There are two equinoxes each year. Although, as the etymology of the word suggests, the term originally referred to the two times of year when night and day are the same length, the modern definition of the equinoxes is the instant at which the center of the Sun crosses declination 0 (i.e., the celestial equator, which is the projection of the Earth's equator onto the sky). With this definition, day and night are not quite the same length on the equinoxes due to (1) refraction of light from the sun as it passes through the Earth's atmosphere and (2) the fact that sunrise and sunset are calculated from the limb (not the center). Both of these effects slightly lengthen "day" relative to "night."

Autumnal Equinox, Egg, Precession of the Equinoxes, Solstice, Summer Solstice, Tropical Year, Vernal Equinox, Winter Solstice