n. [ L. aequator one who equalizes: cf. F. équateur equator. See Equate. ] 1. (Geog.) The imaginary great circle on the earth's surface, everywhere equally distant from the two poles, and dividing the earth's surface into two hemispheres. [ 1913 Webster ]

2. (Astron.) The great circle of the celestial sphere, coincident with the plane of the earth's equator; -- so called because when the sun is in it, the days and nights are of equal length; hence called also the equinoctial, and on maps, globes, etc., the equinoctial line. [ 1913 Webster ]

Equator of the sun or
Equator of a planet (Astron.), the great circle whose plane passes through through the center of the body, and is perpendicular to its axis of revolution. --
Magnetic equator. See Aclinic. [ 1913 Webster ]

a. [ Cf. F. équatorial. ] Of or pertaining to the equator; as, equatorial climates; also, pertaining to an equatorial instrument. [ 1913 Webster ]

n. (Astron.) An instrument consisting of a telescope so mounted as to have two axes of motion at right angles to each other, one of them parallel to the axis of the earth, and each carrying a graduated circle, the one for measuring declination, and the other right ascension, or the hour angle, so that the telescope may be directed, even in the daytime, to any star or other object whose right ascension and declination are known. The motion in right ascension is sometimes communicated by clockwork, so as to keep the object constantly in the field of the telescope. Called also an equatorial telescope. [ 1913 Webster ]

☞ The term equatorial, or equatorial instrument, is sometimes applied to any astronomical instrument which has its principal axis of rotation parallel to the axis of the earth. [ 1913 Webster ]

adv. So as to have motion or direction parallel to the equator. [ 1913 Webster ]

n. An imaginary line paralleling the equator where a magnetic needle has no dip, the dipping needle being horizontal; called also aclinic line. [ WordNet 1.5 ]