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    Re: Position by compass variation
    From: Frank Reed CT
    Date: 2006 Nov 27, 00:21 EST
    Jack Ganssle wrote:
    "And, how, especially long ago, did mariners determine variation with any sort of precision?"
     
    One of the most common daily astronomical observations from two centuries ago has completely vanished from the navigator's modern toolkit. It was the measurement of the Sun's amplitude. At sunrise or sunset, the Sun's azimuth can be read off to the nearest degree or two from the compass. For a known latitude and a rough estimate of longitude (to get the declination right), the actual, true azimuth of sunrise or sunset can be calculated rather accurately, though refraction complicates things. Compared with the magnetic azimuth, the difference between observation and calculation is the variation (landlubbers like geologists, and just about everyone except navigators, call this difference in direction the magnetic "declination", which proves that two different groups of users can invent equally bad names for the same phenomenon <g>).
     
    Records of magnetic variation in old logbooks often provide good examples of excessive precision. It's not uncommon to see something like "Var. per Sun Ampl.: 14d 22' ". They carry the calculation to the minute of arc because they don't realize they shouldn't.
     
    -FER
    42.0N 87.7W, or 41.4N 72.1W.
    www.HistoricalAtlas.com/lunars

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