Astronomy Essentials

All you need to know: March equinox 2021

Three curved, dotted sun tracks across the sky from various heights above the horizon down to the horizon.
The path of the sun across our sky – from about noon to sunset – on 3 different days of the year, an equinox and the summer and winter solstices. Photographer Marcella Giulia Pace said: “I made these observations from Gatto Corvino village, Sicily, Italy. To show as much of the path as possible, I chose a field where the western horizon was clearly visible and always shot from the same spot, every 10 minutes, beginning at true local noon.” Image via Earth Science Picture of the Day (March 19, 2020)/ Universities Space Research Association.

The March equinox – also called the vernal equinox – marks the beginning of the spring season in the Northern Hemisphere and the autumn season in the Southern Hemisphere. The March 2021 equinox will arrive on March 20 at 09:37 UTC or 4:37 a.m. Central Daylight Time.

No matter where you are on Earth, the equinox brings us a number of seasonal effects, which many nature enthusiasts notice.

Equal day and night? At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are often said to be equal in length. In fact, the word equinox comes from the Latin aequus (equal) and nox (night). For our ancestors, whose timekeeping was less precise than ours, day and night likely did seem equal. But we today know it’s not exactly so. Read more: Are day and night equal at the equinox?

Fastest sunsets at the equinoxes. The fastest sunsets and sunrises of the year happen at the equinoxes. We’re talking here about the length of time it takes for the sun to sink below the horizon. Read more: Fastest sunsets happen near equinoxes

Sun rises due east and sets due west? Here’s another equinox phenomenon. You might hear that the sun rises due east and sets due west at the equinox. True? Yes. It’s the case no matter where you live on Earth. At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration below shows. This illustration shows the sun’s location on the celestial equator, every hour, on the day of the equinox. No matter where you are on Earth – except at the Earth’s 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. That’s why the sun pretty much 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. Read more: Sun rises due east and sets due west

Hemispherical grid with red dots around equator.
The day arc of the sun, every hour, at the equinox, as seen on the (imaginary) celestial sphere surrounding Earth. At the equinox, the sun is directly above Earth’s equator. Image via Tau’olunga/ Wikimedia Commons.
Diagram with drawings of Earth in four positions around its orbit, representing equinoxes and solstices.
An equinox is an event that takes place in Earth’s orbit around the sun.

And there are also plenty more effects in play around the time of the March equinox, which all of us can notice. In the Northern Hemisphere, the March equinox brings earlier sunrises, later sunsets, softer winds and sprouting plants.

Meanwhile, you’ll find the opposite season – later sunrises, earlier sunsets, chillier winds, dry and falling leaves – south of the equator.

The equinoxes and solstices are caused by Earth’s tilt on its axis and ceaseless motion in orbit. You can think of an equinox as happening on the imaginary dome of our sky, or as an event that happens in Earth’s orbit around the sun.

The Earth-centered view is that the celestial equator is a great circle dividing Earth’s sky into its Northern and Southern Hemispheres. The celestial equator is an imaginary line wrapping 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 1/2 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.

White ellipse, with central white sun on black background and Earth globe moving around ellipse.
An animation of Earth as it orbits, with points marking both equinoxes and solstices, showing Northern Hemisphere seasons. Image via James O’Donoghue.
Four photos of half-lit Earth, left two at a slant and right two straight up and down.
Here are 4 photos of quarter-Earth seen from space, showing equinoxes and solstices. Image via Geosync.

Since Earth never stops moving around the sun, the position of the sunrise and sunset – and the days of approximately 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 can you look to see signs of the equinox in nature? Everywhere! 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 – permeates all of nature on the northern half of Earth’s globe.

Notice the arc of the sun across the sky each day. You’ll find that 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!

Small red-headed bird sitting on a twig with many 5-petaled pink flowers with prominent stamens.
View at EarthSky Community Photos. | Stephanie Becker in San Francisco Bay Area, Moraga, California, captured this spring-y photo of a finch perched on a cherry tree in bloom on March 1, 2021. She wrote: “A beautiful house finch in the golden sunlight, eating a blossom from our tree.” Thank you, Stephanie!

Bottom line: In 2021, the equinox is on March 20 at 09:37 UTC; translate to your time. In the Northern Hemisphere, notice how daylight begins to lengthen, and in the Southern Hemisphere, how it begins to shorten.

A Chinese perspective on the spring equinox

March 19, 2021
Astronomy Essentials

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