Autumn Equinox: The Science Behind the First Day of Fall

Smileus/iStock via Getty Images
Smileus/iStock via Getty Images

Today, September 23, the whole world will experience a day and night of equal length when the sun shines directly over the equator—the midpoint of Earth. (For 2019, this moment will happen at 3:50 a.m. ET.) In the Northern Hemisphere, we call this the fall or autumn equinox, and it marks the first day of fall. Around the world, people celebrate the day with ceremonies, some of them ancient, and some less so.

You might be wondering two things. Why on almost every other day of the year (the vernal or spring equinox being the exception) do different parts of the world have days and nights of differing length? And, what do they call the fall equinox in the Southern Hemisphere?

How the Fall Equinox Works

Sunlight on yellow fall foliage
allou/iStock via Getty Images

The answer to each of these questions resides in 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. 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.

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 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 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°. 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 spring and fall 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 does fall begin?

Sunlight on golden fall foliage
Kesu01/iStock via Getty Images

The seasons have nothing to do with 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° 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.

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.

Not-So-Fancy Feast: Your Cat Probably Would Eat Your Rotting Corpse

Tycson1/iStock via Getty Images
Tycson1/iStock via Getty Images

Cat enthusiasts often cite the warmth and companionship offered by their pet as reasons why they’re so enamored with them. Despite these and other positive attributes, cat lovers are often confronted with the spurious claim that, while their beloved furry pal might adore them when they’re alive, it won’t hesitate to devour their corpse if they should drop dead.

Though that’s often dismissed as negative cat propaganda spread by dog people, it turns out that it’s probably true. Fluffy might indeed feast on your flesh if you happened to expire.

A horrifying new case study published in the Journal of Forensic Sciences offers the fresh evidence. The paper, first reported by The Washington Post, documents how two cats reacted in the presence of a corpse at Colorado Mesa University’s Forensic Investigation Research Station, or body farm, where the deceased are used to further forensic science for criminal investigations.

The study’s authors did not orchestrate a meeting between cat and corpse. The finding happened by accident: Student and lead author Sara Garcia was scanning surveillance footage of the grounds when she noticed a pair of cats trespassing. The cats, she found, were interested in the flesh of two corpses; they gnawed on human tissue while it was still in the early stages of decomposition, stopping only when the bodies began leaching fluids.

The cats, which were putting away one corpse each, didn’t appear to have a taste for variety, as they both returned to the same corpse virtually every night. The two seemed to prefer the shoulder and arm over other body parts.

This visual evidence joins a litany of reports over the years from medical examiners, who have observed the damage left by both cats and dogs who were trapped in homes with deceased owners and proceeded to eat them. It’s believed pets do this when no other food source is available, though in some cases, eating their human has occurred even with a full food bowl. It’s something to consider the next time your cat gives you an affectionate lick on the arm. Maybe it loves you. Or maybe it has something else in mind.

[h/t The Washington Post]

Wolf Puppies Play Fetch, Too, Study Finds

Christina Hansen Wheat
Christina Hansen Wheat

It took thousands of years of selective breeding for wolves to become the Golden Retrievers you see at dog parks today. Domesticated dogs are very different from their wild counterparts, but according to a new study, they may have a surprising trait in common. Researchers found that some wolf puppies are willing to play fetch with total strangers, suggesting that following human commands is intrinsic to canines.

For their study in the journal iScience, researchers from Stockholm University in Sweden set out to find how domestication affects behaviors in young wolves. They raised litters of wolf and dog pups separately from 10 days old and placed them in various scenarios.

When the scientists tested how the wolf puppies would respond to a game of fetch, they expected to be ignored. Chasing a ball and bringing it back requires understanding human commands and obeying them—abilities that were thought to only have emerged in dogs post-domestication.

The first two wolf groups met expectations by showing little interest in the toy, but something different happened with the third set. Three eight-week old pups went after the ball and brought it back when they were encouraged to do so. This was the case even when the person giving the commands was someone they had never met before.

Even though most of the puppies didn't play fetch, the fact that those who did belonged to the same litter indicates a "standing variation" for a retrieving trait in wolves. "When you talk about a specific trait in the context of standing variation, it means that there is variation for the expression of this trait within a given population," co-author Christina Hansen Wheat tells Mental Floss. "For our study it suggests that, while probably rare, standing variation in the expression of human-directed behavior in ancestral populations could have been an important target for early selective pressures exerted during dog domestication." In other words, ancient people seeking to domesticate wolves might have focused on some wolves' innate ability to follow human commands.

The first dogs were domesticated as far back as 33,000 years ago. Over millennia, humans have selected for traits like loyalty, friendliness, and playfulness to create the modern dog, but these new findings could mean that the dog's earliest canine ancestors were genetically predisposed toward some of these behaviors.

"All three litters were brought up under identical and standardized conditions across years," Hansen says of the pups in the study. "With this significant effort to control the environmental conditions, it is likely that the differences in behavior across litters to some extent have a genetic basis."

After raising the dog and wolf litters for three years and completing that part of their study, the researchers will continue to analyze their data to see if there are any other adorable (or weird) traits the two groups share.

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