aequinoctium

Aequinoctium is a Latin term used in astronomy to denote the equinox. It derives from aequus, equal, and nox, night, and describes the moment when the Sun crosses the celestial equator, placing daylight and darkness in near parity.

There are two aequinoctia each year: the vernal (spring) equinox and the autumnal (fall) equinox. The vernal equinox typically falls on March 20 or 21, signaling the transition from winter to spring in the northern hemisphere; the autumnal equinox normally occurs on September 22 or 23, marking the transition from summer to autumn. The exact moment shifts slightly from year to year due to the eccentricity of Earth's orbit and gravitational perturbations, as well as the effect of atmospheric refraction and the solar disc size.

In practice, the length of day and night is not precisely equal on the equinox. Refraction at sunrise and sunset and the Sun's apparent diameter cause daylight to extend a few minutes past 12 hours. Nevertheless, the equinox remains a conventional marker used in calendars to define seasonal boundaries in many cultures and historical systems, and it has conveyed astronomical significance since antiquity.

Geographically, the concept applies globally, but the observable equality of day and night varies with latitude. Near the equator, day and night remain roughly equal year-round, whereas at higher latitudes the difference between day and night is more pronounced at other times of the year.