At the last public open house I was asked to explain why it took so long to see any stars. I started to go into the light-pollution issues and how all of the unshielded lights were robbing us of our night skies and the persons who had asked me the question stopped me and said, “No, that’s not what we meant. We see sunrise and sunset times in the newspaper but its still light out long after sunset, how come?”
Well, this prompted me to try and explain the different twilights, and I thought I might share the Royal Greenwich Observatory’s official definitions.
Sunrise and Sunset - The times of Sunrise and Sunset refer to the times when the Sun’s upper limb, as affected by refraction, is on the true horizon of an observer at sea-level. This occurs when the Sun’s center is 50 arcminutes below the true horizon, the upper limb then being 34 arcminutes just more than the Sun’s apparent diameter) below the true horizon.
Twilights - There are three different definitions of twilight.
Civil Twilight, when the Sun’s center is 6 degrees below the horizon, is roughly equivalent to between 30 and 60 minutes after sunset. The brightest stars are visible and at sea the horizon is clearly defined.
Nautical Twilight, when the Sun’s center is 12 degrees below the horizon, is to all intents and purposes the time when it is dark. For nautical purposes it is that time when the horizon ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
Astronomical Twilight, when the Sun’s center is 18 degrees below the horizon, is when it is truly dark and no remnant of the Sun’s afterglow can be seen. It is possible to see the Zodiacal light which comes from light from the Sun reflected by small particles between the Earth and the Sun; this can be mistaken for the Sun’s afterglow.
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[I’ll see what I can do. - Jr. ED, CS] [Go here for Mark Explains Again]