Blue hour is the short spell after sunset and before sunrise when the sun is just below the horizon and the sky turns a deep, even blue.

The sun is down, so nothing is lit directly; the sky is lit by sunlight scattered high in the air, and with the reds gone it reads as a saturated blue. City lights come on into it, which is why photographers wait for it. It ends when civil twilight does, at 6° below the horizon; after that the sky loses its colour to the night.

Sol counts it from 4° to 6° below the horizon — ten to twenty minutes at most latitudes — while the ground goes dark and the sky stays lit.