equinoxen

An equinox is one of two points in the year when the Sun appears to cross the celestial equator, producing nearly equal lengths of day and night. The term derives from Latin aequinoctium, meaning “equal night.” There are two equinoxes each year: the vernal (spring) equinox and the autumnal (fall) equinox. In the Northern Hemisphere, the vernal equinox occurs around March 20 or 21 and the autumnal around September 22 or 23. In the Southern Hemisphere, the seasons are opposite, so these dates mark autumn and spring there respectively.

Astronomically, equinoxes arise because Earth’s axis is tilted by about 23.44 degrees relative to its orbit around the Sun. As the planet travels, the Sun appears to move north or south across the sky and crosses the imaginary line of the celestial equator at two points each year. At the vernal equinox the Sun is at 0 degrees ecliptic longitude; at the autumnal equinox it is at 180 degrees.

In practice, day length is not exactly equal to night length on the calendar date of an equinox due to atmospheric refraction and the fact that the Sun is a disk rather than a point. The equality typically occurs only for a moment near local noon and can vary by location.

Culturally, equinoxes have long served as seasonal markers and occasions for rituals and agricultural activity. They are distinct from solstices, which mark the year’s longest and shortest days. Equinoxes thus anchor both astronomical observation and seasonal calendars.