Equinox

An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator. The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). 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, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality, primitive cultures noted the day when the Sun rises due East and sets due West, and indeed this happens on the day closest to the astronomically defined event. In the northern hemisphere, the equinox in March is called the Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox. The dates are slightly variable, dependent as they are on the leap year cycle. Because the Moon (and to a lesser extent the planets) cause the 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 by its declination. The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°. Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the durations of the day and the night are practically equal and the word 'equinox' comes from Latin Aequus, meaning 'equal', and Nox, meaning 'night'. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar, Hindu and the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn. 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. For the same reason, this is also the time when the Sun rises for an observer at one of Earth's rotational poles and sets at the other; for a brief period, both North and South Poles are in daylight. (This is possible because atmospheric refraction 'lofts' the Sun's apparent disk above its true position in the sky.)