In astronomy, the measure of the brightness of a star or other celestial body. The brighter the object, the lower the number assigned as a magnitude. In ancient times six magnitude classes were used, the first containing the brightest stars (see Hipparchus). In the present system a difference of one magnitude is defined as a ratio of brightness of 2.512 times. Thus, a difference of five magnitudes corresponds to a brightness ratio of 100 to 1. Apparent magnitude is an object's brightness as seen from Earth (e.g., 26.7 for the Sun, about 11 for the Moon). Absolute magnitude is an object's brightness as it would be seen at a distance of 10 parsecs (32.6 light-years; e.g., 4.8 for the Sun). See also albedo; photometry.

This entry comes from Encyclopædia Britannica Concise.
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