What is "azimuth compass"

Compass \Com"pass\ (k[u^]m"pas), n. [F. compas, fr. LL. compassus circle, prop., a stepping together; com- + passus pace, step. See Pace, Pass.]

1. A passing round; circuit; circuitous course.

   They fetched a compass of seven day's journey.
   --2 Kings iii. 9.

   This day I breathed first; time is come round, And where I did begin, there shall I end; My life is run his compass.
   --Shak.

2. An inclosing limit; boundary; circumference; as, within the compass of an encircling wall.

3. An inclosed space; an area; extent.

   Their wisdom . . . lies in a very narrow compass.
   --Addison.

4. Extent; reach; sweep; capacity; sphere; as, the compass of his eye; the compass of imagination.

   The compass of his argument.
   --Wordsworth.

5. Moderate bounds, limits of truth; moderation; due limits; -- used with within.

   In two hundred years before (I speak within compass), no such commission had been executed.
   --Sir J. Davies.

6. (Mus.) The range of notes, or tones, within the capacity of a voice or instrument.

   You would sound me from my lowest note to the top of my compass.
   --Shak.

7. An instrument for determining directions upon the earth's surface by means of a magnetized bar or needle turning freely upon a pivot and pointing in a northerly and southerly direction.

   He that first discovered the use of the compass did more for the supplying and increase of useful commodities than those who built workhouses.
   --Locke.

8. A pair of compasses. [R.] See Compasses.

   To fix one foot of their compass wherever they please.
   --Swift.

9. A circle; a continent. [Obs.]

   The tryne compas [the threefold world containing earth, sea, and heaven.
   --Skeat.]
   --Chaucer.

   Azimuth compass. See under Azimuth.

   Beam compass. See under Beam.

   Compass card, the circular card attached to the needles of a mariner's compass, on which are marked the thirty-two points or rhumbs.

   Compass dial, a small pocket compass fitted with a sundial to tell the hour of the day.

   Compass plane (Carp.), a plane, convex in the direction of its length on the under side, for smoothing the concave faces of curved woodwork.

   Compass plant, Compass flower (Bot.), a plant of the American prairies ( Silphium laciniatum), not unlike a small sunflower; rosinweed. Its lower and root leaves are vertical, and on the prairies are disposed to present their edges north and south.

   Its leaves are turned to the north as true as the magnet: This is the compass flower.
   --Longefellow.

   Compass saw, a saw with a narrow blade, which will cut in a curve; -- called also fret saw and keyhole saw.

   Compass timber (Shipbuilding), curved or crooked timber.

   Compass window (Arch.), a circular bay window or oriel window.

   Mariner's compass, a kind of compass used in navigation. It has two or more magnetic needles permanently attached to a card, which moves freely upon a pivot, and is read with reference to a mark on the box representing the ship's head. The card is divided into thirty-two points, called also rhumbs, and the glass-covered box or bowl containing it is suspended in gimbals within the binnacle, in order to preserve its horizontal position.

   Surveyor's compass, an instrument used in surveying for measuring horizontal angles. See Circumferentor.

   Variation compass, a compass of delicate construction, used in observations on the variations of the needle.

   To fetch a compass, to make a circuit.

Azimuth \Az"i*muth\, n. [OE. azimut, F. azimut, fr. Ar. as-sum?t, pl. of as-samt a way, or perh., a point of the horizon and a circle extending to it from the zenith, as being the Arabic article: cf. It. azzimutto, Pg. azimuth, and Ar. samt-al-r[=a]'s the vertex of the heaven. Cf. Zenith.] (Astron. & Geodesy)

1. The quadrant of an azimuth circle.

2. An arc of the horizon intercepted between the meridian of the place and a vertical circle passing through the center of any object; as, the azimuth of a star; the azimuth or bearing of a line surveying.

   Note: In trigonometrical surveying, it is customary to reckon the azimuth of a line from the south point of the horizon around by the west from 0[deg] to 360[deg].

   Azimuth circle, or Vertical circle, one of the great circles of the sphere intersecting each other in the zenith and nadir, and cutting the horizon at right angles.
   --Hutton.

   Azimuth compass, a compass resembling the mariner's compass, but having the card divided into degrees instead of rhumbs, and having vertical sights; used for taking the magnetic azimuth of a heavenly body, in order to find, by comparison with the true azimuth, the variation of the needle.

   Azimuth dial, a dial whose stile or gnomon is at right angles to the plane of the horizon.
   --Hutton.

   Magnetic azimuth, an arc of the horizon, intercepted between the vertical circle passing through any object and the magnetic meridian. This is found by observing the object with an azimuth compass.

An azimuth compass (or azimuthal compass) is a nautical instrument used to measure the magnetic azimuth, the angle of the arc on the horizon between the direction of the sun or some other celestial object and the magnetic north. This can be compared to the true azimuth obtained by astronomical observation to determine the magnetic declination, the amount by which the reading of a ship's compass must be adjusted to obtain an accurate reading. Azimuth compasses were important in the period before development of the reliable chronometers needed to determine a vessel's exact position from astronomical observations.