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Definitions

Civil Twilight, Dawn & Dusk

Civil twilight is defined as the time when the sun is 6 degrees below the horizon and up to the time it has risen, or the time between when it has set to when it is 6 degrees below the horizon. In the morning this time period is known as dawn—in the evening it is called dusk.

The end of dusk and the start of dawn are the limits at which illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at these times, 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.

The time displayed on this site is when the sun is 6 degrees below the horizon.

Nautical Twilight

Nautical twilight is defined when the sun is from 6 to 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.

The time displayed on this site is when the sun is 12 degrees below the horizon.

Astronomical Twilight

Astronomical twilight is defined when the sun is from 12 to 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 the beginning of morning astronomical twilight to the beginning of morning nautical twilight, and from the end of evening nautical twilight to the end of evening astronomical twilight sky illumination is so faint that it is practically imperceptible.

The time displayed on this site is when the sun is 18 degrees below the horizon.

Solar Noon

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

Time Zones: Greenwich Mean Time (GMT) & Universal Time

GMT, as used on this site, is simply the time zone for Greenwich, England—for civil, non-high-precision purposes, GMT is considered equivalent to UTC. 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.

′ and ″ used in degrees- and hours-minutes-seconds items

Liteware

Software that is distributed freely, with that version having less capability than the full for-sale version. The product is usually limited in its features or capacity; it is designed to provide a potential customer with a sample of the “look-and-feel” of that product.

 

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