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 parsec s (32.6 light-years; e.g., 4.8 for the Sun). See also albedo ; photometry .