The New International Encyclopædia/Meridian

MERID′IAN (Lat meridianus, relating to midday, from meridies, for *medidies, midday, from medius, middle + dies, day). Any great circle of a sphere passing through its poles. A terrestrial meridian is the intersection of the earth's surface made by a plane passing through the poles. A celestial meridian is a great circle of the celestial sphere passing through the celestial poles. The prime meridian is the one from which longitude is measured. The magnetic meridian at any point is the horizontal direction at the point of a freely suspended compass needle undisturbed by influences other than the earth's magnetic force. To distinguish the terrestrial or geographical meridian from the magnetic, the former is called the true meridian, as its direction is that of true north or south. It is evident from the explanation given that the so-called magnetic meridian is not a great circle of the earth, but merely a direction at a particular point. The line joining the points of equal variation are not therefore coincident with the magnetic meridian; they are called isogonic lines, and, owing to the non-uniform character of the earth's magnetic force, these lines are very irregular curves. See Meridian Circle; Meridian Measurement.