"Twilight" can refer to civil twilight, nautical twilight, and astronomical twilight. The brightest of these is civil twilight and occurs when the sun is closest to the horizon, with astronomical twilight being the darkest when the sun is farthest from the horizon. Nautical twilight occurs in the middle. The length of these twilight times varies slightly depending on the distance from the equator and the time of year.
The Definition of Civil Twilight
Civil twilight occurs when the center of the sun is between 6 degrees below the horizon to just at the edge of the horizon. For example, at sunset civil twilight begins as the sun disappears and lasts until the sun's center is 6 degrees below the horizon. For midlatitudes, civil twilight lasts a bit longer in summer and winter and is a bit shorter in spring and fall. In spring and fall, the sun rises and sets more directly in the east and west and therefore makes a straighter path downward (or upward), reaching the 6 degree mark in a shorter period of time. In summer and winter, the sun arcs across the sky, cutting across the horizon at an angle. This angle is more pronounced in summer, which is why civil twilight lasts the longest in summer. Civil twilight in midlatitudes can last, on average, a half an hour.
Compare this to tropical regions. At the equator, the length of civil twilight hardly varies. The sun around the equator makes a path across the sky that cuts cleanly down toward the horizon at sunset in a nearly perpendicular fashion, therefore the sun and its rays disappear faster, giving equatorial regions a shorter twilight than in the higher latitudes. Near the poles, twilight times last much longer.
During civil twilight, objects are still easily distinguishable and activities can carry on without artificial light. The brightest stars and planets appear during civil twilight.
The Definition of Nautical Twilight
Nautical twilight picks up where civil twilight left off, when the center of the sun is 6 degrees below the horizon to 12 degrees below the horizon. The horizon appears indistinct and terrestrial objects can be made out but artificial light is required to carry on activities. More bright stars appear during this time, which was important in the early days of navigation as directional cues. For polar regions, the summer sun does not set below 12 degrees below the horizon, therefore these regions have no astronomical twilight for a portion of the year. For midlatitudes, nautical twilight can last from about a half hour in spring, winter, and fall to about 45 minutes in summer.
The Definition of Astronomical Twilight
Astronomical twilight is the darkest form of twilight. It occurs when the center of the sun reaches 12 degrees below the horizon to 18 degrees below the horizon. In midlatitudes, astronomical twilight can last about a half hour from fall through spring but up to an hour in summer. Any sky illumination might not even be noticeable during astronomical twilight. Astronomical twilight begins about an hour to an hour and a half after sunset for midlatitudes. For stargazers, this is the time when fainter stars, clusters, and other sky objects appear and become good observing targets.
Twilight Occurs Twice A Day
Twilight is the hours just after the sun sets and before the sun rises. When the sun is closest to the horizon to 6 degrees it is Civil Twilight, 6 to 12 degrees below is Nautical Twilight, and 12 to 18 degrees below is Astronomical Twilight, which is the best twilight time to observe the heavens.
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