Twilight refers to the period of time before sunrise and after sunset when the atmosphere is partially illuminated by the sun, being neither totally dark nor completely lit. There are three categories of twilight that are defined by how far the sun is below the horizon: civil, nautical, and astronomical twilight. The different types of twilight are defined as follows:

• Civil Twilight: This occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.

• Nautical Twilight: This occurs when the Sun is between 6 and 12 degrees below the horizon. During nautical twilight, the horizon is still visible, but it is too dark to see the horizon clearly.

• Astronomical Twilight: This occurs when the Sun is between 12 and 18 degrees below the horizon. During astronomical twilight, the sky is completely dark, and no remnant of the Suns afterglow can be seen.

The duration of twilight depends on the latitude and the time of the year. The apparent travel of the Sun occurs at the rate of 15 degrees per hour, but sunrise and sunset happen typically at oblique angles to the horizon, and the actual duration of any twilight period will be a function of that angle, being longer for more oblique angles. Twilight is popular with photographers and painters, who often refer to it as the blue hour, after the French expression lheure bleue.