EQUATOR, an imaginary great circle of the celestial vault or on the surface of the earth. As used in astronomy the term signifies a great circle of the celestial vault at right angles to its axis, and dividing it into a northern and a southern hemisphere. It is constituted by the plane of the earth's equator, produced in every direction till it reaches the itnaginary celestial sphere. The sun is twice a year tn the celestial equator—namely, at the equinoxes, whence the equator is also known as the equinoctial line, or simply the equinoctial (see EQuirrox). The point in the equator which touches the meridian is raised above the true horizon by an arc which is the complement 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 equator of the heavenly body. In geography, the equator is a great cir cle on the surface of the earth equidistant from its poles, and dividing it into two hemispheres.
Its latitude is zero; it is therefore marked on maps as O. Other parallels of latitude are counted from it, augmenting in their numerical designation as their distance from it north or south increases, the poles being 90 degrees. The plane of the terrestrial equator is a plane perpen dicular to the earth's axis and passing through its centre. The magnetic equator is a somewhat irregular line, nearly but not quite a great circle of the earth, in which there is no dip of the mag netic needle. It is hence called also the actinic line. It is not far from the geographical equa tor, but its situation slowly alters year by year, there being a slow oscillation of the magnetic poles, while the geographical equator and poles are almost fixed.