Equinox

An equinox in astronomy is the event when the Sun can be observed to be directly above the equator. The event occurs twice a year, around March 20 and September 23. More technically, the equinox happens when the Sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. In a wider sense, the equinoxes are the two days each year when the center of the Sun spends an equal amount of time above and below the horizon at every location on Earth. The word equinox derives from the Latin words aequus (equal) and nox (night).

In practice, at the equinox, the day is longer than the night. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. This is firstly because the Sun is not a single point of light, but appears as a disc. So when the center of the Sun is still below the horizon, the upper edge is already visible and sheds light. Furthermore, the atmosphere refracts light downwards, so even when the upper limb of the Sun is still below the horizon, its rays already reach around the horizon to the ground. These effects together make the day about 14 minutes longer than the night (at the equator, and more towards the poles). The real equality of day and night happens a few days towards the winter side of the equinox.

Note: Equal nights is an abstraction strictly speaking only true when Earth and Sun are considered particles, that is ignoring atmospheric effects, parallax, perturbations, et cetera, (as discussed below). It is not supposed to be exactly measurable.