Three Types of Twilight

We’re all familiar with the term twilight, that period of time just before sunrise or just after sunset. Did you know that there are actually three different twilight definitions?
  • Civil Twilight: the time at which the sun is 6 degrees below the horizon. At this time, there is enough light for objects to be clearly distinguishable and that outdoor activities can commence (dawn) or end (dusk) without artificial illumination. Civil twilight is the definition of twilight most widely used by the general public.
  • Nautical Twilight: the time when the center of the sun is 12 degrees below the horizon, and only general or vague outlines of objects are visible. During the evening this is when it becomes too difficult to perceive the horizon, and in the morning this is the point when the horizon becomes distinguishable. This term goes back to the days when sailing ships navigated by using the stars.
  • Astronomical Twilight: the time at which the sun is 18 degrees below the horizon. It is that point in time at which the sun starts lightening the sky. Prior to this time during the morning, the sky is completely dark. During the evening, this is the point where the sky completely turns dark.
So you can see the sequence, here are the times for these twilights at Louisville on January 8, 2006:

Start of Morning Astronomical Twilight: 6:26 AM EST
Start of Morning Nautical Twilight: 6:58 AM EST
Start of Morning Civil Twilight: 7:31 AM EST
Sunrise: 8:00 AM EST

Sunset: 5:40 PM EST
End of Evening Civil Twilight: 6:09 PM EST
End of Evening Nautical Twilight: 6:42 PM EST
End of Evening Astronomical Twilight: 7:14 PM EST

These data were provided from the U.S. Naval Observatory. You can get twilight information for other locations at their website.

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