<P> An equinox is commonly regarded as the moment the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun, which occurs twice each year, around 20 March and 22 - 23 September . In other words, it is the point in which the center of the visible sun is directly over the equator . </P> <P> However, because the moon (and to a lesser extent the other planets) cause the true motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun's more regular ecliptic longitude rather than latitude . The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0 ° and 180 ° . There are tiny (up to 11⁄4 arcsecond) variations in the Sun's latitude (discussed below), which means the Sun's center is rarely precisely over the equator under the official definition . The two understandings of the equinox can lead to discrepancies of up to 69 seconds . </P> <P> On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet . They are not exactly equal, however, due to the angular size of the Sun and atmospheric refraction . The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). </P> <P> The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator . As a result, the northern and southern hemispheres are equally illuminated . The word comes from Latin Aequus, meaning "equal", and Nox, meaning "night". </P>

What is it called when day and night are equal length