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Re: Sunset, sunrise, civil & nautical twilight
From: Dan Allen
Date: 2002 Jan 30, 2:21 PM
From: Dan Allen
Date: 2002 Jan 30, 2:21 PM
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@LISTSERV.WEBKAHUNA.COM]On Behalf Of daveweilacher@earthlink.net Sent: Wednesday, January 30, 2002 1:04 PM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM 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? -------------------------------------------------------------------- mail2web - Check your email from the web at http://mail2web.com/ .
