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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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!

Bill

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