A measure of the apparent or absolute brightness of a celestial object, first used around 120 bc; with the brightest stars referred to as first magnitude and the dimmest as sixth. This was put on a scientific basis by Pogson in 1854, who showed that equal magnitude steps were in logarithmic progression: a magnitude difference of one unit corresponds to a brightness ratio of 2-512; five magnitudes to 100 in actual brightnesses. The zero of the magnitude scale is essentially arbitrary. The apparent magnitude is the brightness measured at the Earth and it depends on distance and luminosity. More useful physically is the absolute magnitude, in which the observed apparent magnitude is corrected to that which it would have if placed at an arbitrary distance of 10 pc 32-6 light- years from the Earth. The observed magnitude will depend on the technique used to measure the brightness: visual magnitude refers to observation by eye; photographic mag

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