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Subject: Re: Sunset, sunrise, civil & nautical twilight
From: Dan Allen (danallen46@XXX.XXX)
Date: Wed Jan 30 2002 - 16:20:44 EST
The canonical angles are
Sun rise/set: 0 degrees
Civil twilight: 6 degrees below
Nautical twilight: 12 degrees below
Astronomical twilight: 18 degrees below
The definitions have to do with the amount of light visible. The first one is self explanatory (but due to refraction the sunrise
or sunset is actually at 50 minutes beneath the horizon...)
Civil twilight is the time when cars are supposed to have their headlights on by. One can no longer see details of things well at
this time. People common say at this time that it is dark outside.
Nautical twilight is when the horizon at sea is no longer visible (but there still may be some light in the sky). That is, if one
was out to sea the line of the horizon cannot be made out any more, and thus sextant observations without artificial horizons could
no longer be taken.
Astronomical twilight is when it is as dark as it is ever going to get. There is no more light in the sky from the sun. (The moon
may light things up though!)
These definitions hold true in any latitude at any time of year. However as you observed the conditions do not allow some of these
events to occur at certain combinations of times and places, i.e., in the summer in the Arctic none of these occur at all.
Specificially, on the summer solstice (near June 20th) any place north of 66 degrees north latitude will not have the sun set, let
alone enjoy any twilights: the sun is above the horizon all night long.
If you try and do the math to solve for such things you will end up taking the arccos of 2 or something like that -- which is not
defined for real numbers.
Dan Allen
-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@XXX.XXX]On Behalf Of
daveweilacher@XXX.XXX
Sent: Wednesday, January 30, 2002 1:04 PM
To: NAVIGATION-L@XXX.XXX
Subject: Sunset, sunrise, civil & nautical twilight
A definition I've read for these is:
Sunset occurs right when the top of the sun disappears over the horizon. Civil twilight occurs when the sun is 6 degrees below the
horizon, and nautical twilight occurs at 12 degrees.
Sunset might make sense but the 6 and 12 degree notion doesn't seem right to me.
The closer to a pole you are, the longer twilight lasts (yes or no?) in which case the degrees don't work.
Would someone be willing to clarify this for me?
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