One of the two points of intersection of the celestial equator and the ecliptic. The Sun passes through the vernal equinox on about 21 March and through the autumnal equinox on about 22 September.
One of the two points on the celestial sphere where the celestial equator intersects the ecliptic, that is when the apparent ecliptic longitude of the Sun is 0° or 180°. At equinox, the length of the
day and the night are equal all over the globe. The equinox is not a fixed point; it moves due to precession and nutation. If only precession is considered, we deal with the mean equinox of date. If nutation is also taken into account, then we are concerned with the true equinox.