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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.
在遥远的将来，居住在银河系中心附近小行星上的外星人与你取得联系，想要拜访你。
And so you tell them, "Of course! I'm free any day this week."
所以你告诉他们，“当然可以！我这个星期每天都有空。”
But they don't know what that means - they live in an asteroid belt and have a totally different kind of calendar and,
但是他们并不知道这是什么意思—他们生活在小行星带，他们用的日历和我们的完全不同，
to them, the concept of a "day" is very… alien.
对他们来说，‘天’这个概念非常陌生。
So you tell them that a day is how long it takes for the Earth to complete a full rotation about its axis.
所以你就跟他们解释，一天就是地球绕地轴旋转一周所需的时间。
And as they input that into their computer simulation, you notice a fatal flaw in your explanation:
当他们把这个输入进他们的计算机模拟时，你发现你的解释中有一个致命的缺陷：
as the earth rotates relative to the distant aliens, it moves a little bit around the sun,
当地球相对于遥远的外星旋转时，它会绕着太阳转，
and by the time it makes its way to the other side of the sun, our "daytime" and "nighttime" have somehow switched,
并且当它到达太阳的另一边时，我们的“白天”和“夜晚”不知怎么就调换了，
with the sun directly overhead when one day changes to the next, rather than in the middle of the day!
当昼夜交换之际，太阳直接转到了我们的头顶而不是中间！
This is not what we mean when we talk about calendar days.
这不是我们所说的日历天。
What you've actually described to the aliens is called a Stellar day
你向外星人描述的那一天叫做“恒星日”
and it's measured with respect to a distant, more or less stationary reference point far off in space -
并且它是相对于一个遥远的，或多或少固定的参考点来测量的—
but our concept of a day has more to do with the sun, not the galactic center. So you try again.
但是我们关于一天的概念更多的是以太阳为参考点，而不是银河系中心。所以你又试着解释一遍。
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.
这一次，你告诉他们，当地球人仰望天空时，地球每转一次，太阳就有一段时间是在最高点。
And you say that a day is the time it takes for the Sun to get back to the highest point.
并且你告诉他们一天是太阳回到最高点所需的时间。
And so the Asteroid-ians tap away on their instruments,
所以小行星群敲击着他们的仪器，
calibrating them to your insightful specifications until you notice that their day counter isn't staying in sync with your clock!
将他们的时间和你的校准，直到你发现他们的天数计数器和你的时钟不同步！
It's starting the new day earlier, and earlier and earlier each… day. And then later, and later, and later.
新的一天开始得越来越早，一天比一天早。然后越来越晚。
This isn't a bug in their programming - it's a feature… of the Earth's orbit.
这不是他们程序中的漏洞，而是地球轨道的一个特性。
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.
你向他们描述的实际上是太阳日，和钟表上的日子不一样。
Solar days use the sun as a reference point for when "noon" is,
太阳日以太阳作为“正午”的参考点，
but the length of time between when the sun is highest isn't constant -
但太阳何时到达最高点的时间间隔是变动的
it changes up or down by a minute over the course of the year.
它在一年周期中的变化幅度为一分钟。
This discrepancy is due to the complications of the earth's orbit being elliptical and the earth's spin axis being tilted.
这种差异是由于地球的轨道是椭圆形的，而地球的自转轴是倾斜的。
If we used solar days in everyday life, we'd either need to have calendars and clocks that changed the number of minutes
如果我们在日常生活中使用太阳日，我们要么需要日历和时钟
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
根据一年中的时间来改变一天中的分钟和秒数，或者我们需要根据一年的时间
(or changed the number of seconds in an hour) depending on the time of year.
来改变一秒长度的时钟（或者改变一小时内秒数）。
And sundials kind of automatically do this! But they have other… drawbacks.
日晷可以自动做到这一点！但它们也有其他的缺陷。
Anyway, changing the length of a second or the number of seconds in an hour isn't particularly appealing for regular – or interplanetary – use.
无论如何，改变一秒的长度或一小时内的秒数不论对我们还是他们都不是特别有吸引力。
And so you tell the asteroidaliens that a day is - more or less - an invented time period that is 24 hours long,
所以你可以告诉小行星外星人一天—或多或少—是一个虚构的时间段，其长度为24小时，
where each hour is 33 trillion oscillations of a special kind photon emitted by a cesium atom.
其中每小时有33万亿个由铯原子发射的特殊光子振荡。
And if they want to know why a day is just defined to be a fixed time period
如果他们想知道为什么一天是一个固定的时间段
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.
以及这个时间段和地球自转有什么关系，你可以把MinuteLabs的互动视频给他们。
It will guide anyone and everyone through the details of solar, stellar, and standard 24 hour days;
它将指导任何人和每个人了解太阳、恒星和标准24小时的细节；
how they're related; and how the orbit of the earth affects them.
它们之间的关联以及地球轨道对它们的影响。
Not only that, but it also lets you play around with different orbits to see how it changes the length of those days!
不仅如此，它还可以让你在不同的轨道上玩耍，看看它是如何改变那些天的长度的！
The link is in the video description, or you can just go to minutelabs.io and look for the "What is a Day?" lab,
连接在视频描述下方或者你可以登陆minutelabs.io寻找"What is a Day?"实验室，
and you'll be fully prepared to coordinate a visit with aliens… no matter what day that may be.
无论什么时候，你都要做好充分的准备去拜访外星人。
