Difference between revisions of "Magnitude"
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Magnitude
(symbol m)
</dt>
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>
References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use
