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 geometric center of the Sun's disk. 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 ‘equinox’ is derived from
the Latin. 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 equatorial 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. As a
consequence, according to a properly constructed and aligned sundial, the
daytime duration is 12 hours.
In the Northern Hemisphere, the
March equinox is called the vernal or spring equinox while the September
equinox is called the autumnal or fall equinox. In the Southern Hemisphere, the
reverse is true. The dates slightly vary due to leap years and other factors.
Hemisphere-neutral names are
northward equinox for the March equinox, indicating that at that moment the
solar declination is crossing the celestial equator in a northward direction,
and southward equinox for the September equinox, indicating that at that moment
the solar declination is crossing the celestial equator in a southward
direction.
Since the Moon (and to a lesser extent the planets) causes Earth's orbit to slightly vary from a perfect ellipse, the equinox is 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°.