目视量
目视星等

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.
[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]