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  • In the distant future, aliens who live on asteroids near the center of the galaxy get

  • in touch and want to come visit you.

  • And so you tell them, “Of course!

  • I’m free any day this week.”

  • But they don’t know what that means - they live in an asteroid belt and have a totally

  • different kind of calendar and, to them, the concept of a “dayis veryalien.

  • So you tell them that a day is how long it takes for the Earth to complete a full rotation

  • about its axis.

  • And as they input that into their computer simulation, you notice a fatal flaw in your

  • explanation: as the earth rotates relative to the distant aliens, it moves a little bit

  • around the sun, and by the time it makes its way to the other side of the sun, ourdaytime

  • andnighttimehave somehow switched, with the sun directly overhead when one day

  • changes to the next, rather than in the middle of the day!

  • This is not what we mean when we talk about calendar days.

  • What youve actually described to the aliens is called a Stellar day and it’s measured

  • with respect to a distant, more or less stationary reference point far off in space - but our

  • concept of a day has more to do with the sun, not the galactic center.

  • So you try again.

  • This time you tell them that when Earthlings look up at the sky, for each turn of the earth

  • there’s a time when the sun is highest.

  • And you say that a day is the time it takes for the Sun to get back to the highest point.

  • And so the Asteroid-ians tap away on their instruments, calibrating them to your insightful

  • specifications until you notice that their day counter isn’t staying in sync with your

  • clock!

  • It’s starting the new day earlier, and earlier and earlier eachday.

  • And then later, and later, and later.

  • This isn’t a bug in their programming - it’s a featureof the Earth’s orbit.

  • What you actually described to them is called a Solar day, and it’s not the same thing

  • as a day kept by a clock.

  • Solar days use the sun as a reference point for whennoonis, but the length of

  • time between when the sun is highest isn’t constant - it changes up or down by a minute

  • over the course of the year.

  • This discrepancy is due to the complications of the earth’s orbit being elliptical and

  • the earth’s spin axis being tilted.

  • If we used solar days in everyday life, we’d either need to have calendars and clocks that

  • changed the number of minutes and seconds in a day depending on the time of year, or

  • we’d need to have clocks that changed the length of a second (or changed the number

  • of seconds in an hour) depending on the time of year.

  • And sundials kind of automatically do this!

  • But they have otherdrawbacks.

  • Anyway, changing the length of a second or the number of seconds in an hour isn’t particularly

  • appealing for regularor interplanetaryuse.

  • And so you tell the asteroidaliens that a day is - more or less - an invented time period

  • that is 24 hours long, where each hour is 33 trillion oscillations of a special kind

  • photon emitted by a cesium atom.

  • And if they want to know why a day is just defined to be a fixed time period and how

  • that time period actually relates to the rotation of the earth, you can send them over to our

  • interactive video over on MinuteLabs.

  • It will guide anyone and everyone through the details of solar, stellar, and standard

  • 24 hour days; how theyre related; and how the orbit of the earth affects them.

  • Not only that, but it also lets you play around with different orbits to see how it changes

  • the length of those days!

  • The link is in the video description, or you can just go to and look for

  • theWhat is a Day?”

  • lab, and youll be fully prepared to coordinate a visit with aliensno matter what day

  • that may be.

In the distant future, aliens who live on asteroids near the center of the galaxy get


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B1 中級

なぜ24時間ではない日があるのか (Why Some Days Aren’t 24 Hours)

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    林宜悉 に公開 2021 年 01 月 14 日