Twilight is 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 twilight, nautical twilight, and astronomical twilight. The three stages of twilight are:

• Civil Twilight: This is the period from when the sun disappears until the suns center is 6 degrees below the horizon. It is the brightest form of twilight, and there is enough natural sunlight during this period that artificial light may not be required to carry out outdoor activities.

• Nautical Twilight: This is the period when the center of the sun is between 6 degrees and 12 degrees below the horizon. It is less bright than civil twilight, and artificial light is generally required for outdoor activities. Nautical dawn occurs when the sun is 12 degrees below the horizon.

• Astronomical Twilight: This is the period of time when the center of the sun is 12 degrees below the horizon to 18 degrees below the horizon. It is the darkest twilight stage, and you probably wont notice any illumination left in the sky at this time. For stargazers, this is the time when fainter stars, clusters, and other sky objects appear and become good observing targets.

Twilight is also a term used metaphorically to imply that something is losing strength and approaching its end.