Tuesday, July 13, 2010
Dawn, Sunrise, Twilight, Sunset and Dusk
In last Thursday's blog, I described early morning walks with my wife after we rise at dawn. I promised to describe what "dawn" means.
Dawn is the time that marks the beginning of the twilight before sunrise. Per the definitions below, there is astronomical dawn, nautical dawn, and civil dawn.
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east.
Twilight is the time between dawn and sunrise, and between sunset and dusk. Per the definitions below, there is astronomical twilight, nautical twilight, and civil twilight.
Sunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation.
Dusk is the beginning of darkness in the evening. Per the definitions below, there is astronomical dawn, nautical dawn and civil dusk.
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon (the point of civil dusk). In general, civil twilight is the point where artificial illumination is required to read outside.
Nautical twilight is the time when the center of the sun is between 6° and 12° below the horizon. In general, nautical twilight ends when navigation via the horizon at sea is no longer possible.
Astronomical twilight is the time when the center of the Sun is between 12° and 18° below the horizon. In general, the end of astronomical twilight is the point where the sky is no longer illuminated by the sun and is dark enough for all astronomical observations.
All of this is summarized in the graphic above.
So when I say that I walk with my wife at dawn, I mean that we rise at civil dawn and begin our walk in civil twilight, ending it after sunrise.
Never again will you confuse dawn with sunrise or sunset with dusk!
Posted by John Halamka at 3:00 AM