The word equinox is derived from the Latin word aequinoctium meaning equal night. An Equinox occurs two times during the year when 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 length or 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 vernal or spring equinox will occur either on March 20 or March 21, depending on the earth's position in a given year. This day also marks the beginning spring. In the northern hemisphere, the autumnal equinox will occur either on September 22 or September 23, again depending on the earth's position in a given year. This day also marks the beginning of autumn.
http://webclipart.about.com/od/flowerclipart/ss/The-Four-Seasons.htm">The Four Seasons