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Why Are There Only 28 Days in February?

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Thirty days hath September,
April, June, and November.
All the rest have 31,
Except for February,
Which got the short stick because it's cold and no one likes it.  

Well, something to that effect. Some believe February once boasted 29 days and that Augustus Caesar stole a day so he could add it to August, which was named for him. (If there’s a month named after you, why not milk it?) But that’s a myth. Rather, February has 28 days because, to the Romans, the month was an afterthought. In the 8th century BCE, they used the Calendar of Romulus, a 10-month calendar that kicked the year off in March (with the spring equinox) and ended in December. January and February didn’t even exist:

Martius: 31 days
Aprilius: 30 days
Maius: 31 days
Junius: 30 days
Quintilis: 31 days
Sextilis: 30 days
September: 30 days
October: 31 days
November: 30 days
December: 30 days

Tally up those numbers, and you’ll see a problem—the year is only 304 days long. Back then, winter was a nameless, monthless period that no one cared for much. (Planters and harvesters used the calendar as a timetable. To them, winter was useless and wasn’t worth counting.) So for 61 days out of the year, Romans could ask “What month is it?” and you could correctly answer, “None!”

King Numa Pompilius thought that was stupid. Why have a calendar if you’re going to neglect one-sixth of the year? So in 713 BCE, he lined the calendar up with the year’s 12 lunar cycles—a span of about 355 days—and introduced January and February. The months were added to the end of the calendar, making February the last month of the year.

But no Roman calendar would be complete without some good old-fashioned superstition mixed in! The Romans believed even numbers were unlucky, so Numa tried to make each month odd. But to reach the quota of 355, one month had to be even. February ended up pulling the short stick, probably because it was simply the last month on the list. (Or as Cecil Adams puts it, “If there had to be an unlucky month, better make it a short one.”) Numa’s calendar ended up looking like this:

Martius: 31 days
Aprilius: 29 days
Maius: 31 days
Iunius: 29 days
Quintilis: 31 days
Sextilis: 29 days
September: 29 days
October: 31 days
November: 29 days
December: 29 days
Ianuarius: 29 days
Februarius: 28 days

Of course, a 355-day calendar had its bugs. After a few years went by, the seasons and months would fall out of sync. So to keep things straight, the Romans would occasionally insert a 27-day leap month called Mercedonius. The Romans would erase the last couple days of February and start the leap month on February 24—further evidence no one ever cared much for the month.  

This caused headaches everywhere. The leap month was inconsistent, mainly because Rome’s high priests determined when it would arrive. Not only did they insert Mercedonius haphazardly, but the priests (being politicians) abused the power, using it to extend the terms of friends and trim the terms of enemies. By Julius Caesar’s time, the Roman people had no clue what day it was.

So Caesar nixed the leap month and reformed the calendar again. (To get Rome back on track, the year 46 BCE had to be 445 days long!) Caesar aligned the calendar with the sun and added a few days so that everything added up to 365. February, which by now was at the top of the calendar, kept its 28 days. We can only imagine it’s because Caesar, like everyone before and after him, just wanted it to be March already. 

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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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Big Questions
How Are Speed Limits Set?
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When driving down a road where speed limits are oppressively low, or high enough to let drivers get away with reckless behavior, it's easy to blame the government for getting it wrong. But you and your fellow drivers play a bigger a role in determining speed limits than you might think.

Before cities can come up with speed limit figures, they first need to look at how fast motorists drive down certain roads when there are no limitations. According to The Sacramento Bee, officials conduct speed surveys on two types of roads: arterial roads (typically four-lane highways) and collector streets (two-lane roads connecting residential areas to arterials). Once the data has been collected, they toss out the fastest 15 percent of drivers. The thinking is that this group is probably going faster than what's safe and isn't representative of the average driver. The sweet spot, according to the state, is the 85th percentile: Drivers in this group are thought to occupy the Goldilocks zone of safety and efficiency.

Officials use whatever speed falls in the 85th percentile to set limits for that street, but they do have some wiggle room. If the average speed is 33 mph, for example, they’d normally round up to 35 or down to 30 to reach the nearest 5-mph increment. Whether they decide to make the number higher or lower depends on other information they know about that area. If there’s a risky turn, they might decide to round down and keep drivers on the slow side.

A road’s crash rate also comes into play: If the number of collisions per million miles traveled for that stretch of road is higher than average, officials might lower the speed limit regardless of the 85th percentile rule. Roads that have a history of accidents might also warrant a special signal or sign to reinforce the new speed limit.

For other types of roads, setting speed limits is more of a cut-and-dry process. Streets that run through school zones, business districts, and residential areas are all assigned standard speed limits that are much lower than what drivers might hit if given free rein.

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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