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    Re: Almanac
    From: Chuck Taylor
    Date: 2002 Mar 25, 08:10 US/PACIFIC

    Anders Reed Mohn wrote:
    
    > I think someone once told me that the Almanac repeats every
    > four years.
    >
    > Is that correct?
    
    Approximately.  The major factors which change from year to
    year are
    
    1. The position of the earth in its orbit around the sun.
    2. Precession of the earth about its axis (wobble).
    3. Nutation (wobble on the wobble).
    4. Proper motion of the stars.
    
    Of these, the first is by far the largest influence. Another
    name for this effect would be "the Leap Year effect".  If the
    earth took precisely 365.00 days to orbit around the sun, it
    would be in exactly the same position relative to the sun every
    year at the same time. In fact the number is closer to (but not
    exactly) 365.25, so every four years the earth is in approximately
    the same place at the same time of year.
    
    In other words, the next best thing to the current almanac is a
    four-year-old almanac. Still, a current almanac is better.
    
    For comparison, here are a few excerpts for March 24 from the
    1998 and 2002 nautical almanacs
    
                         1998          2002
    GHA Aries
    1200                1 45.6       1 47.3
    1300               16 48.0      16 49.7
    1400               31 50.5      31 52.2
    
    GHA Sun
    1200              358 25.1     358 25.3
    1300               13 25.2      13 25.5
    1400               28 25.4      28 25.7
    
    Dec Sun
    1200             N  1 26.9    N  1 27.5
    1300             N  1 27.9    N  1 28.5
    1400             N  1 28.9    N  1 29.5
    
    SHA
    Alioth            166 30.4     166 27.7
    Arcturus          146 06.1     146 03.5
    Canopus           264 01.3     264 00.1
    
    Dec
    Alioth           N 55 58.1    N 55 56.8
    Arcturus         N 19 11.4    N 19 10.1
    Canopus          S 52 42.1    S 52 42.1
    
    The orbits of the moon and planets are on different cycles,
    so a four-year-old almanac is of no help whatsoever in
    predicting their positions.
    Chuck Taylor
    Everett, WA, USA
    
    
    

       
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