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The Collaborative International Dictionary of English v.0.48 (gcide)

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.
[1913 Webster]

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.
[PJC]

{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]