EQUINOCTIAL (aquus, equal, and nox, night), in Astronomy, a great 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 immoveable; the equator being drawn about the convex surface of the sphere, and the equinoctial on the concave surface of the magnus orbis.
When the sun, in its progress through the ecliptic, comes to this circle, the days and nights are equal all over the globe. 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 the night, which happens twice a-year, viz., about the 21st of March and the 23d of September.