What is "variation of the needle"

Variation \Va`ri*a"tion\, n. [OE. variatioun, F. variation, L. variatio. See Vary.]

1. The act of varying; a partial change in the form, position, state, or qualities of a thing; modification; alternation; mutation; diversity; deviation; as, a variation of color in different lights; a variation in size; variation of language.

   The essences of things are conceived not capable of any such variation.
   --Locke.

2. Extent to which a thing varies; amount of departure from a position or state; amount or rate of change.

3. (Gram.) Change of termination of words, as in declension, conjugation, derivation, etc.

4. (Mus.) Repetition of a theme or melody with fanciful embellishments or modifications, in time, tune, or harmony, or sometimes change of key; the presentation of a musical thought in new and varied aspects, yet so that the essential features of the original shall still preserve their identity.

5. (Alg.) One of the different arrangements which can be made of any number of quantities taking a certain number of them together.

   Annual variation (Astron.), the yearly change in the right ascension or declination of a star, produced by the combined effects of the precession of the equinoxes and the proper motion of the star.

   Calculus of variations. See under Calculus.

   Variation compass. See under Compass.

   Variation of the moon (Astron.), an inequality of the moon's motion, depending on the angular distance of the moon from the sun. It is greater at the octants, and zero at the quadratures.

   Variation of the needle (Geog. & Naut.), the angle included between the true and magnetic meridians of a place; the deviation of the direction of a magnetic needle from the true north and south line; -- called also declination of the needle.

   Syn: Change; vicissitude; variety; deviation.