The vernal equinox occurs on Friday, March 20th, 2009, at 5:44 am, MDT. This is the official beginning of spring in the northern hemisphere. At this time the Sun is crossing the celestial equator from the southern half of the sky to the northern half of the sky. On the date of the equinox, the Sun will rise due east and set due west.
We notice that days are getting longer and warmer, nights are shorter and less chilly.
In the southern hemisphere, seasons are reversed. March 20th marks the first day of autumn. The Sun is still crossing from the southern half of the sky to the northern half, but this results in longer nights, colder days and the coming of winter.