The new astronomical season is coming. What exactly happens with the Earth on the day of the equinox? What is the difference for the two hemispheres? We’ll explain it to you in today’s article.
What happens during an equinox?
Although we already explained the term “equinox” in our previous articles, let’s go through it briefly one more time. There are two imaginary lines — the celestial equator and the ecliptic. The celestial equator divides the Earth’s celestial sphere into the Northern and the Southern hemispheres, and the ecliptic marks the annual path of the Sun. The moments when these lines coincide are called equinoxes.
In other words, the equinox marks the moment when the Sun crosses the celestial equator, and the length of night and day become nearly equal. This day, the Sun rises to due east and sets to due west, no matter your location.
How many equinoxes are there in a year?
There is a common delusion that there are four equinoxes a year, but in fact, there are only two of them. What is commonly mistaken for the equinox is called a solstice. The solstices bring the longest and the shortest days of a year, while on an equinox the amount of daylight and nighttime is approximately equal. To see if you can tell an equinox from a solstice, take our astronomical quiz.
What are the dates of equinoxes?
The first equinox of 2021 happens on March 20, 2021, at 05:37 a.m. EDT (09:37 GMT). By the way, the date can vary from year to year: the March equinox can take place on March 19, 20, or 21. For example, do you remember the last time it occurred on March 21? It’s quite challenging since the last time this happened was in 2007, and the next time it will happen in 2102!
Equinoxes in different hemispheres
In astronomical terms, the March equinox marks the first day of spring for those who live in the Northern Hemisphere and the first day of autumn for people in the Southern Hemisphere. This is why it’s called the vernal equinox in the northern latitudes. Vice versa, the Southern Hemisphere considers it as the autumnal equinox. The same happens with the September equinox.
There are a lot of differences for stargazers from the Northern and Southern hemispheres. Did you know that people in different parts of the world see different skies? Here is a quite informative article about observing the night skies from the Northern and Southern hemispheres.
The noon shadow experiment
Do you want to witness something you can see only twice a year? You’ll need a straight stick or a long wooden ruler, a protractor, and the Star Walk 2 app.
Find an open space, for example, a park or a parking lot. Open Star Walk 2, go to the menu, open the settings, and find the coordinates of your current location — you’ll need only the first number, before the “°” sign. Subtract this number from 90 — this will be the angle you need to place the stick in the ground at.
Then go back to the main screen of the app. Look at the compass icon in the upper left corner of your screen and turn your device to find south, if you’re in the Northern Hemisphere, and then point the stick in that direction. If you’re in the Southern Hemisphere, do the same to the north. Use the protractor to fix the stick in the ground at the angle you just calculated.
Wait till noon and see how the shadow of the stick disappears — at noon, the stick will have no shadow at all! You can only do this experiment twice a year — on the March equinox and the September equinox.
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