UPDATE: In 2015, the autumnal equinox is on September 23 in the Northern Hemisphere. The examine moment is 8:21 UTC, or 3:21 a.m. here in Chicago, which is located in the Central Time Zone. This post was written in 2013 and the date and time is for that year. In 2016, the autumnal equinox will take place on September 22nd.
The autumnal equinox marks the first day of fall. The start of fall is marked by the Autumnal Equinox. My tween wasn't so clear on what an equinox is or means. Apparently she missed that day in science. If you did, too, or just need a refresher, check out these eight facts about the equinox.
What is the autumnal equinox?
It is the time at which the sun is directly over the equator. After that point, the sun's direct rays then shift into the southern hemisphere for the next six months.
When is the autumnal equinox this year?
The precise start of fall is actually 4:44 p.m. Eastern Daylight Time (3:44 Central Daylight Time here in Chicago), on September 22 is the exact moment of the autumnal equinox.
Is it autumn everywhere?
Nope. There's actually a movement to start calling this event the September Equinox to "avoid North Hemisphere bias." because it is only autumnal in the Northern Hemisphere. In the Southern Hemisphere it marks the start of spring. The Spring Equinox is also called the Vernal Equinox. Between equinoxes are solstices. Read about the Summer Solstice here.
Why not? And why is it getting colder?
After the September equinox, due to the tilt of the Earth, the sun will shine more directly on the southern half of our planet than it does on the northern half, meaning that the Northern Hemisphere gets colder after this equinox as the Southern Hemisphere warms up, according to space.com. Less direct sunlight means less heat.
What does the word "equinox" mean?
The word "equinox" comes from the Latin words aequus (equal) and nox (night) according to almanac.com. Day and night are both approximately 12 hours each on an equinox.
Where does the sun rise and set on the equinox?
According to EarthSky.org, the sun rises in the due east and sets in the due west on the equinox. That's because, on the equinox, the sun is above the celestial equator, the imaginary line above the true equator of the Earth.
Do the days get shorter after the equinox?
Yes. They've actually been getting shorter since after the Summer Solstice. Here in Chicago, we will lose almost 3 minutes of daylight each day until the Winter Solstice. Check out the chart in this Washington Post piece to see how cities farther north lose even more daylight, but cities to the South lose less. For example, Miami loses only 90 seconds of daylight per day in the fall.
Need a more visual explanation?
Check out this National Geographic video here or this video featuring Dr. Laura Danly explains the Autumnal Equinox at the Griffith Observatory in Los Angeles:
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