Civil Twilight
Civil twilight is defined when the sun is 6 degrees below the horizon. In the morning this is known as dawn, in the evening it is called dusk. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.

Nautical Twilight
Nautical twilight is defined when the sun is 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.

Astronomical Twilight
Astronomical twilight is defined when the sun is 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.

Solar Noon
Solar noon is the time when the sun appears highest in the sky (toward the north pole or south pole) for that location. Read more in the Astronomy FAQs.

Greenwich Mean Time (GMT)
GMT, as used by this site, is simply the time zone for Greenwich, England. All other time zones are adjusted to GMT by either adding or subtracting time so the times shown are local times. The convention used on this site is negative for west of Greenwich and positive for east, for example: -5 is the adjustment for the Eastern time zone of the United States and +3 for Moscow, Russia.

Read more about GMT on the Wikipedia web site.

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