What is "declension of the needle"

Declension \De*clen"sion\, n. [Apparently corrupted fr. F. d['e]clinaison, fr. L. declinatio, fr. declinare. See Decline, and cf. Declination.]

1. The act or the state of declining; declination; descent; slope.

   The declension of the land from that place to the sea.
   --T. Burnet.

2. A falling off towards a worse state; a downward tendency; deterioration; decay; as, the declension of virtue, of science, of a state, etc.

   Seduced the pitch and height of all his thoughts To base declension.
   --Shak.

3. Act of courteously refusing; act of declining; a declinature; refusal; as, the declension of a nomination.

4. (Gram.)

   1. Inflection of nouns, adjectives, etc., according to the grammatical cases.

   2. The form of the inflection of a word declined by cases; as, the first or the second declension of nouns, adjectives, etc.

   3. Rehearsing a word as declined.

      Note: The nominative was held to be the primary and original form, and was likened to a perpendicular line; the variations, or oblique cases, were regarded as fallings (hence called casus, cases, or fallings) from the nominative or perpendicular; and an enumerating of the various forms, being a sort of progressive descent from the noun's upright form, was called a declension.
      --Harris.

      Declension of the needle, declination of the needle.