Equinox is either of the two times during the year that the sun crosses the celestial equator, an imaginary line through the sky, and appears directly above the equator, the imaginary line that divides the earth into the northern and southern hemispheres. When this occurs, the length of the day and the night are approximately equal at every place on earth. While the earth orbits around the sun, the position of the sun changes in relation to the equator. Between the March, or vernal, equinox and the September, or autumnal, equinox, the sun appears north of the equator. It appears south of the equator in the time between the September equinox and the March equinox.
In the northern hemisphere, the autumnal equinox will occur either on September 22 or September 23, depending on the earth's position in a given year. This day also marks the beginning of autumn. The word equinox is derived from the Latin word aequinoctium (equal night).
On the autumnal equinox, many Pagans celebrate Mabon as one of the eight sabbats (a celebration based on the cycles of the sun). Mabon celebrates the second harvest and the beginning of winter preparations.