Civil twilight

Posted on 20 November 2008

Q: What is civil twilight, and why should I care?

A: You’ve probably always known about civil twilight; it’s just that you didn’t know that it had an “official” name. As a boater or outdoors person, you’ve always known that there’s a time before the sun comes up and after it goes down that you can see well enough to do things outside.

Civil twilight is particularly important to boaters. If you’re like me, there have been many times when you’ve wanted to push it to the limit as to when you start out on your boat and when you get back. It may be relevant to when you leave that good fishing spot, when you leave the dock, when you head out the inlet, when you start pulling up the anchor and many more activities. If you’ve done overnight passages on a black, moonless night, you’ve wondered just how long the lonely darkness was going to last and when you’d start to see a pink glow to the east in the morning.

But the regularly broadcast times of sunrise and sunset that you get on the VHF, television weather programs and other popular sources only allow you to guess when it’s going to be really dark. I usually guess at around 30 to 45 minutes before and after. I have a strong tendency to be very optimistic with my guessing because I want to have as much on-the-water time as I can squeeze into life. But guessing wrong can lead to trouble. To be safer, there’s a better way, and Mel, my wife, found out about it because she was tired of my optimism getting us into trouble.

The term “civil twilight” is generally used to refer to a certain period in the morning as well as in the evening. In the morning it refers to that period of time when the center of the sun is less than 6 degrees below the horizon. It ends at sunrise. Civil twilight in the evening begins when the sun sets and ends when the sun’s center is more than 6 degrees below the horizon. (You’ll also see the terms “civil dawn” and “civil dusk” used to refer to these times.) During civil twilight, natural illumination is supposed to be sufficient to see objects outdoors well enough to get along without artificial lighting.

There’s another important aspect of this. At the beginning of civil twilight in the morning and at the end of civil twilight in the evening, we should be able to see not only the brightest stars but also the horizon. (Only when there is no moon, of course, since a bright enough moon would dim the stars.) This allows mariners to take sights of those stars for celestial navigation. But keep in mind that clouds, rain, fog, mist and other atmospheric conditions will vary what you can see.

There’s also a “nautical twilight,” which, for the purposes of most of us, means that it’s absolutely dark out there when it begins in the morning or ends in the evening. Nautical twilight is the period of time when the sun’s center is between 6 and 12 degrees below the horizon. During nautical twilight you can make out some outlines, but the horizon is indistinct and detailed outdoor activities are not possible without artificial illumination. There’s even an “astronomical twilight,” with 12 to 18 degrees being the defining limits, during which some stars are visible and some are not yet.

As you might imagine, this concept can be carried to great detail, and the experts specializing in this can add many nuances. But this is enough for me. It gives Mel one more tool to keep me from screwing up, and to keep us safer. If you want to read more about this concept, including the exact times of civil, nautical and astronomical twilights, check out (the Astronomical Applications Department of the U.S. Naval Observatory) and enter “civil twilight” in the search window.

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