Autumn Equinox: The Science Behind the First Day of Fall

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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.

What Happened to the Physical Copy of Martin Luther King's 'I Have a Dream' Speech?

AFP, Getty Images
AFP, Getty Images

On August 28, 1963, Martin Luther King Jr. stood on the steps of the Lincoln Memorial and gave a speech for the ages, delivering the oratorical masterpiece "I Have a Dream" to nearly 250,000 people.

When he was done, King stepped away from the podium, folded his speech, and found himself standing in front of George Raveling, a former Villanova basketball player who, along with his friend Warren Wilson, had been asked to provide extra security around Dr. King while he was speaking. "We were both tall, gangly guys," Raveling told TIME in 2003. "We didn't know what we were doing but we certainly made for a good appearance."

Moved by the speech, Raveling saw the folded papers in King’s hands and asked if he could have them. King gave the young volunteer the speech without hesitation, and that was that.

“At no time do I remember thinking, ‘Wow, we got this historic document,’” Raveling told Sports Illustrated in 2015. Not realizing he was holding what would become an important piece of history in his hands, Raveling went home and stuck the three sheets of paper into a Harry Truman biography for safekeeping. They sat there for nearly two decades while Raveling developed an impressive career coaching NCAA men’s basketball.

In 1984, he had recently taken over as the head coach at the University of Iowa and was chatting with Bob Denney of the Cedar Rapids Gazette when Denney brought up the March on Washington. That's when Raveling dropped the bomb: “You know, I’ve got a copy of that speech," he said, and dug it out of the Truman book. After writing an article about Raveling's connection, the reporter had the speech professionally framed for the coach.

Though he displayed the framed speech in his house for a few years, Raveling began to realize the value of the piece and moved it to a bank vault in Los Angeles. Though he has received offers for King’s speech—one collector wanted to purchase the speech for $3 million in 2014—Raveling has turned them all down. He has been in talks with various museums and universities and hopes to put the speech on display in the future, but for now, he cherishes having it in his possession.

“That to me is something I’ll always be able to look back and say I was there,” Raveling said in the original Cedar Rapids Gazette article. “And not only out there in that arena of people, but to be within touching distance of him. That’s like when you’re 80 or 90 years old you can look back and say ‘I was in touching distance of Abraham Lincoln when he made the Gettysburg Address.’"

“I have no idea why I even asked him for the speech,” Raveling, now CEO of Coaching for Success, has said. “But I’m sure glad that I did.”

Mardi Gras King Cake Ice Cream Is Coming to a Grocery Store Near You

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iStock.com/fstop123

Each year, Blue Bell Creamery celebrates Mardi Gras with a limited-edition ice cream that captures the spirit of the festival. Now, for the first time, the once-regional flavor will be available wherever Blue Bell ice cream is sold, KXXV reports.

Blue Bell debuted Mardi Gras King Cake in 2012, and for years it could only be found in places like Louisiana and Alabama. Exclusively available in the months leading up to Mardi Gras, or Shrove Tuesday, the ice cream has become a seasonal favorite in that part of the country. Blue Bell recently announced it's expanding the flavor in response to nationwide interest to cover its entire distribution area in the southern U.S.

Mardi Gras King Cake combines two old Blue Bell flavors: Mardi Gras, which came out in 2004, and King Cake, which launched in 2006. It features pastry pieces, cream cheese swirls, and colorful sprinkles in cinnamon cake-flavored ice cream. (The traditional plastic baby is missing from this version).

Half-gallons of Blue Bell's Mardi Gras King Cake ice cream can be found in stores starting the first week of 2019.

Carton of Blue Bell Mardi Gras King Cake ice cream.
Courtesy of Blue Bell

[h/t KXXV]

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