By Deborah Byrd
EarthSky Communications

Friday, March 20, 2009
The March equinox — when the sun crosses celestial equator, moving from south to north — happens today according to U.S. clocks. It happened at 11:44 Universal Time, or 5:44 a.m. Mountain Standard Time this morning for us in the Rocky Mountains.

The equinox is a hallmark in Earth’s orbit, but it’s also an event that happens on the imaginary dome of Earth’s sky. The celestial equator is an imaginary line on our sky. It wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator. All these imaginary components, and yet what happens at every equinox is very real, as real as the sun’s passage across the sky each day and as real as the change of the seasons.
 
Our ancestors couldn’t have understood the equinoxes as we do. They didn’t understand them as events that occur in the course of Earth’s yearly orbit around the sun. But if they were observant — and some were very observant indeed — they surely marked today as being midway between the sun’s lowest path across the sky in winter and highest path across the sky in summer.
 
If they thought in terms of four directions, they might also have learned a fact of nature that occurs whenever there’s an equinox, whenever the sun crosses the celestial equator. Since the celestial equator intersects the horizon at due east and due west, the sun rises due east and sets due west on the day of the equinox, as seen from everywhere on the globe.