The intersection of the ecliptic and celestial equator where the Sun crosses the equator from north to south A time when every place on Earth has 12 hours of daylight and 12 hours of night
The sun is on the equator moving south, and marks the beginning of autumn, (in the sothern hemisphere this is the beginning of spring) At both equinoxes the day is the same length (12 hours on the earth) everywhere except at the poles