NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Definition of Arctic Circle
From: Lu Abel
Date: 2000 Oct 08, 4:33 PM
From: Lu Abel
Date: 2000 Oct 08, 4:33 PM
I've looked in several sources for a precise definition of the Arctic Circle. I can find its latitude (approx 66d 33m) but I can't find its definition in terms of astronomical phenomena. Given that its latitude seems to be geometric complement of the angle of tilt of the earth's axis, I going to guess that it simply the point of tangency where a plane parallel to the plane of the ecliptic touches the northern hemisphere. The reason for my question is to settle a friendly argument about whether one can see 24 hours of daylight *south* of the Arctic Circle. If the above definition (geometric complement of the tilt of the earth's axis) is correct, the one certainly can see the sun never set while south of the Circle. First, with the above definition, to an observer on the Arctic Circle at the summer solstice the sun would never drop below its center. That means one could travel south by an amount equivalent to the semidiameter of the Sun and still see the upper limb of the sun at all times. An even more important phenomenon would be atmospheric refraction. I suspect most on this list know the sun is really well below the horizon when it appears to set, but atmospheric refraction "lifts" it up so we see it. If I interpret the tables in the front of the Nautical Almanac correctly, the combination of refraction and semidiameter add up to almost a full degree, which means one could see the sun for 24 hours upwards of a full degree below the Arctic Circle. Similar arguments can be used to claim that to experience the sun never rising one would have to be at least a degree north of the Circle on the winter solstice. By the way, I am well aware that one can be well below the Arctic Circle and be in a "twilight-or-better" zone where one never experiences true darkness. Again, if I'm interpreting the Nautical Almanac correctly, this can be as much as 12 degrees south of the Arctic Circle. (Which, to give some geographic meaning, encompasses Scotland, Norway, and all but the southern tip of Sweden) Comments?? Thanx Lu Abel