Dawn (from an Old English verb dagian "to become day") is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the Sun itself is still below the horizon. Dawn should not be confused with sunrise, which is the moment when the leading edge of the Sun itself appears above the horizon.

During dawn (and dusk) it is usually possible (provided that the sky is cloud-free) to see approximately in which direction the Sun lies, though it is 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 dawn[edit]

Astronomical dawn is defined as the moment after which the sky is no longer completely dark. This occurs when the Sun is 18 degrees below the horizon in the morning.[1] Though it is possible to localize the direction of the Sun during astronomical dawn and dusk, people in general experience astronomical dawn and dusk as night, even without clouds. The zenith is dark and more than just the brightest stars can be seen (except low above the horizon in the direction of the sun).

Clouds begin to glow with colors at nautical dawn.

Main article: Twilight § Nautical twilight

Nautical dawn is the time at which there is enough sunlight for the horizon and some objects to be distinguishable; formally, when the Sun is 12 degrees below the horizon in the morning.[1]

Civil dawn in Southern California, United States

Main article: Twilight § Civil twilight

Civil dawn is the time at which there is enough light for objects to be distinguishable, so that outdoor activities can commence; formally, when the Sunis 6 degrees below the horizon in the morning.[1] At civil dawn there is no darkness in any direction, nor at zenith. The sky is bright, even when cloudy.

Effects of latitude[edit]

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

Effects of latitude (equatorial)[edit]

Panoramic view of a dawn inHiguerote, Venezuela

All phases of dawn and dusk are shortest at the equator, where the Sun at equinox rises and sets at a right angle to the horizon; the steps between civil, nautical, and astronomical dawn or dusk correspond to only 24 minutes each. At all places on the earth, dawn and dusk times are fastest around the equinoxes and slowest at the summer and winter solstices.

Effects of latitude (polar)[edit]

As the calendar approaches the summer/winter solstices the days/nights get shorter, which can have a potential impact on the time and duration of dawn and dusk. This effect is more pronounced closer to the poles, where the Sun rises at the spring equinox and sets at the autumn equinox; with a long period of dawn/dusk, lasting for a few weeks .

The polar circle (at 66°30′ N) is defined as the first latitude at which the Sun does not set during the summer solstice. Therefore the angular radius of the polar circle is equal to the angle between the plane of Earth's equator and that of the ecliptic. This period of time with no sunset extends closer to the North Pole.

Near the summer solstice, latitudes higher than 54°30′ get no darker than nautical dawn/dusk; the "darkness of the night" varies greatly in these latitudes.

At latitudes higher than about 59°20, summer nights get no darker than civil dusk or dawn. This period of "bright nights" is longer at higher latitudes (further north).