The term Equinox (or Equinoxes) is the time when the sun crosses the plane of the earth’s equator, making night and day equal length — throughout all time zones, occurring around March 21st and September 22nd.
The point at which the Sun’s path crosses the celestial equator moving from south to north is called the vernal equinox. The vernal equinox marks the zero point in both the equatorial and ecliptic coordinate systems; horizontal angular distances are measured eastward from this point.
The vernal equinox is also known as the first point of Aries because when first devised some 2,000 years ago this point occurred at the beginning of Aries in the zodiac. Because of the westward precession of the equinoxes, the vernal equinox is now located at the beginning of Pisces. The point at which the Sun’s path crosses the celestial equator moving from north to south is called the autumnal equinox.
On the day of an equinox, the Sun looks like a disk rather than a point of light.
It happens twice a year, every year.