Autumn Equinox: The Science Behind the First Day of Fall

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iStock

On September 22, the Sun will shine directly over the equator—the midpoint of the Earth. (For 2018, this moment will happen at 9:54 p.m. ET.) The whole world will thus experience a day and night of equal length. In the Northern Hemisphere, we call this the autumn equinox. It marks the first day of fall. Around the world, people are marking the day with ceremonies, some of them ancient (and some less so).

You might be wondering two things: 1. Why on almost every other day of the year (the vernal equinox being the other exception) do different parts of the world have days and nights of differing length? 2. What do they call the day in the Southern Hemisphere?

A DAY AT THE BEACH

The answer to each of these questions resides in the Earth's axial tilt. The easiest way to imagine that tilt is to think about tanning on the beach. (Stay with me here.) If you lay on your stomach, your back gets blasted by the Sun. You don't wait 30 minutes then flop over and call it a day. Rather, as you tan, every once in a while, you shift positions a little. Maybe you lay a bit more on one side. Maybe you lift a shoulder, move a leg a little. Why? Because you want the Sun to shine directly on a different part of you. You want an even tan.

It might seem a little silly when you think about it. The Sun is a giant fusion reactor 93 million miles away. Solar radiation is hitting your entire back and arms and legs and so on whether or not you adjust your shoulder just so. But you adjust, and it really does improve your tan, and you know this instinctively.

People light candles during the autumn equinox celebration at Neris River waterfront in Vilnius, Lithuania after sunset on September 21, 2013.
People light candles during the autumn equinox celebration at Neris River waterfront in Vilnius, Lithuania after sunset on September 21, 2013.
PETRAS MALUKAS, AFP/Getty Images

The Earth works a lot like that, except it's operating by physics, not instinct. If there were no tilt, only one line of latitude would ever receive the most direct blast of sunlight: the equator. As the Earth revolved around the Sun, the planet would be bathed in sunlight, but it would only be the equator that would always get the most direct hit (and the darkest tan). But the Earth does have a tilt. Shove a pole through the planet with one end sticking out the North Pole and one end sticking out the South, and angle the whole thing by 23.5 degrees. That's the grade of Earth's tilt.

Now spin our little skewered Earth and place it in orbit around the Sun. At various points in the orbit, the Sun will shine directly on different latitudes. It will shine directly on the equator twice in a complete orbit—the fall and spring equinoxes—and at various points in the year, the most direct blast of sunlight will slide up or down. The highest latitude receiving direct sunlight is called the Tropic of Cancer. The lowest point is the Tropic of Capricorn. The poles, you will note, are snow white. They have, if you will, a terrible tan—and that's because they never receive solar radiation from a directly overhead Sun (even during the long polar summer, when the Sun never sinks below the horizon).

WHEN DO THE SEASONS CHANGE?

A Maya priestess conducts an autumn equinox ceremony at El Salvador's Cihuatan Archeological Park.
A Maya priestess conducts an autumn equinox ceremony at El Salvador's Cihuatan Archeological Park.
Jose CABEZAS, AFP/Getty Images

The seasons have nothing to do with the Earth's distance from the Sun. Axial tilt is the reason for the seasons. The Sun is directly over the Tropic of Cancer (66.5 degrees latitude in the Northern Hemisphere) on June 21 or 22. When that occurs, the Northern Hemisphere is in the summer solstice. The days grow long and hot. As the year elapses, the days slowly get shorter and cooler as summer gives way to autumn. On September 21 or 22, the Sun's direct light has reached the equator. Days and night reach parity, and because the Sun is hitting the whole world head-on, every latitude experiences this simultaneously.

On December 21 or 22, the Sun is directly over the Tropic of Capricorn in the Southern Hemisphere, meaning the Northern Hemisphere is receiving the least sunlight it will get all year. The Northern Hemisphere is therefore in winter solstice. Our days are short and nights are long. Parity will again be reached on March 21 or 22, the vernal equinox for the Northern Hemisphere, and the whole process will repeat itself.

Members of The Druid Order of London conduct a ceremony on Primrose Hill to celebrate the Autumn Equinox on September 22, 2008 in London, England.
Members of The Druid Order of London conduct a ceremony on Primrose Hill to celebrate the Autumn Equinox on September 22, 2008 in London, England. The Druid Order of London, which was founded in Oxford in 1245, has been conducting the Autumn Equinox ceremony on Primrose Hill since 1717.
Matt Cardy, Getty Images

Now reverse all of this for the Southern Hemisphere. When we're at autumnal equinox, they're at vernal equinox. Happy first day of spring, Southern Hemisphere!

And welcome to fall, Northern Hemisphere! Enjoy this long day of sunlight, because dark days are ahead. You'll get less and less light until the winter solstice, and the days will grow colder. Take solace, though, in knowing that the whole world is experiencing the very same thing. Now it's the Southern Hemisphere's turn to get ready to spend some time at the beach.

This story first ran in 2016.

The 3 Best Mattresses You Can Order Online Right Now

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iStock.com/diego_cervo

So you're ready to ditch your lumpy old mattress for a newer, more comfortable model, huh? (Hello, memory foam.) If you don’t feel like fighting off the crowds while shopping at your local mattress outlet, there are plenty of great online deals you can take advantage of—and you don't have to sift through a thousand reviews to find the best one. Check out our buying guide to the three comfiest mattresses on the market right now from Leesa, Allswell, and Linenspa. Several models are on sale for Memorial Day weekend, so grab a deal while you can.

1. Leesa’s Universal Adaptive Feel Memory Foam Cooling Mattress

Leesa's universal adaptive feel memory foam cooling mattress
Leesa, Amazon

Do you kick off the covers in the middle of the night because you get too hot? Try Leesa’s 10-inch-tall cooling mattress. It has three unique foam layers: The first layer is designed to keep you cool, while the second offers body-contouring comfort, and the third helps relieve pressure and provide core support. The “adaptive feel” label refers to the fact that it adapts to your body no matter what your preferred sleeping position may be. The mattress comes in twin through California king sizes, with prices starting at $425.

Buy it on Amazon or from Leesa's website. The latter is currently offering 15 percent off plus two free pillows with any purchase for Memorial Day.

2. Allswell’s Luxe Hybrid Mattress

This extra-thick mattress is so popular that it frequently sells out from Allswell and at other retailers. It packs 12 inches of high-end material into one mattress, including quilted memory foam and an outer layer designed keep you cool in hot weather. Similar mattresses tend to go for thousands of dollars, but the Luxe Hybrid retails for far less. Prices start at $345 for a twin, $485 for a full, $585 for a queen, or $745 for a king. (Not sure about the difference in mattress sizes? We've got you covered.)

Buy it from Allswell's website or at Walmart starting at $345. Through May 27, you can get 15 percent off mattresses and 30 percent off bedding using the code SUMMERTIME.

3. Linenspa’s memory foam and innerspring hybrid mattress

The Linenspa memory foam and innerspring hybrid mattress
Linenspa, Amazon

Amazon customers swear by Linenspa’s hybrid mattress, which comes in sizes twin ($144) through California king ($288). Ideal for people who like medium-firm mattresses, it combines the benefits of memory foam with the support one gets from a traditional innerspring mattress. The standard version is 8 inches thick, but Linenspa also offers a 10-inch version in all sizes. (We also love the company's incredibly affordable down-alternative duvet, which is only $30 on Amazon.)

Buy it on Amazon, where the 8-inch queen mattress is regularly on sale for up to 20 percent off. It's $225 right now.

Mental Floss has affiliate relationships with certain retailers and may receive a small percentage of any sale. But we choose all products independently and only get commission on items you buy and don’t return, so we’re only happy if you’re happy. Thanks for helping us pay the bills!

What's the Difference Between Memorial Day and Veterans Day?

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iStock/flySnow

It may not be easy for some people to admit, but certain national holidays often get a little muddled—namely, Memorial Day and Veterans Day. In fact, the U.S. Department of Veterans Affairs sees the confusion often enough that they spelled out the distinction on their website. The two days are held six months apart: Veterans Day is celebrated every November 11, and Memorial Day takes place on the last Monday of May as part of a three-day weekend with parades and plenty of retail sales promotions. You probably realize both are intended to acknowledge the contributions of those who have served in the United States military, but you may not recall the important distinction between the two. So what's the difference?

Veterans Day was originally known as Armistice Day. It was first observed on November 11, 1919, the one-year anniversary of the end of World War I. Congress passed a resolution making it an annual observance in 1926. It became a national holiday in 1938. In 1954, President Dwight D. Eisenhower changed the name from Armistice Day to Veterans Day to recognize veterans of the two world wars. The intention is to celebrate all military veterans, living or dead, who have served the country, with an emphasis on thanking those in our lives who have spent time in uniform.

We also celebrate military veterans on Memorial Day, but the mood is more somber. The occasion is reserved for those who died while serving their country. The day was first observed in the wake of the Civil War, where local communities organized tributes around the gravesites of fallen soldiers. The observation was originally called Decoration Day because the graves were adorned with flowers. It was held May 30 because that date wasn't the anniversary for any battle in particular and all soldiers could be honored. (The date was recognized by northern states, with southern states choosing different days.) After World War I, the day shifted from remembering the fallen in the Civil War to those who had perished in all of America's conflicts. It gradually became known as Memorial Day and was declared a federal holiday and moved to the last Monday in May to organize a three-day weekend beginning in 1971.

The easiest way to think of the two holidays is to consider Veterans Day a time to shake the hand of a veteran who stood up for our freedoms. Memorial Day is a time to remember and honor those who are no longer around to receive your gratitude personally.

Have you got a Big Question you'd like us to answer? If so, send it to bigquestions@mentalfloss.com.

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