magnitude

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magnitude in astronomy, measure of the brightness of a star or other celestial object. The stars cataloged by Ptolemy (2d cent. AD), all visible with the unaided eye, were ranked on a brightness scale such that the brightest stars were of 1st magnitude and the dimmest stars were of 6th magnitude. The modern magnitude scale was placed on a precise basis by N. R. Pogson (1856). It was found by photometric measurements that stars of the 1st magnitude were about 100 times as bright as stars of the 6th magnitude, i.e., 5 magnitudes lower. Pogson defined a difference of 5 magnitudes to be exactly equal to a hundredfold change in brightness, so that stars differing by 1 magnitude differ in brightness by a factor of 2.512 (the 5th root of 100). The modern magnitude scale permits a precise expression of a star's relative brightness and extends to both extremely bright and very dim objects. Thus, an object 2.512 times as bright as a 1st-magnitude star is of 0 magnitude; brighter objects have negative magnitudes. The sun's magnitude, for example, is -26.8. On the other hand, a faint star of 16th magnitude is only 1/10,000 as bright as a 6th-magnitude star, the dimmest that can be seen with the naked eye. Magnitudes determined on the basis of an object's relative brightness as seen from the earth are known as apparent magnitudes. Astronomers also assign a star an absolute magnitude, which is the magnitude that a star would have if it were located at a standard distance of 10 parsecs (32.6 light-years). Absolute magnitude is a measure of the intrinsic luminosity of the star, i.e., its true brightness. Since in modern times magnitudes are measured with photometers and electronic detectors, which may be more sensitive to light at one wavelength than at another wavelength, it is necessary to specify the method and the filter used when comparing two or more magnitudes. The magnitude usually referred to is the visual, or photovisual, magnitude, measured with a photometer.

magnitude, as it applies to nautical astronomy, refers to the apparent brightness or luminosity of any of the navigational stars or planets. First to classify stars according to their apparent brightness was Hipparchus (c.190–after 126 bc), the prince of ancient astronomers. His system comprised six magnitude classes, the small group of some fourteen of the brighter stars forming the group of first-magnitude stars, and the relatively large number of faint stars just visible to the naked eye forming the group of sixth-magnitude stars. This rough and ready classification was improved during the 19th century with the introduction of a decimal scale of magnitudes, and the extension of the magnitude scale to include telescopic stars whose magnitude numbers are more than 600, and fractional and negative magnitudes. Sir John Herschel, son of the astronomer Sir William Herschel, is credited with having suggested that a star of magnitude 1.0 should be regarded as being 100 times as bright as a star of magnitude 6.0.

Nautical almanacs include tables providing for the navigational use of selected stars of the first and second magnitude, but those of lower magnitudes are never used for observation although they may form part of the constellation which enable the navigational stars to be identified.

See also celestial navigation.

magnitude A measure of the brightness of a star. Ancient Greek astronomers defined the brightest stars as being of the first magnitude because they were the first to appear after sunset. The magnitude scale continued in steps of decreasing brightness down to sixth magnitude, for those stars which were visible only in total darkness. From its crude beginnings, the magnitude scale has been extended and is now on a strictly defined footing (see Pogson Scale) so that a difference of one magnitude corresponds to a difference in brightness of a factor of 2.512, and 5 magnitudes equals a brightness difference of exactly a hundredfold. Ancient magnitude estimates depended solely on the human eye, corresponding roughly to the modern V magnitude. The apparent magnitude of a star is its brightness as seen from Earth, whereas the absolute magnitude is a measure of its actual (i.e. intrinsic) brightness; the two differ because the intensity of light falls off with distance, and because of interstellar absorption. When the brightness is measured over all wavelengths, rather than just visible wavelengths, it is known as the bolometric magnitude.

mag·ni·tude / ˈmagnəˌtoōd/ • n. 1. the great size or extent of something: they may feel discouraged at the magnitude of the task before them. ∎  great importance: events of tragic magnitude. 2. size: electorates of less than average magnitude. ∎  a numerical quantity or value: the magnitudes of all the economic variables could be determined. 3. the degree of brightness of a star. The magnitude of an astronomical object is now reckoned as the negative logarithm of the brightness; a decrease of one magnitude represents an increase in brightness of 2.512 times. A star with an apparent magnitude of six is barely visible to the naked eye.See also apparent magnitude, absolute magnitude. ∎  the class into which a star falls by virtue of its brightness. ∎  a difference of one on a scale of brightness, treated as a unit of measurement. PHRASES: of the first magnitudesee first.

magnitude In astronomy, numerical value expressing the brightness of a celestial object on a logarithmic scale. Apparent magnitude is the magnitude as seen from Earth, determined by eye, photographically or photometrically. It ranges from positive through zero to negative values, the brightness increasing as the magnitude decreases. Absolute magnitude indicates intrinsic luminosity, and is defined as the apparent magnitude of an object at a distance of 10 parsecs from the object.

magnitude greatness XIV; (relative) size XVI. — L. magnitūdō, f. magnus great, large, rel. to Gr. mégas (cf. MEGA-), Skr. mahā́nt- great, Gmc. *mikil- MUCH; see -TUDE.

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