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    Definition of Arctic Circle
    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
    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)
    Lu Abel

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