Twilight is the faint light which appears a little before sunrise and again after sunset before it gets really dark. On the Moon there is no twilight. Darkness comes suddenly as soon as direct sunlight ceases to reach the moon’s surface.
This does not happen on Earth because of the halo of air, called the atmosphere, which surrounds it. When the Sun goes below the horizon, its light leaves the Earth but is reflected downward from the upper atmosphere, illuminating the lower atmosphere so that Earth’s surface is neither completely lit nor completely dark. The word is also used to denote the periods of time when this illumination occurs.
The farther the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18° below the horizon, the twilight’s brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight.
Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, it has long been popular with photographers, who sometimes refer to it as “sweet light”, and painters, who often refer to it as the blue hour, after the French expression l’heure bleue.
