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(symbol m) 

1. The relative luminance of a celestial body. The smaller (algebraically) the number indicating magnitude, the more luminous the body. Also called stellar magnitude. See absolute magnitude. </dd>
The ratio of relative luminosity of two celestial bodies differing in magnitude by 1.0 is 2.512, the fifth root of 100.
Decrease of light by a factor of 100 increases the stellar magnitude by 5.00; hence, the brightness objects have negative magnitudes (Sun: -26.8; mean full moon: -12.5; Venus at brightest: -4.3; Jupiter at opposition: -2.3; Sirius: -1.6; Vega: 0.2; Polaris: 2.1). The faintest stars visible to the naked eye on a clear dark night are of about the sixth magnitude (though on a perfectly black background the limit for a single luminous point approaches the eighth magnitude). The faintest stars visible with a telescope of aperture a (in inches) is one approximately of magnitude <math>9 + 5 log10 a</math>. The magnitude of the faintest stars which can be photographed with the 200-inch telescope is about +22.7.
The expression first magnitude is often used somewhat loosely to refer to all bodies of magnitude 1.5 or brighter, including negative magnitudes. </dd>
2. Amount; size; greatness. See order of magnitude. </dd>


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use