# How Many Light Years Away From the Sun Are We?

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Viktor T. Toth:

You could look up the numbers easily enough: The astronomical unit (the average distance between the Earth and the Sun) is defined as exactly 149,597,870,700 meters, whereas the light year is defined as the distance covered by light in a Julian year (365.25 days), which means exactly 9,460,730,472,580,800 meters. The rest is a simple division.

But instead of typing 149597870700/9460730472580800 into Google for you (yes, Google can serve as your calculator) let me show you how you can estimate the answer to this question reasonably accurately without looking up anything or using any calculator, so long as you remember two useful numbers.

It is not hard to remember that the Earth-Sun distance is roughly 150 billion meters; that is, the digits 15 are followed by 10 zeroes.

It is also not too hard to remember that a light year is roughly 10,000 trillion meters; that is, the digit 1 is followed by 16 zeroes.

So 15 followed by 10 zeroes divided by 1 followed by 16 zeroes … that's the same as 15 followed by no zeroes at all divided by 1 followed by 6 zeroes; or 15 parts in a million. And that answer is correct within about 5.5 percent.

[The error is due mainly to the fact that the light year is overestimated by more than 5 percent, so we underestimated the ratio. "Boosting" that 15 parts in a million by an extra 5 percent gives 15.8 parts in a million, which is actually quite accurate. But at this point, you’re probably better off using a calculator after all.]

# What's the Difference Between Cement and Concrete?

Picture yourself walking down a city block. The sidewalk you follow may be obscured by shuffling feet and discarded gum, but it’s clearly made from something hard, smooth, and gray. What may be less clear is the proper name for that material: Is it concrete or cement? Is there even a real difference between the two words?

Though they’re often used interchangeably, concrete and cement describe different yet related elements of the blocks, flooring, and walls that make up many everyday structures. In simple terms, concrete is the name of the gray, gritty building material used in construction, and cement is an ingredient used in concrete.

Cement is a dry powder mixture that looks much different from the wet stuff poured out of so-called cement trucks. It’s made from minerals that have been crushed up and mixed together. Exactly what kind of minerals it’s made from varies: Limestone and clay are commonly used today, but anything from seashells to volcanic ash is suitable. After the ingredients are mixed together the first time, they’re fired in a kiln at 2642°F to form strong new compounds, then cooled, crushed, and combined again.

Cement
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This mixture is useless on its own. Before it’s ready to be used in construction projects, the cement must be mixed with water and an aggregate, such as sand, to form a moldable paste. This substance is known as concrete. It fills whatever mold it’s poured into and quickly hardens into a solid, rock-like form, which is partly why it’s become the most widely-used building material on Earth.

So whether you’re etching your initials into a wet sidewalk slab, power-hosing your back patio, or admiring some Brutalist architecture, you’re dealing with concrete. But if you ever happen to be handling a chalky gray powder that hasn’t been mixed with water, cement is the correct label to use.

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# Why Do You Stop Feeling Tired As Soon As You Climb Into Bed?

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There are few situations more frustrating: After a day spent nodding off at your desk, on the train, and on your couch, you suddenly can't sleep the moment you crawl into bed. It's not that you aren't tired or have insomnia, necessarily. Like a curse designed just to torture you, the sleeplessness only seems to occur when you're in your own bed at home, a.k.a. the place where you'd prefer to do your sleeping.

Doing things that aren't sleeping in bed isn't the only way to develop conditioned arousal. If there are other factors keeping you up at night—like thoughts about your day, or that cup of coffee you had at 8 p.m.—they can lead to the same result. Your brain starts to associate being in bed with tossing and turning all night, so even if those mental and physical stimulants go away, the muscle memory of being awake in bed remains.

Conditioned arousal is a vicious cycle that can't be broken in one night. The only way to manage it, according to the American Psychological Association (APA), is to minimize behaviors that contribute to poor sleep habits and to reserve your bed for sleeping (though sex is OK, according to the APA).

If you're a nighttime scroller, browse apps in a different room before getting into bed, or skip checking your phone at the end of the day altogether. When you spend more than 20 minutes struggling to fall asleep in bed, get up and move to a different part of the house until you get sleepy again; this will stop your brain from strengthening the association between your bed and feeling restless. The results won't be instant, but by sticking to a new sleep routine, you should eventually train your body to follow healthier patterns.

Of course, combating conditioned arousal alone isn't always effective. In people with conditions like anxiety and insomnia, intrusive thoughts and genetic factors can prevent them from falling asleep even under ideal circumstances. In such cases, the help of a medical professional may be required to sleep more soundly.

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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