Apparent magnitude

Magnitude \Mag"ni*tude\, n. [L. magnitudo, from magnus great.
   See {Master}, and cf. {Maxim}.]
   1. Extent of dimensions; size; -- applied to things that have
      length, breadth, and thickness.
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            Conceive those particles of bodies to be so disposed
            amongst themselves, that the intervals of empty
            spaces between them may be equal in magnitude to
            them all.                             --Sir I.
                                                  Newton.
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   2. (Geom.) That which has one or more of the three
      dimensions, length, breadth, and thickness.
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   3. Anything of which greater or less can be predicated, as
      time, weight, force, and the like.
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   4. Greatness; grandeur. "With plain, heroic magnitude of
      mind." --Milton.
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   5. Greatness, in reference to influence or effect;
      importance; as, an affair of magnitude.
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            The magnitude of his designs.         --Bp. Horsley.
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   6. (Astron.) See {magnitude of a star}, below.
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   {Apparent magnitude}

   1. (Opt.), the angular breadth of an object viewed as
      measured by the angle which it subtends at the eye of the
      observer; -- called also {apparent diameter}.

   2. (Astron.) Same as {magnitude of a star}, below.

   {Magnitude of a star} (Astron.), the rank of a star with
      respect to brightness. About twenty very bright stars are
      said to be of first magnitude, the stars of the sixth
      magnitude being just visible to the naked eye; called also
      {visual magnitude}, {apparent magnitude}, and simply
      {magnitude}. Stars observable only in the telescope are
      classified down to below the twelfth magnitude. The
      difference in actual brightness between magnitudes is now
      specified as a factor of 2.512, i.e. the difference in
      brightness is 100 for stars differing by five magnitudes.
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