EQUATOR 38 EQUINOX the difference between the time at which an astronomical occurrence takes place and that at which a fallible observer notes that it does so; also, the correction of personal differences between particu- lar individuals as to exactness in obser- vations with astronomical instruments; equation of payments, a rule for ascer- tc^ining at what time a person should in equity pay the whole of a debt contracted in different portions to be repaid at different times. 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 (g. 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 0. 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 12°, 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. EQUATORIAL, an astronomical in- strument designed to note the course of the stars as they move through the sky. A strong axis is constructed and perma- nently fixed in a slanting position so as to point exactly to the North pole of the heavens. It turns on its axis, carrying with it a telescope which, if it retained its relative position to that of the re- volving portion of the instrument, would enable an observer looking through it to see no more than a single great circle of the sky. It is not, however, fixed to the revolving portion of the instrument, but may be moved up or down so that with it an astronomer can follow the entire course of a circumpolar star in its pas- sage around the sky. It is of importance to ascertain not only the course of a star, but the apparent rapidity of its move- ment. This end is attained by attaching to the axis of the equatorial a racked wheel in which works an endless screw or worm, the whole put in motion by an apparatus furnished with centrifugal balls, like those of the governor of a steam engine, and which render the mo- tion uniform. EQUATORIAL CURRENT, a current in the ocean which crosses the Atlantic from Africa to Brazil, having a breadth varying from 160 to 450 nautical miles. Its waters are cooler by 3° or 4° than those of the ocean under the line. Its effect, therefore, is to diminish the heat of the tropics. EQUATORIAL TELESCOPE, a tele- scope so mounted as to have a motion in two planes at right angles to each other; one parallel to the axis of the earth, and the other to the equator. Each axis has a graduated circle, one for measur- ing declination and the other right ascen- sion. EQUINOCTIAL, the same as the celestial equator. The equinoctial points are those in which the equinoctial and the ecliptic intersect. Equinoctial time is time reckoned from the moment in each year when the sun passes the vernal equinox. This instant is selected as a convenient starting-point of a uniform reckoning of time for the purposes of astronomical observers. EQUINOCTIAL GALE, a gale popu- larly supposed to occur at the time of the spring or autumn equinox. Long-con- tinued observations, however, are decisive against this popular belief. EQUINOX, in astronomy, either one of the two points at which the sun, in its annual apparent course among the stars, crosses the equator ; so called because the days and nights are nearly equal when the sun is at these points. The vernal equinox occurs about March 20. When but one equinox is referred to, vernal equinox is meant. The autumnal equinox occurs about Sept. 23.
