The question that remainsIf the dark of the night sky is the space held by light not yet arrived, is that darkness an absence or a not-yet?