Once the sun sets behind the horizon, there is still some sunlight visible. This is because the Earthís atmosphere reflects and refracts some sunlight.
This means light from the sun hits the Earthís atmosphere and is scattered, or its path is bent. This allows light to reach us after the sun has dropped below the horizon. This period is twilight.
There are three types of twilight: Civil, nautical and astronomical. Each is defined by how far the sunís center is below the horizon. Civil twilight ends when the sunís center is 6 degrees below the horizon. Nautical twilight ends when it is 12 degrees below and astronomical twilight ends when the sunís center is 18 degrees below the horizon.
During twilight, the added light prevents astronomers from observing very faint objects.