Autumn Equinox: The Science Behind the First Day of Fall

iStock
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.

10,000 People Gathered at Stonehenge to Welcome the Summer Solstice

Finnbarr Webster, Getty Images
Finnbarr Webster, Getty Images

There are plenty of reasons to welcome the start of summer. Today, people visiting Stonehenge took that celebration to a whole new level.

The BBC reported that an estimated 10,000 people made the pilgrimage to the 5000-year-old site to partake in summer solstice festivities. "Stonehenge was built to align with the Sun, and to Neolithic people, the skies were arguably as important as the surrounding landscape," Susan Greaney, a senior historian at English Heritage, said in a statement. "At solstice we remember the changing daylight hours, but the changing seasons, the cycles of the Moon, and movements of the Sun are likely to have underpinned many practical spiritual aspects of Neolithic life."

These spiritual aspects are just one of the many fascinating facts about the summer solstice; the day is an extremely old calendar event recognized by ancient cultures across the globe. They include the Druids and other pagans, whose tradition of observing the solstice at Stonehenge has long been upheld by modern revelers.

Scientifically speaking, Stonehenge is an optimal viewing place for the solstice due to its structure. According to TIME, the site’s architects appeared to have kept both the summer and winter solstices in mind during its construction, as the positions of the stones are specifically tuned to complement the sky on both occasions.

The solstices were sacred to the pagans, whose modern-day followers continue to honor their rituals. Pagans in particular refer to the day as Litha, and mark it with activities such as meditation, fire rites, and outdoor yoga.

“What you’re celebrating on a mystical level is that you’re looking at light at its strongest," Frank Somers, a member of the Amesbury and Stonehenge Druids, said in 2014. "It represents things like the triumph of the king, the power of light over darkness, and just life—life at its fullest."

Those who were unable to make the journey can head over to the Stonehenge Skyscape project's website, where English Heritage’s interactive live feed fully captured the experience.

11 Patriotic Products to Celebrate the Fourth of July

Amazon
Amazon

Whether you’ll be lounging by the beach or grilling in your backyard this Independence Day, don’t forget to show off your all-American spirit with a little swag. Here are 11 products to help you celebrate Fourth of July—all of which would pair nicely with your collection of presidential bobbleheads.

1. Declaration Of Independence Signatures Mug; $15

Mug decorated with the signatures from the Declaration of Independence.
CafePress, Amazon

The names of the 56 signers of the Declaration of Independence are on display on this ceramic coffee mug. Studying the signatures each morning might inspire you to perfect your own John Hancock.

Find it: Amazon

2. Stars And Stripes Cufflinks; $55

Cufflinks with images of stars and stripes on them

Cufflinks Inc., Nordstrom

Brighten up your wardrobe with a splash of American pride. One half of this cufflinks set is striped red and white and the other is blue and spangled with stars. Together they make a patriotic accent piece.

Find it: Nordstrom

3. Fireworks Light Show Projector; $41

Portable fireworks light projector.
Uncle Milton, Amazon

If you want a low-hazard Fourth of July that still delivers thrills, consider getting a light show projector. When it’s pointed at a wall, the device displays vibrant fireworks animations accompanied by realistic sound effects. And it works indoors, so you can count on your Independence Day party ending with a light show no matter the weather forecast. (There's also a Disney version available for just $20.)

Find it: Amazon

4. Inflatable Cooler; $14

Inflatable American flag cooler filled with ice and drinks.
Fun Express, Amazon

When preparing for a picnic or barbecue this Fourth of July, don’t bother breaking out your tiny roll-away cooler. This 54-by-28-inch inflatable trough holds enough bottles and cans to keep your party going well past sunset. Once the cooler has been drained and deflated, it folds neatly for easy storage.

Find it: Amazon

5. American Trivia Cards; $15

Even players who paid attention in American history class might find this game challenging. Each box comes with 150 cards of United States trivia. Some questions—like what was the first state to grant women the right to vote—highlight important moments in our nation’s history. Others—like how many columns the Lincoln Memorial features—are a little more random.

Find it: UncommonGoods

6. U.S. Navy Blanket; $200

The U.S. military has been going to Faribault Woolen Mill Co. for its blankets for over 100 years. This one is crafted from 100 percent wool and weighs 3.5 pounds. The design is inspired by the blankets supplied on Navy ships—and if it’s tough enough for the Navy, you can bet it will withstand a Fourth of July picnic. We like the gray version, but it comes in several different styles and colors.

Find it: Faribault Woolen Mill Co.

7. American Flag Cornhole; $90

Two American flag cornhole boards with bean bags.
GoSports, Amazon

Baseball faces some stiff competition from cornhole for the title of No. 1 American pastime. This lawn set includes four blue bean bags, four red bean bags, and two boards decorated to resemble the American flag.

Find it: Amazon

8. U.S. Map Cutting Board; $20

Wooden cutting board in the shape of the U.S. map.
Totally Bamboo, Amazon

This 100 percent bamboo board eschews the traditional rectangle shape in favor of the outline of the American mainland. You can either keep it in the kitchen and use it as a cutting board or bring it out to the party as a serving platter for fruits, cheeses, and other hors d'oeuvres.

Find it: Amazon

9. Boston Tea Party Tea Sampler; $15

Six piles of loose-leaf tea.
Solstice Tea Traders, Amazon

A sip of one of the teas in this sampler will put you in touch with your inner revolutionary. Each of the six loose-leaf varieties—bohea black tea, oolong, congou black tea, souchong, singlo, and hyson—was among those tossed over the sides of British ships during the Boston Party. Whether you drink the tea or chuck it into the nearest harbor is up to you. (If you're looking for something to put on display, UncommonGoods also sells a slightly pricier, more elegant option with five teas.)

Find it: Amazon

10. State Map Prints; $27 and Up

A poster featuring a colorful map of Wisconsin
Bri Buckley, Society6

These unique prints by Bri Buckley highlight the beauty of each U.S. state in vibrant color. Each state map is available in both a modern and a vintage style, and like most Society6 art, the design is also available on tapestries, shirts, pillows, tote bags, and more.

Find it: Society6

11. Bald Eagle Pool Float; $15

A man and a woman ride a bald eagle pool float
Swimline, Amazon

Cool down the patriotic way this Fourth of July. This massive 8-foot-by-6-foot bald eagle pool float can fit up to two people comfortably, and will look great floating around your pool during your backyard barbecue.

Find it: Amazon

A version of this article first ran in 2017. It has been updated for 2019.

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