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Why Do the Sun and the Moon Appear to be the Same Size?

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It’s just kind of a strange coincidence. The diameter of the Sun is about 400 times larger than the Moon's, but it is also roughly 400 times farther away from Earth. These two qualities almost cancel each other out, and the Sun usually winds up looking either the same size or just a wee bit bigger than the moon to us.

How do we know how far away and how big the Sun and Moon are in the first place?

Since we can’t just wrap a giant measuring tape around a celestial body, we have to get a little creative.

Look up at the sky and imagine it as a big dome, which goes all away around the Earth, with the Sun, Moon and stars projected onto its surface. This is a neat little hypothetical that astronomers call the celestial sphere. If we think of the sphere as being 360 degrees, like a circle, we can talk about the things we see on the dome in terms of angular size (that is, the “visual diameter” of the object measured as an angle) and angular distance (the distance separating two objects measured as an angle).

We’ve been able to get a better handle on the actual distance thanks to spaceflight and laser and radar technology. For the Moon, Apollo astronauts left retro-reflectors on its surface, which we bounced lasers off of from 1969 to 2009. By timing how long it took for the laser beam to make the trip there and back, astronomers could work out the Moon’s physical distance.

For the Sun, we tried the same sort of thing and bounced radio waves off it, but the star’s outer atmosphere scattered the waves too much for accurate measurements. Astonomers switched to a more reliable method that uses German astronomer Johannes Kepler’s Third Law of Planetary Motion, observations of the other planets’ movements, and radar measurements of their distances to work out an indirect, but accurate, measurement for the distance of the Sun.

Once we have the angular size of something in the sky and its actual distance, we can use them to calculate the object’s physical diameter.

But don't get used to them looking this way

The Sun and Moon didn’t always look so close in size from Earth, though, and they won’t continue to do so forever. The Moon is moving away from Earth at a rate of about 4 centimeters per year. In a few billion years, our descendants won’t be looking up at the same sky we are, and the size difference between the Sun and Moon will be much more noticeable.

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Big Questions
What Are Curlers Yelling About?
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WANG ZHAO/AFP/Getty Images

Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

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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Why You Should Never Take Your Shoes Off On an Airplane
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What should be worn during takeoff?

Tony Luna:

If you are a frequent flyer, you may often notice that some passengers like to kick off their shoes the moment they've settled down into their seats.

As an ex-flight attendant, I'm here to tell you that it is a dangerous thing to do. Why?

Besides stinking up the whole cabin, footwear is essential during an airplane emergency, even though it is not part of the flight safety information.

During an emergency, all sorts of debris and unpleasant ground surfaces will block your way toward the exit, as well as outside the aircraft. If your feet aren't properly covered, you'll have a hard time making your way to safety.

Imagine destroying your bare feet as you run down the aisle covered with broken glass, fires, and metal shards. Kind of like John McClane in Die Hard, but worse. Ouch!

Bruce Willis stars in 'Die Hard' (1988)
20th Century Fox Home Entertainment

A mere couple of seconds delay during an emergency evacuation can be a matter of life and death, especially in an enclosed environment. Not to mention the entire aircraft will likely be engulfed in panic and chaos.

So, the next time you go on a plane trip, please keep your shoes on during takeoff, even if it is uncomfortable.

You can slip on a pair of bathroom slippers if you really need to let your toes breathe. They're pretty useless in a real emergency evacuation, but at least they're better than going barefoot.

This post originally appeared on Quora. Click here to view.

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