After sunset, there is period of time where the sky gradually darkens, and before sunrise there is a period of time during which the sky gradually brightens.
Twilight, the time between day and night, is caused by scattered sunlight in the atmosphere that illuminates the sky and the ground for a while.
Few know that "twilight" has strict astronomical definitions.
• Civil twilight is the time when the center of the sun is geometrically less than 6 degrees below the horizon. During this time, the brightest stars and planets appear.
• At nautical twilight the sun is between 6 and 12 degrees below the horizon. Sailors used to use this time for reliable sightings of well-known stars for navigation.
• Astronomical twilight takes place when the center of the sun is more than 12 and less than 18 degrees below the horizon. The sky appears nearly fully dark, but very faint objects may still be hidden by the dim sunlight in the atmosphere.
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On Friday, astronomical twilight starts at 4:07 a.m., nautical twilight starts at 4:41 a.m., and civil twilight starts at 5:13 a.m. Sunrise is at 5:40 a.m. Sunset is at 7:20 p.m., evening civil twilight ends at 7:45 p.m., while evening nautical twilight ends at 8:18 p.m., and astronomical twilight ends at 8:52 p.m.
The length of twilight is very dependent on one's latitude. At high latitudes it can last hours, but at the equator it can be as short as 20 minutes.