March equinox! Happy spring or fall
What is an equinox? Each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless motion in orbit. The equinox is also an event you can think about as happening on the imaginary dome of our sky.
The Earth-centered view is that the celestial equator is a great circle dividing Earth’s sky into Northern and Southern Hemispheres. The celestial equator wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator, to enter the sky’s Northern Hemisphere.
The Earth-in-space view is that, because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern hemispheres trade places in receiving the sun’s light and warmth most directly, as Earth orbits the sun. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun.
At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are approximately equal in length. The word equinox comes from the Latin aequus (equal) and nox (night).
But, since Earth never stops moving around the sun, these days of equal sunlight and night will change quickly.
The video below was the Astronomy Picture of the Day for March 19, 2014. APOD explained:
At an equinox, the Earth’s terminator – the dividing line between day and night – becomes vertical and connects the north and south poles. The time-lapse video [below] demonstrates this by displaying an entire year on planet Earth in 12 seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere — and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken — and will take — around the sun.
Where should I look to see signs of the equinox in nature? Forget about the weather for a moment, and think only about the daylight. In terms of daylight, the knowledge that spring is here – and summer is coming – is everywhere now, on the northern half of Earth’s globe.
If you live in Earth’s Northern Hemisphere, you’ve likely been noticing the earlier dawns and later sunsets for some weeks now.
Also notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the north. Responding to the change in daylight, birds and butterflies are migrating back northward, too, along with the path of the sun.
The longer days do bring with them warmer weather. People are leaving their winter coats at home. Trees are budding, and plants are beginning a new cycle of growth. In many places, spring flowers are beginning to bloom.
Meanwhile, in the Southern Hemisphere, the days are getting shorter and nights longer. A chill is in the air. Fall is here, and winter is coming!
Does the sun rise due east and set due west at the equinox? Yes, it does. And that’s true no matter where you live on Earth, because we all see the same sky.
No matter where you are on Earth, 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.
At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration above shows. This illustration 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.
So enjoy the 2017 spring equinox on March 20 – an event that happens on our sky’s dome – and a seasonal marker in Earth’s orbit around the sun!
Bottom line: In 2017, the vernal equinox came on March 20 at 10:29 UTC, or 5:29 a.m. Central Daylight Time for us in the central U.S. Translate to your time zone here.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. “Being an EarthSky editor is like hosting a big global party for cool nature-lovers,” she says.
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