EQUINOCTIAL, or AQUINOCTIAL, in Astronomy, a great and immovable circle of the sphere, under which the equator moves in its diurnal motion.
The equinoctial or equinoctial line is ordinarily confounded with the equator; but there is a difference; the equator being moveable, and the equinoctial immovable; and the equator being drawn about the convex surface of the sphere, but the equinoctial on the concave surface of the magnus orbis.
Whenever the sun in his progress through the ecliptic comes to this circle, it makes equal days and nights all round the globe; as then rising due east and setting due west, which he never does at any other time of the year. And hence the denomination from aequus and nox, "night;" quia aequat diem nocti.
The equinoctial then is the circle which the sun describes, or appears to describe, at the time of the equinoxes; that is, when the length of the day is everywhere equal to that of night, which happens twice a-year. See EQUINOX.