or AEQUINOCTIAL, in astronomy, a great and immoveable 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; 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 around 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 aquae and nox, "night," quia aquat diem noet.

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.

in geography. See Equator.

The shadows of those who live under this circle are cast to the southward of them for one half of the year, and to the northward of them during the other half; and twice in a year, viz. at the equinoxes, the sun at noon casts no shadow, being in their zenith.

From this circle is the declination or latitude of places accounted in the degrees of the meridian.

Equinoctial Points, are the two points wherein the equator and ecliptic intersect each other: the one being in the first point of Aries, is called the vernal point or equinox; and the other in the first point of Libra, the autumnal point or equinox.

Equinoctial Dial, is that whose plane lies parallel to the equinoctial. See Dial.