7.2.6. Dawn and Twilight

The brief period between sunrise and full daylight is called dawn and that between sunset and complete darkness is termed twilight. This is caused by the fact that during the period the dawn and twilight the earth receives diffused or refracted light from the sun whilst it is still below the horizon. Since the sun rises and sets in a vertical path at the equator the period during which refracted light is received is short. But in temperate latitudes, the sun rises and sets in an oblique path and the period of refracted light is longer. It is much longer still at the poles, so that the winter darkness is really only twilight most of the time.