What is "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.

   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.

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

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

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

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

   The magnitude of his designs.
   --Bp. Horsley.

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.