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    Re: Sunset, sunrise, civil & nautical twilight
    From: R.H. van Gent
    Date: 2002 Jan 30, 2:40 PM

    "daveweilacher@earthlink.net" wrote:
    > 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.
    These are the standard astronomical (and nautical, I guess) definitions
    for the various dusk and dawn intervals tabulated in the (local)
    astronomical almanacs.
    For sunset/sunset, the centre of the Sun is assumed to be 90 degrees 50
    arc minutes distant from the zenith (the 50 arc minutes accounts for
    both refraction and the Sun's semi-diameter, in an average sense). Note
    that this is in fact only correct at sea-level, for non-zero altitudes a
    small correction can be made if this is deemed to be necessary.
    Civil twilight is when the Sun's centre is 96 degrees distant from the
    zenith and nautical and astronomical twilight when the Sun's centre is
    102 and 108 degrees distant from the zenith.
    > 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.
    That is true (twilight lasts longer) and at certain latitudes some of
    the darker twilight periods do not even occur during certain periods of
    the year.
    For instance, for latitudes higher than 48.5 degrees astronomical
    twilight does not occur around the summer solstice. This is of some
    concern to Muslim communities in Northern Europe and elsewhere as the
    lawful period for one their five compulsory prayers can never occur.
    For latitudes higher than 54.5 degrees you will likewise have a period
    with no nautical twilight around the summer solstice and higher than
    60.5 degrees no civil twilight either.
    The same of course also occurs on the southern hemisphere near to the
    (northern hemisphere) winter solstice.
    * Robert H. van Gent                                  *
    * E-mail: r.h.vangent@astro.uu.nl                     *
    * Homepage: http://www.phys.uu.nl/~vgent/homepage.htm *

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