apparent diameter

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. [1913 Webster]

2. (Geom.) That which has one or more of the three dimensions, length, breadth, and thickness. [1913 Webster]

3. Anything of which greater or less can be predicated, as time, weight, force, and the like. [1913 Webster]

4. Greatness; grandeur. ``With plain, heroic magnitude of mind.'' --Milton. [1913 Webster]

5. Greatness, in reference to influence or effect; importance; as, an affair of magnitude. [1913 Webster]

The magnitude of his designs. --Bp. Horsley. [1913 Webster]

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]