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    Re: Nautical Almanac clarification
    From: Bill B
    Date: 2007 Mar 09, 20:49 -0500

    > Gary LaPook adds:
    > A little background on "magnitudes."
    Nicely stated.  I recalled an article in my archives on the topic that is a
    bit less user friendly than Gary's excellent explanation (but not by an
    order of magnitude ;-)
    Magnitude is the degree of brightness of a star. In 1856, British astronomer
    Norman Pogson proposed a quantitative scale of stellar magnitudes, which was
    adopted by the astronomical community. He noted that we receive 100 times
    more light from a first magnitude star as from a sixth; thus with a
    difference of five magnitudes, there is a 100:1 ratio of incoming light
    energy, which is called luminous flux.
    Because of the nature of human perception, equal intervals of brightness are
    actually equal ratios of luminous flux. Pogson's proposal was that one
    increment in magnitude be the fifth root of 100. This means that each
    increment in magnitude corresponds to an increase in the amount of energy by
    2.512, approximately. A fifth magnitude star is 2.512 times as bright as a
    sixth, and a fourth magnitude star is 6.310 times as bright as a sixth, and
    so on. The naked eye, upon optimum conditions, can see down to around the
    sixth magnitude, that is +6. Under Pogson's system, a few of the brighter
    stars now have negative magnitudes. For example, Sirius is �1.5. The lower
    the magnitude number, the brighter the object. The full moon has a magnitude
    of about �12.5, and the sun is a bright �26.51!
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