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Why equinox sun rises due east and sets due west

The ecliptic and celestial equator intersect at the spring and autumn equinox points. The ecliptic represents the sun's apparent yearly path in front of the constellations of the Zodiac.  The celestial equator is simply a circle drawn around the sky, above Earth's equator.

The ecliptic and celestial equator intersect at the spring and autumn equinox points. The ecliptic represents the sun’s apparent yearly path in front of the constellations of the Zodiac. The celestial equator is simply a circle drawn around the sky, above Earth’s equator.

Flattened sunset photo above by Helio de Carvalho Vital.

Why 2016 has the earliest spring since 1896.

The March 2016 equinox happens on March 20 at 4:30 Universal Time, which is March 19 at 11:30 p.m. Central Daylight Time for us in the central U.S. The March equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It’s the earliest spring since 1896! Plus, on this day, the sun rises due east and sets due west.

We get many questions at this time of year about the sun’s due east and due west rise and set. It seems counterintuitive. But it’s true no matter where you live on Earth, because the sun shines on the celestial equator – Earth’s equator projected onto the great dome of sky – as seen from everywhere around the globe.

The equinox represents a point on Earth’s orbit, but it’s also an event that happens on the imaginary dome of Earth’s sky. It marks that special moment when the sun crosses the celestial equator going from south to north. 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 this equinox, the sun crosses the celestial equator, to enter the sky’s Northern Hemisphere. All these components are imaginary, 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.

No matter where you are on Earth (except the North and South Poles), you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth. Read more: Understanding celestial coordinates

At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration at left shows. This illustration (which is by Tau’olunga) shows the sun’s location on the celestial equator, every hour, on the day of the equinox.

That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west.

This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.

If you do this, you’ll be able to use those landmarks to find those cardinal directions in the weeks and months ahead, long after Earth has moved on in its orbit around the sun, carrying the sunrise and sunset points northward.

Seasonal Diagram

The seasons result from the Earth's rotational axis tilting 23.5 degrees out of perpendicular to the ecliptic - or Earth's orbital plane.

Our ancestors may not have understood the equinoxes and solstices 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.

Here's the moon on the night of March 18, 2016 as seen in India.  From around the world now, the moon is waxing to full.  This full moon will bring a subtle kind of lunar eclipse, a penumbral eclipse on March 23.

Here’s the moon on the night of March 18, 2016 as seen in India. From around the world now, the moon is waxing to full. This full moon will bring a subtle kind of lunar eclipse, a penumbral eclipse on March 23.

Bottom line: The 2016 March equinox comes on March 20 at 4:30 Universal Time – or on March 19 at 11:30 p.m. Central Daylight Time for us in the central U.S. The March equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. On this day, the sun rises due east and sets due west.

A Chinese perspective on the spring equinox

Everything you need to know: Vernal or spring equinox 2016

Hamal: Ancient equinox star

Bruce McClure

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