equinoccio

An equinox is an astronomical event in which the plane of Earth's equator passes through the center of the Sun, causing the Sun to be directly above the equator. The term derives from Latin aequinoctium, from aequus meaning equal and nox meaning night. On an equinox, day and night are approximately equal in length, though not exactly, due to atmospheric refraction and the Sun’s apparent disk.

There are two equinoxes each year: the vernal (spring) equinox and the autumnal (fall) equinox. The vernal equinox signals the start of spring in the Northern Hemisphere and occurs around March 20 or 21. The autumnal equinox marks the start of autumn around September 22 or 23. In the Southern Hemisphere, the seasons are opposite, so the dates correspond to autumn and spring there.

Astronomically, an equinox occurs when the Sun’s apparent declination is 0°, meaning the Sun crosses the celestial equator. The equinox is distinct from the solstices, which occur when the Sun reaches its northernmost or southernmost position relative to the celestial equator.

Culturally, equinoxes have long played a role in calendars and rituals, serving as markers for agricultural cycles and seasonal celebrations in many regions. The exact dates shift slightly from year to year due to calendar adjustments and orbital variabilities.