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Equinox sun rises due east, sets due west

Flattened sunset photo above by Helio de Carvalho Vital.

The March 2018 equinox happens on March 20 at 16:15 Universal Time, which is 11:15 a.m. Central Daylight Time for us in the central U.S. Translate to your time zone. The March equinox signals spring in the Northern Hemisphere and autumn in the Southern Hemisphere. On this day, the sun rises due east and sets due west.

It may seem counterintuitive. But it’s true no matter where you live on Earth.

To understand the due-east and due-west rising and setting of an equinox sun, you have to think of the reality of Earth in space. First think of why the sun’s path across our sky shifts from season to season. It’s because our world is tilted on its axis with respect to its orbit around the sun.

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

Now think about what an equinox is. It’s 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. And it also, of course, represents a point on Earth’s orbit.

The 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. At the March equinox, the sun crosses the celestial equator to enter the sky’s Northern Hemisphere.

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

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 for 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 great circle on the http://oneminuteastronomer.com/9905/celestial-equator-poles/celestial sphere above the true equator of the Earth.

And that’s why the sun rises due east and sets due west, for all of us, at the equinox. The equinox sun is on the celestial equator. No matter where you are on Earth, the celestial equator intersects your horizon at due east and due west.

Where does the celestial equator intersect your horizon? No matter what your latitude is, it intersects your horizon at points due east and due west. Read more about how an observer’s latitude affects your visible sky.

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.

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: each middway point between the sun’s lowest and highest path.

That is, the sun rises due east and sets due west on the day of the equinox, as seen from everywhere on the globe.

The day arc of the sun, every hour, during the equinox as seen on the celestial dome, from the pole. Image via Tau’olunga at Wikimedia Commons.

Bottom line: The 2018 March equinox comes on March 20 at 16:15 Universal Time – or at 11:15 a.m. Central Daylight Time for us in the central U.S. 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 2018

Hamal: Ancient equinox star

Read more: Understanding celestial coordinates

Bruce McClure

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