Dawn

Dawn (from an Old English verb dagian "to become day") or astronomical dawn is the time that marks, depending on the specific usage, the beginning of the twilight before sunrise, the period of the pre-sunrise twilight or the time of sunrise. When identified as the beginning of or the period of twilight, it is recognized by the presence of weak sunlight, while the Sun itself is still below the horizon.

Different definitions exist for the start of dawn. The difference between these definitions is the amount of sunlight that must be present. This can be correlated with the angular distance of the centre of the Sun (degrees) below the horizon, in the morning:

Astronomical twilight begins at the moment after which the sky is no longer completely dark after astronomical dawn. This occurs when the Sun is 18 degrees below the horizon in the morning. Though it is possible to localize the direction of the Sun during astronomical dawn and dusk, astronomical dawn and dusk are night, even without clouds. The zenith is extremely dark and more than just the brightest stars can be seen (except low above the horizon in the direction of the sun).

Nautical twilight begins at the time at which there is enough sunlight for sailors to distinguish the horizon via sea but artificial light will be needed to perform outdoor activities; formally, when the Sun is 12 degrees below the horizon in the morning. Nautical Dawn is still quite dark and is considered night.

Civil twilight begins at the time at which there is enough light for most objects to be distinguishable, so that some outdoor activities but not all can commence; formally, when the Sun is 6 degrees below the horizon in the morning. At civil dawn there is some darkness and at the zenith. The sky is still a bit dark and civil dawn is still considered night.

The duration of the twilight period between first light and sunrise varies greatly depending on the observer's latitude, from a little over twenty minutes in equatorial regions, to many hours in polar regions, to several weeks at the poles.

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