Twilight explained
Twilight is the soft light before sunrise and after sunset, when the Sun is below the horizon but still lighting the sky from beneath it. It comes in three bands, civil, nautical and astronomical, defined by how far the Sun has sunk below the horizon.
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The three bands of twilight
Civil twilight runs from the horizon to 6 degrees below it. There is enough light for most outdoor activity, and the brightest planets appear. Nautical twilight runs from 6 to 12 degrees below: sailors can still make out the horizon against the sky to take star sightings. Astronomical twilight runs from 12 to 18 degrees below; the sky looks dark to the eye but a faint glow still washes out the dimmest stars. Below 18 degrees it is true night.
Why twilight is longer in summer and near the poles
Twilight length depends on the angle at which the Sun crosses the horizon. Near the equator the Sun drops almost straight down, so twilight is brief. At high latitudes, and especially in summer, the Sun skims the horizon at a shallow angle and takes far longer to reach 18 degrees below, stretching twilight into hours. Above the polar circles in midsummer it may never get there at all, giving white nights.
Frequently asked questions
What are the three types of twilight?
Civil (0 to 6 degrees below the horizon), nautical (6 to 12) and astronomical (12 to 18). Below 18 degrees is full night.
What is civil twilight?
The brightest band of twilight, from the horizon to 6 degrees below. There is enough natural light for most outdoor activity without artificial lighting.
Why is twilight longer in summer?
The Sun crosses the horizon at a shallower angle, so it takes longer to sink the 18 degrees needed for full night. The effect is strongest at high latitudes.
When does true night begin?
At the end of astronomical twilight, when the Sun is more than 18 degrees below the horizon and the sky no longer brightens.