Declination

Declination (1) of a star, in Astronomy, is the arc on the star's meridian in the celestial sphere, intercepted between the star and the celestial equator. It is, therefore, the altitude of the star measured from the equator. With this and the right ascension the relative position of the star in the heavens is determined. (2) In Magnetism. When a magnetic needle is suspended by a fibre or pivot at its centre it assumes a definite direction by reason of the earth's magnetic force. It lies in a line that is called the magnetic meridian at that spot on the earth's surface, and the more northerly end points to what is called the magnetic north. As a rule the magnetic north at any place will not coincide with the true or geographical north - the magnetic meridian will not be identical with the geographical meridian. The angle between these two directions is called the declination. Thus in London the declination is about 17° west of north; it increases up to a certain limit as one travels westwards, subsequently diminishing to zero in the region of Hudson's Bay. Beyond this the declination is east of north. There are slight daily variations, as well as steady long-period changes, in the declination at any place. Lines drawn on the earth's surface to mark those points where the declination is of the same value, are called isogonic lines. Lines of no declination, where the magnetic and geographic meridians coincide, are called agonic lines. Along these the needle points to the true north; one of them traverses Hudson's Bay, the United States, and Brazil; a second crosses Spitzbergen, Russia, Asia Minor, the Indian Ocean, and Western Australia; and a third encloses an oval patch in Eastern Asia. Within this oval the declination is westerly; between it and the other two agonic lines the declination is easterly, and is to the west again over the remaining portions of the globe. Accurate declination charts, or variation charts as they are termed by sailors, are of great importance to navigation.