速成課堂 天文篇- 03 天空中的循環 Cycles in the Sky

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Under a dark sky, you can see thousands of stars.

在黑暗的天空下，你可以看到成千上萬的星星。

If you watch for a few hours, you can see them rise and set as the Earth rotates once a day.

如果你觀察上幾個小時，就可以看到它們會隨著地球每天的自轉而上升和下降。

And if you go outside the next night at the same time, you'll see that things'll look pretty much the same as they did the night before.

如果你第二天晚上在同一時間出門，你會發現情況和前一天晚上差不多。

The stars rise and set, Polaris hangs to the north, and so on.

星星會有起落，北極星掛在北方，等等。

One day hardly makes any difference to the sky's appearance.

一天的時間對天空的樣子幾乎沒有任何影響。

But what if you wait for another night?

但如果你再等一個晚上呢?

Or a week?

或者是一個星期？

If you're that patient, and observant, you can spot subtle changes in the sky.

如果你是那種有耐心、善於觀察的人，你就能發現天空中細微的變化。

Let's say a couple of weeks have passed.

假設幾周過去了。

Remember that star that was just over a tree in the east when the Sun set — the one that made you first notice the stars are rising and setting?

還記得當太陽落山時，東方的一棵樹上掛著的那顆星星嗎？是它讓你第一次注意到星星正在升起和落下。

Go look at it again.

再去看看它。

If you happen to be out at the same time, you'd expect that star to be in the same place.

如果你恰好在同一時間離開，你可能會料想那顆星在同一位置上。

But it's not.

但並非如此。

It's actually a bit higher above the tree.

它實際上比那棵樹高一點。

And if you look west, stars that were well off the horizon just after sunset last week are now lower.

如果你往西看，會發現上周太陽剛剛落山時還在地平線上的星星現在更低了。

If you wait a month, this effect is even more pronounced; all new constellations will be visible in the sky after sunset.

如果你等上一個月，這會更加明顯；日落之後，所有新的星座都將出現在天空中。

This is because the Earth is going around the Sun, literally changing our viewpoint on the sky.

這是因為地球繞著太陽轉，改變了我們視野之中天空的景象。

The Earth takes a year to orbit the Sun once.

地球繞太陽一周需要一年的時間。

Every day, it moves a little bit along its orbit.

每天，它都會沿著軌道移動一點點。

And as it does, from Earth's perspective, distant stars appear to move their positions relative to the sun.

從地球的角度來看，遙遠的恆星似乎在相對於太陽移動它們的位置。

So, one day we might see a star very near the Sun, but the next day the angle is a bit bigger.

所以，某天我們可能會看到一顆星星離太陽很近，但第二天它們之間的角度會變大一點。

At some point, about six months after we first saw it, the star is directly opposite the Sun in the sky.

在某個時刻，大約是在我們第一次看到它的六個月後，這顆恆星就會正對著天空中的太陽。

Then the angle starts to shrink again as the star approaches the Sun from the opposite side, until, after a full year, the cycle repeats.

然後，它又會從另一邊接近太陽，角度再次開始縮小，直到一年後，這個周期又會重複。

What this means to you, the naked eye observer, is that the stars appear to rise and set at different times over the course of the year.

對於你們這樣的肉眼觀察者來說，這意味著一年中星星似乎在不同的時間升起和落下。

Stars in the east rise about four minutes earlier every night, and stars in the west set four minutes earlier.

東方的星星每晚大約提前四分鐘升起，而西方的星星則提前四分鐘落下。

A constellation that was entirely below the eastern horizon at sunset one month might be completely visible after sunset the next month.

某個月日落時完全在東方地平線以下的星座，可能在下個月日落後就完全可見了。

Another way to think about it is that the stars appear to be fixed, and as the Earth circles the Sun, the Sun moves through the stars over the course of the year, making a complete circle around the sky once per year.

另一種思考方式是，恆星似乎是固定的，而隨著地球公轉，太陽在星星中運動，它的軌跡每年都在天球中形成一個圓圈。

The path it takes is a reflection of the Earth's path around the Sun, a line in the sky.

這條軌跡實際上反映了地球繞太陽轉動的路線。

We call that line the ecliptic.

我們稱這條線為黃道線。

That means the Sun passes through the same constellations in the sky every year.

這意味著太陽每年都會經過天空中同樣的一系列星座。

We give those constellations a special name: the zodiac.

我們給這些星座起了一個特別的名字：黃道十二宮。

Every year, during a given month, the Sun will appear to be in a certain zodiacal constellation, from Sagittarius through Scorpius, Libra, Virgo, Leo, Cancer, and the rest.

每年，在特定的月份裡，太陽將會出現在一個特定的黃道帶星座之中，依次是射手座、天蠍座、天秤座、處女座、獅子座、巨蟹座，以及其他星座。

Eventually, over a year, the Sun returns to Sagittarius, and the cycle starts again.

最終，在一年後，太陽會回到射手座，周而復始。

But even though we talk about this process in terms of the sun's movement, it's really the path traveled by the Earth that creates this effect, as our perspective moves with it.

但是，雖然我們從太陽運動的角度來討論這個過程，實際上卻是我們的視角隨著地球公轉運動而造成了這一現象。

And of course, the planets move in the sky as well.

當然，天空之中的的行星也會運動。

Mercury, Venus, Mars. . . they orbit the Sun, too, and they do so in approximately the same plane the Earth does.

水星、金星、火星……它們也繞太陽公轉，它們的公轉軌道與地球的公轉軌道大致相同。

If you could see the solar system from the side, it would look flat!

如果你從側面觀察太陽系，它看起來就是扁平的。

So to us, on Earth, the planets go around the sky over the course of a year, and they also appear to change their positions relative to the Sun and the stars.

所以對我們在地球上的人來說，行星每年在天空中運動，看起來就像是改變了它們與太陽和其它星星的相對位置。

The inner planets, Mercury and Venus, move so rapidly you can see their motion after a single night.

水星和火星這種內行星的移動速度很快，以至於一個晚上就能看出變化。

The outer planets are more leisurely, but wait long enough and they too will be seen to move, sliding through the constellations.

外行星的移動速度就悠閒的多，但只要等得夠久，就能看到它們划過星座。

By the way, the word "planet" is Greek for "wanderer."

順便說一下，「行星（planet）」這個詞在希臘語中是「流浪者」的意思。

There's another aspect of all this you might notice over time.

關於星星的運動，你可能會逐漸注意到另一點。

You've probably seen a globe, and noticed that the axis of it is tilted; that is, it's not straight up-and-down, perpendicular to how it sits.

你多半見過地球儀，而它的軸是傾斜的，即並不從上到下垂直於坐落的平面。

That's because a globe is modeling the Earth and the Earth is tilted.

這是因為地球儀代表的地球本身就是傾斜的。

The Earth spins on its axis once per day, and orbits the Sun once per year.

地球每天繞地軸自轉一周，每年繞太陽公轉一周。

But the Earth's axis is tilted with respect to its orbital plane by 23.5 degrees.

但地軸相對於其軌道平面傾斜了23.5度。

And this has a profound effect on our planet.

這對我們的星球產生了深遠的影響。

Imagine for a moment that the Earth's axis were exactly perpendicular to its orbit, straight up and down.

想像一下，如果地球自轉軸與公轉平面垂直。

If that were the case, every day, the Sun would take the same path across the sky.

如果那樣的話，太陽每天會以同樣的路徑經過天空。

If you were on the equator, the Sun would rise, go exactly overhead, and then set.

如果你在赤道上，太陽會從你頭頂正上方升起落下。

If you're on the pole, the Sun will appear to go around the horizon every day, neither rising nor setting — it would always be twilight.

如果你在極點，太陽看起來就每天沿著地平線轉，它不會升起或落下——整天都只是微亮狀態。

But that's not the case.

但事實並非如此。

The Earth is tilted.

地球是傾斜的。

In the months of June and July, the Earth's north pole is tipped toward the Sun.

六、七月時，地球的北極朝向太陽。

Six months later, it's pointed away.

六個月後，它就指向了相反的方向。

This affects the path the Sun takes across our sky.

這會影響太陽在空中的軌跡。

Instead of it taking the same path every day, in the northern summer, when we're tipped toward the Sun, the Sun takes a higher path in the sky.

它不再每天遵循同樣的軌跡，在北半球的夏季，當我們向太陽傾斜時，太陽的軌道會更高一些。

Because that path is longer, the days are longer, too.

而由於這條軌道比較長，白晝也更長。

Six months later, in December and January, the Earth's pole is tipped away.

六個月後，在十二月和一月時，地球的兩極傾斜。

The Sun takes a lower path in the sky, and because the path is shorter, days are shorter too.

太陽的軌道變低，也更短，白晝也因此變短。

That's why we have seasons!

這就是我們有四季的原因。

When the Sun is up high in the sky, it shines straight down on the ground, heating it better, and days are longer so it has more time to heat us up. It gets hot.

當太陽位置很高時，陽光直射地面，加熱效果更強，又因為白晝更長，照射的時間也更長，天氣就變熱。

In the winter, it's the reverse: the Sun is lower so it can't warm us up as efficiently, and it has less time to do so. It gets cold.

而在冬天，情況則正好相反：太陽較低，所以加熱效率打折扣，再加上日照時間變短了，天氣就變冷。

There you go: seasons.

就這樣，季節產生了。

The Earth's axis is tipped.

原因就是傾斜的地軸。

If it weren't, the seasons wouldn't occur, and the temperature of the Earth wouldn't change month to month.

若非如此，季節就不會產生，地球的溫度也不會每月變化。

There's a common misconception that the Earth has seasons because it orbits the Sun on an ellipse, and so it's closer to the Sun in summer and farther in winter.

有一種常見的誤解，即認為地球四季的產生是因為地球的公轉軌道是個橢圓，所以夏天離太陽近，冬天離太陽遠。

While it's true the orbit is elliptical, Earth is closer to the sun in January — on the order of five million kilometers or so — than it is in July.

雖然公轉軌道確實是橢圓形的，但地球在一月份比七月份離太陽更近——差距在500萬公裡上下。

It's the angle of the sun's rays that makes winter cold and summer hot, not our distance from the sun.

陽光入射的角度在真正決定了冬天和夏天的冷熱差別，而非距太陽的遠近。

Also, you may know that when it's summer in the northern hemisphere, it's winter in the southern.

另外，你也許知道當北半球是夏天時，南半球是冬天。

When the north pole is tipped toward the Sun, the south pole is tipped away, so northern and southern hemisphere seasons are opposite each other.

當北極點偏向太陽時，南極就會遠離太陽，因此北半球和南半球的季節是相反的。

But nothing in astronomy is permanent.

但天文學中沒有什麼是一塵不變的。

The north pole's not always going to point toward the sun in June, and Polaris is not always going to be the North Star.

北極並不會一直在六月偏向太陽，北極星也不會永遠指向北極。

That's because our planet's axis is actually moving.

這是因為地軸其實也在變化。

Have you ever seen a spinning top start to wobble, its axis moving in a slow circle even as the top itself spins?

你見過旋轉的陀螺開始搖晃，即使頂點正在旋轉，而它的軸在緩緩地劃著圓圈嗎？

This is called precession, and the Earth does it too!

這叫做進動，地球也一樣！

Our planet spins on its axis once per day, but the axis wobbles, making a very slow circle that takes 26,000 years to complete.

地球每天繞地軸自轉一次，但地軸本身會搖晃，它會花上兩萬六千年才完成一圈。

This affects a lot of what we see in the sky.

這也會影響我們在天空中看到的很多東西。

For example, Polaris won't always be the pole star!

比如，北極星不會永遠處於北極。

Every year, the pole points a little farther from that star, making a big circle 47 degrees across.

每年地球北極都會遠離北極星一點點，形成一個47度的大圈。

For ancient Egyptians, the star Thuban was the pole star, and in about 11,000 years that position will be held by the bright star Vega.

對古埃及人來說，紫微右垣曾經是他們的北極星，大約11000年後，這一位置將會被另一個亮星織女星佔據。

Also, the date the Sun is in a particular zodiac constellation changes slowly due to precession as well.

此外，由於進動的關係，太陽在某一宮的日期也會慢慢變化。

When the ancients first thought up this idea, the Sun was in Aries on March 22, the vernal equinox (what some people call the first day of spring).

當古人第一次產生劃分星座的想法時，太陽在3月22日進入白羊座，也就是春分日（有些人稱之為春季的第一天）。

But due to precession, it's now in Pisces!

但因為進動的影響，現在3月22日太陽在雙魚座。

That's why your astrological sign doesn't match where the Sun actually is in the sky; 2000 years of precession has changed them. . . one of the many reasons astrology is silly.

這就是為什麼你的星座和實際太陽位置不相符的原因，兩千年的歲差改變了它們……這也是佔星術沒什麼道理的原因之一。

It's incredible to think about: The Earth, the Sun, the stars: they allow us to tell the time and time of year just by looking up and paying attention.

想到這一點就覺得很奇妙：只需要注意觀察地球、太陽和星星就可以判斷時間和季節。

This is why the stars were so important to ancient humans.

這也是為什麼對古人來說星星非常重要的原因。

The stars were like a clock and a calendar in the sky, long before we had invented either.

在我們還沒發明鐘錶和日曆之前，它們就像掛在空中的時鐘和日曆。

We've actually learned a lot about the sky just by looking at it.

其實僅僅通過觀察，我們就已經對星空有了很多了解。

Of course, some of the stuff I've explained we've learned through other means — the Earth is spinning, stars have different intrinsic brightnesses, and so on.

當然，我之前解釋的一部分現象是通過其他途徑獲知的，比如地球自轉、星星有不同的本徵亮度等等。

But all of that knowledge, and far more, got its start by people who went outside and looked up.

但所有這些知識，以及更多的知識的獲得，都是從人們走出去抬頭觀察開始的。

Later, as we applied math and physics to what we observed we learned even more, and could then go back and explain what we saw.

隨後，當人們把數學和物理學應用到觀察的現象上，我們就學到了更多，並能解釋那些之前看到的現象。

So don't discount naked eye astronomy; it's all we had for thousands of years.

所以不要小看裸眼天文觀察，幾千年來這曾是我們僅有的方式。

In fact, I think we lost something when we started using clocks and calendars, and moving to cities with bright lights that washed away the stars from the sky.

實際上，我覺得當我們開始使用鐘錶和日曆，當我們搬入被明亮的燈光衝淡星空的城市時，我們也丟失了一些東西。

Those folks long ago were tied to the sky; they knew it like you know the streets in your neighborhood.

古人曾和星空緊密聯繫在一起，就像你熟悉你家附近的大街小巷。

They could see the stars rise and set, they knew the glory of the Milky Way sprawled across the heavens, even if they didn't know exactly what it was.

他們熟悉跨越天空的銀河的絢爛，哪怕他們並不真正理解它是什麼。

We do know, now, with our knowledge gained over the centuries.

但現在，經過很多世紀的知識積累，我們知道了。

But it comes at the cost of losing touch with the sky, not living under it as much as we once did.

卻也付出了失去這種緊密聯繫的代價，不再像曾經那樣生活在星空之下。

I've spent thousands of hours over my life at night just simply looking up, watching the stars, appreciating the Universe as I can see it.

我曾花了我生命中的上千小時僅僅是抬頭看著星空，欣賞宇宙之美。

The things I have witnessed have shaped my life, and instilled in me a permanent and endless sense of wonder and joy.

我的所見塑造了我的人生，也是我的敬畏和歡樂永恆不竭的源泉。

The Universe belongs to everyone.

宇宙屬於每個人。

Go outside and, if you can, soak up your share.

如果可以的話，你也應該走出去享受屬於你的那部分。

Today we talked about cycles: As the Earth goes around the Sun we see stars rising and setting at different times.

今天我們講了星星運動的周期：當地球繞著太陽旋轉時，我們會看到星星在不同的時間升起落下。

The Sun moves along a line in the sky called the ecliptic, through a set of constellations called the zodiac — really a reflection of the Earth's motion around the Sun — that the planets move more or less along the ecliptic as well, and that seasons are caused by the tilt of the Earth's axis together with its annual orbit around the Sun.

太陽在空中走過的路徑被稱為黃道，它經過的一系列星座被稱為十二宮——這些都是地球公轉運動的映射其它行星大致也都沿黃道運動，還講了季節變化是由於地球軸線的傾斜和地球的公轉運動共同引起的。

Crash Course is produced in association with PBS Digital Studios.

Crash Course 是與 PBS 數字工作室聯合製作的。

This episode was written by me, Phil Plait.

我是菲爾·普萊特，這一集是由我創作的。

The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.

腳本由 Blake de Pastino 編輯，我們的顧問是 Michelle Thaller 博士。

It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics team is Thought Café.

本片由尼古拉斯·詹金斯(Nicholas Jenkins)和麥可·阿蘭達(Michael Aranda)聯合執導，視頻畫麵團隊是 Thought Cafe。