Courtesy of EarthSky
A Clear Voice for Science
The March equinox marks that special moment when the sun crosses celestial equator, going from south to north. It happens today at 17:32 Universal Time, or 11:32 a.m. Mountain Daylight Time. The March equinox signals the beginning of spring in the northern hemisphere and autumn in the southern hemisphere.
The equinox is a hallmark in Earth’s orbit, but it is also an event that happens on the imaginary dome of Earth’s sky. The imaginary celestial equator is a great circle dividing the imaginary celestial sphere into its northern and southern hemispheres. The celestial equator wraps the sky directly above Earth’s equator, and at the equinox today, the sun crosses the celestial equator, to enter the sky’s northern hemisphere. 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 could not have understood the equinoxes as we do. They did not 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 is 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.
Written by Deborah Byrd