eSky: Vernal Equinox
Here is a clear and concise explanation of the vernal equinox, the first day of spring, which falls on March 20 at 5:15 GMT, but on March 19 in most of the United States, depending on your time zone. You will find a diagram of the earth and its positions at the solstices and equinoxes throughout the year as well as a table showing the time of the vernal equinoxes through 2020. There are also links to definitions of many of the specialized vocabulary words used in describing this yearly occurrence.
http://www.glyphweb.com/esky/concepts/vernalequinox.html