visual magnitude

From The Collaborative International Dictionary of English v.0.48:

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.
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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.

   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.
      [1913 Webster +PJC]
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