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Wednesday, June 12, 2024

Sky Tonight—March 20, The 2011 March equinox

Courtesy of EarthSky
A Clear Voice for Science

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The March equinox happens today at 23:21 Universal Time, which is 17:21 (5:21 p.m.) Mountain Daylight Time for us in the U.S. The March equinox signals the beginning of spring in the northern hemisphere and autumn in the southern hemisphere. It marks that special moment when the sun crosses the celestial equator going from south to north.

How do I translate Universal Time into my time?

Celebrate the change of seasons, by watching the moon team up with the star Spica and the planet Saturn tonight. Spica is the brightest star in the constellation Virgo the Maiden.

The equinox represents a point on 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.


Earth, as seen from the sun, on March equinox

Graphic on right from Earth and Moon Viewer

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.

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U.S. Naval Observator Astronomical Information center

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Simostronomy Blog

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The York County Astronomical Society

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James S McDonnell Planetarium

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