EQUATOR, an imaginary great circle of the celestial vault or on the surface of the earth.
In Astronomy.—A great circle of the celestial vault at right angles to its axis, and dividing it into a northern and southern hemisphere. It is constituted by the plane of the earth's equator, pro duced in every direction till it reaches the concave of the celestial sphere. In his progress north and south, and vice versa, the sun is twice a year in the celestial equator—viz., at the EQUINOXES (q. v.). The point in the equator which touches the meridian is raised above the true horizon by an arc which is the comple ment of the latitude. The sun and planets all have equators. They rotate around their several axes, and the plane at right angles in each case is the equa tor of the heavenly body.
In Geography.—A great circle on the surface of the earth equidistant from its poles, and dividing it into two hemi spheres. Its latitude is zero; it is there fore marked on the maps as O. Other parallels of latitude are counted from it, augmenting in their numerical designa tion as their distance from it north or south increases, the poles being 90°. The
plane of the equator is a plane perpen dicular to the earth's axis, and passing through its center.
In Magnetism.--A somewhat irregular line, nearly but not quite a great circle of the earth, in which there is no dip of the magnetic needle. It is hence called also the aclinic line. It is inclined to the equator at an angle of and cuts it at two points almost exactly opposite to each other, the one in the Atlantic and the other in the Pacific. It is not far from the geographical equator, but its situation slowly alters year by year, there being a slow oscillation of the mag netic poles, while the geographical equa tor and poles are fixed. The two points in which the magnetic equator cuts the equator seem traveling at present from E. to W.