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The Science of the Solstice

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Today at 4:48 UTC (Coordinated Universal Time) we marked the winter solstice. (Those of you in Eastern Standard Time experienced it at 11:48 p.m. on December 21.) It's the longest night and shortest day of the year in the Northern Hemisphere. So what is a solstice, and why is it affecting your day?

The Earth doesn't spin vertically like a top as it orbits the Sun. Instead, imagine a line drawn directly through the Earth at its poles. Tilt the Earth along that line at 23.5 degrees. That is the Earth's axial tilt. The direction of the Earth's axial tilt doesn't change as it orbits the Sun (for our timescale, at least), which means the Sun is directly overhead different parts of the Earth at different times of the year. For half the year, the Southern Hemisphere is "closer" to the Sun and its radiation; for the other half, the Northern Hemisphere is closer. You might recognize this phenomenon as the reason we have seasons.

The axial tilt also affects the length of days. Consider a map of the Earth, with horizontal lines dividing it latitudinally, and vertical lines longitudinally. Among the best-known latitudes are the equator, at 0 degrees; the Northern Hemisphere's Tropic of Cancer, at 23.5 degrees; and the Southern Hemisphere's Tropic of Capricorn, at -23.5 degrees. Recognize that number? Now we're getting somewhere.

The winter solstice in the Northern Hemisphere occurs when the Tropic of Capricorn experiences an overhead Sun, which makes that latitude's day 13 hours, 27 minutes long. Because less of the Northern Hemisphere is in direct sunlight, the higher up the Earth you get from the Tropic of Capricorn, the shorter the day. The day is 12 hours long at the equator; just over 10.5 hours at the Tropic of Cancer; and 0 hours at the Arctic Circle, which is in total darkness.

If the lack of sunlight on the shortest day of the year has you weeping into your whiskey, take heart: Beginning tomorrow, the days slowly start to lengthen again. It may be the first day of winter, but spring is on the way. On March 20, 2016, the Sun will be directly "over" the equator. This is the vernal equinox. The Sun is again over the equator in September—the autumnal equinox. And exactly halfway between them is the day that sun worshippers probably consider the best one of the year: the summer solstice, when the Sun is directly over the Tropic of Cancer. 

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Here's What You Need to Know Before Getting Inked or Pierced, According to Doctors
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Getting inked or pierced is a rite of passage for many teens and young adults. But before getting that belly ring or butterfly on your back, experts want you to be aware of the risks, which are reviewed in a new clinical report from the American Academy of Pediatrics (AAP). According to NPR, it's the first set of recommendations the professional association has ever released on the practices.

Forthcoming in the October 2017 issue of Pediatrics and available online, the report provides a general assessment of the types and methods used to perform body modifications, along with potential health and social consequences. Here are a few main takeaways:

—It's unclear how often tattoos cause health complications, but they're generally believed to be rare, with the greatest risk being infection. One recent study found that nanoparticles in ink can travel to and linger in lymph nodes for an extended period. That said, you should check with your doctor to make sure all of your immunizations are up to date before getting either a tattoo or piercing, and that you're not taking any immunity-compromising medicines.

—Before shelling out your hard-earned cash on a tattoo, make sure it's something you'll likely still appreciate in five to 10 years, as it costs anywhere from $49 to $300 per square inch to remove a tattoo with lasers. (This might provide all the more incentive to opt for a small design instead of a full sleeve.)

—About half of people 18 to 29 years of age have some kind of piercing or tattoo, according to Dr. Cora Breuner, who is chair of the AAP committee on adolescence. Many individuals don't regret getting one, with some reporting that tattoos make them feel sexier. But while millennials appear to be cool with metal and ink, hiring managers might not be too pleased: In a 2014 survey of 2700 people, 76 percent said they thought a tattoo or piercing had hindered their chances of getting hired, and nearly 40 percent thought tattooed employees reflected poorly on their employers.

—Not all tattoo parlors are created equal, as each state has different regulations. Keep a close eye on whether your artist uses fresh disposable gloves, fresh needles, and unused ink poured into a new container. This helps prevent infection.

—The advice is similar for getting pierced: Make sure the piercer puts on new, disposable gloves and uses new equipment from a sterile container. Tongue piercings can cause tooth chippings, so be careful of that—and remove any piercings before you play contacts sports.

The full report is available online.

[h/t NPR]

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The Body
7 Essential Facts About the Pelvis
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The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

The pelvis, which crooner Elvis was famous for thrusting around in ways that raised eyebrows, is not actually a single body part but a term that refers to a collection of bones, muscles and organs below the waist. We spoke to Katherine Gillogley, department chair of obstetrics and gynecology with Mercy Medical Group in Sacramento, California, for these seven facts about the pelvis.

1. SO WHAT IS THE PELVIS, EXACTLY?

"The pelvis refers to the lower abdominal area in both men and women," Gillogley says. "An important function of the pelvis region is to protect organs used for digestion and reproduction, though all its functions are crucial," she says. It protects the bladder, both large and small intestines, and male and female reproductive organs. Another key role is to support the hip joints.

2. THE PELVIC BONES FORM A BASIN.

Four bones come together to form a bowl-like shape, or basin: the two hip bones, the sacrum (the triangle-shaped bone at the low back) and the coccyx (also known as the tailbone).

3. YOUR PELVIC FLOOR IS LIKE A TRAMPOLINE.

At the bottom of the pelvis lies your pelvic floor. You don't have to worry about sweeping it, but you might want to do Kegel exercises to keep it strong. The pelvic floor is like a "mini-trampoline made of firm muscle," according the Continence Foundation of Australia. Just like a trampoline, the pelvic floor is flexible and can move up and down. It also creates a surface (floor) for the pelvic organs to lie upon: the bladder, uterus, and bowels. It has holes, too, for vagina, urethra, and anus to pass through.

4. IT PLAYS A KEY PART IN WALKING.

Anyone who has ever broken a pelvic bone or pulled a pelvic muscle will know just how key a role the pelvis plays in such functions as walking and standing. "The pelvis also acts as a solid foundation for the attachment of the spinal column and legs," says Gillogley.

5. THE FEMALE PELVIS STARTS OUT LARGER, BUT NARROWS OVER TIME.

Gillogley says that the female pelvis "tends to be larger and wider" than the male, most likely to accommodate a baby during pregnancy and to make childbirth possible. However, women's pelvises narrow as they age, suggesting that they start out wider to accommodate childbearing and then shift when that is no longer necessary. A shifting pelvis shape is thought to be a key part of our evolutionary history, as it changed as when we began walking upright.    

6. PREGNANCY CHANGES THE PELVIS FOREVER.

During pregnancy the body secretes a hormone known as relaxin to help the body accommodate the growing baby and soften the cervix. However, what happens is, "the joints between the pelvic bones actually loosen and slightly separate during pregnancy and childbirth," Gillogley says. Sometimes, however, relaxin can make the joints too loose, causing a painful syndrome known as symphysis pubis dysfunction (SPD), causing the pelvic joint to become unstable, causing pain and weakness in the pelvis, perineum and even upper thighs during walking and other activities. Many women with the condition have to wear a pelvic belt. It usually resolves after pregnancy is over, though physical therapy may be necessary.

7. IT'S ACCIDENT PRONE.

According to the American Association for the Surgery of Trauma, about 8 to 9 percent of blunt trauma includes pelvic injury, Gillogley says. "These accidents include falls, motor vehicle crashes, bicycle accidents, and pedestrians being struck by moving vehicles. With these serious injuries, pelvic bones can fracture or dislocate and sometimes bladder injury even occurs." So take care with your pelvis—in worse-case scenarios, breaks of the pelvic bones can require pins, rods, and surgery to fix.

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