The vernal equinox, or March equinox, occurs on Sunday, March 20th, 2011 at 5:21 pm MDT. This marks the astronomical beginning of spring in the northern hemisphere.
It is on this day that the Sun crosses the celestial equator in its annual trek around the celestial sphere. Equinox means ‘equal night,’ and on the equinox we notice that day and night are each approximately 12 hours long. We also note that the Sun rises due east and sets due west.
As we move through spring into summer, we will notice that the Sun rises a little further north of east each morning, is progressively higher in the sky at noon, and sets a little further north of west each evening. We will also see that the Sun continues to rise a little earlier each morning and set a little later each evening, and the days getting longer and longer.
In the southern hemisphere the seasons are reversed. March 20th marks the beginning of autumn. The Sun is still rising and setting further northward from this day on, but for the southern hemisphere, that means the Sun is getting lower and lower in the sky, the days are getting shorter and shorter, and winter is fast approaching.