The equinox is an event that takes place in Earth’s orbit around 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. So Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. And we all know Earth doesn’t orbit upright but is instead tilted on its axis by 23 1/2 degrees. If you think of it from an Earth-in-space perspective, you have to think of Earth in orbit around the sun. Image via Tau’olunga/ Wikimedia Commons ( CC BY-SA 2.5). That is – as seen from the equator on the day of an equinox – a tree stands in the center of its own shadow. And see that the tree’s shadow at noon is cast straight down. Note that the sun is at its highest point at noon. Also showing are twilight suns (in red) down to -18 degrees altitude. This illustration shows the day arc of the equinox sun, as seen from Earth’s equator. At the equinox, the sun crosses the celestial equator to enter the sky’s Northern Hemisphere. The celestial equator is an imaginary line wrapping the sky directly above Earth’s equator. If you think of it from an Earth-centered perspective, you can think of the celestial equator as a great circle dividing Earth’s sky into its Northern and Southern Hemispheres. 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 equinoxes and solstices are caused by Earth’s tilt on its axis and ceaseless motion in orbit. Meanwhile, you’ll find the opposite season – later sunrises, earlier sunsets, chillier winds, dry and falling leaves – south of the equator. In the Northern Hemisphere, the March equinox brings earlier sunrises, later sunsets and sprouting plants. More March equinox effectsĪnd there are also plenty more effects in play around the time of the March equinox that all of us can notice. At the equinox, the sun is directly above Earth’s equator. Read more: Sun rises due east and sets due west The day arc of the sun, every hour, at the equinox, as seen on the (imaginary) celestial sphere surrounding Earth. Voila! The sun rises due east and sets due west. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west. That point marks the intersection of your horizon with the celestial equator: the imaginary line above the true equator of the Earth. 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. This illustration shows the sun’s location on the celestial equator, every hour, on the day of the equinox. At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration below shows. In fact, it’s the case no matter where you live on Earth, with the exception of the North and South Pole. You might hear that the sun rises due east and sets due west at the equinox. Read more: Fastest sunsets happen near equinoxes Sun rises due east and sets due west? We’re talking here about the length of time it takes for the whole sun to sink below the horizon. The fastest sunsets and sunrises of the year happen at the equinoxes. Read more: Are day and night equal at the equinox? Fastest sunsets at the equinoxes For our ancestors, whose timekeeping was less precise than ours, day and night likely did seem equal. In fact, the word equinox comes from the Latin aequus (equal) and nox (night). Night and day are often said to be equal in length. Equal day and night on the equinox?Īt the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Last chance to get a moon phase calendar! Only a few left. No matter where you are on Earth, the equinox brings us a number of seasonal effects, noticeable to nature lovers around the globe. Note: The March equinox – aka the vernal equinox – marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. When is the next one? It’ll happen at 21:24 UTC (4:24 p.m. What is it? The March equinox marks the sun’s crossing above the Earth’s equator, moving from south to north. Happy equinox, y’all! Here are satellite views of Earth on the solstices and equinoxes, via NASA Earth Observatory.
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