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Why is This The Last Time Hanukkah and Thanksgiving Will Occur on The Same Day?

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We measure time by the way things rotate. It takes one day for the Earth to spin one full rotation on its own axis. It takes a little less than 28 days for the moon to rotate once around the Earth. It takes a little more than 365 days for the Earth to go around the sun. It would be nice if all this rotating was perfectly synced up, but it isn’t, and since we can’t control the motions of the planets, we are stuck with a messy calendar system that needs a bit of adjusting now and then.

Leap years are one way we correct for mismatches. Having months of varying lengths is another. In the calendar we use, the months are only roughly inspired by moon cycles. If we measured months by the moon and the year by the sun, the seasons would start to drift and eventually the Fourth of July would be in the dead of winter.

The Hebrew calendar, used to determine the dates of Jewish holidays, follows the cycles of the moon more closely. Months are 29 or 30 days, but complicated adjustments are made, including the addition of an extra month every so often, in order to ensure that certain holidays show up in the correct season. Hanukkah is supposed to start on the 25th of Kislev, which after drift and adjustment, ends up falling between the end of November and the end of December. This year it’s on November 28th, the earliest date it can be, which also happens to be Thanksgiving.

The 25th of Kislev will fall on November 28th again, but not when November 28th is also Thanksgiving (fourth Thursday of the month). Also, there is a tiny difference in the way adjustments to align the seasons are made in the two calendars, so tiny that they only matter after they accumulate for a few hundred years. By the time enough years have passed to get to the point where Hanukkah would fall on a November 28th that was also Thanksgiving again, that tiny divergence will have pushed Hanukkah over the line to the next day. And there’s no going back to the earlier date. The earliest date for Hanukkah will slowly keep getting later...

...unless another big adjustment by decree is made, which is what has happened every couple thousand years in the history of calendar keeping. If Hanukkah and Thanksgiving coincide again, it won’t be according to the systems as they are in place now, but through our improvised efforts to keep up with the sloppy, syncopated spinning of the spheres.

If you want more specific details on the adjustments, here is a more in-depth explanation of the numbers.

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Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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