declination-¹(astronomy) The angle at the center of the celestial sphere between the radius passing through a celestial body and the plane of the celestial equator. Astronomic declination is measured by the arc of the hour circle between the celestial body and the equator; it is plus when the body is north of the equator, and minus when south of it. It corresponds to latitude on the earth, and with right ascension forms a pair of coordinates which define the position of a body on the celestial sphere. ² The bearing (reckoned east or west from the north branch of the celestial meridian plane) of magnetic north as determined by the positive pole of a freely suspended magnetic needle which is subject to no transient artificial disturbance. In nautical and aeronautical navigation, the term magnetic variation is used instead of magnetic declination, and the angle is termed variation of the compass or magnetic variation.

declination arc-¹A graduated arc on a surveyor’s solar compass or on the solar attachment of an engineer’s transit, on which the declination of the sun (corrected for refraction) is set off. This represents one side (polar distance) of the astronomical triangle which is solved mechanically by the solar compass or attachment. ² A graduated arc attached to the alidade of a surveyor’s compass or transit, on which the magnetic declination is set off. When the magnetic declination is set off on the declination arc of a surveyor’s compass or transit, a reading of the needle will give a bearing corrected for that declination.

declination of grid north—See gisement.

–

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

Part of LearnCST’s exam text bundle.