We humans of planet Earth benefit from a great coincidence.
我們這些地球上的人類都獲益於一個巨大的巧合。
It's a coincidence of time, and of space. And of math.
他是一個時間上、空間上和數字上的巧合。
The coincidence is this: the Sun is about 400 times wider than the Moon, and it's also on average about 400 times farther away than the Moon.
這個巧合就是：太陽的寬度大約是月球的400倍，太陽到地球的距離也大約是月球的400倍。
The apparent size of an object in the sky depends on how big it is and how far away it is. . . so these numbers being equal means the Sun and the Moon appear to be about the same size in the sky.
天空中物體看上去的尺寸取決於它實際的大小和距離遠近……所以這兩個數字相等意味著太陽和月亮在天空中看起來是一樣大的。
And that's where another interesting thing comes in: Sometimes, the Moon passes directly between the Earth and the Sun.
這也導致了另一個有趣的現象：有時候，月亮會正好處於地球和太陽之間。
It doesn't happen all that often, but when it does, you get magic.
這並不常見，但一旦發生，你就會看到魔法般的現象。
Or even better: you get SCIENCE.
更美妙的是：理解這背後的科學原理。
You get an eclipse.
這個現象就是日/月蝕。
An eclipse is a generic term in astronomy for when one object passes into the shadow of another object, darkening or blocking it.
蝕是天文學中的一個通用術語，指的是一個物體由於進入另一個物體的陰影而被減暗或遮蔽。
A solar eclipse is when the Moon blocks the Sun, casting a shadow on the Earth, and a lunar eclipse is when the Earth blocks the Sun, casting a shadow on the Moon.
日食是月亮擋住太陽，在地球上投下陰影，月食是地球擋住太陽，在月球上投下陰影。
But how do they work?
但這是如何發生的呢？
Well, the Moon orbits the Earth once per month, and the Earth orbits the Sun once per year.
月球每個月繞地球一周，地球每年繞太陽一周。
If the Moon's orbit were perfectly aligned with the Earth's, essentially sharing the same plane, we'd get a solar eclipse every new Moon and a lunar eclipse every full Moon!
如果月球的公轉軌道與地球的公轉軌道完全平行，並在同一個平面上，那麼每到朔月就會發生日食，每到滿月就會發生月食！
But we don't.
然而事實並非如此。
That's because the Moon's orbit is tilted with respect to Earth's, by about 5°.
這是因為月球的公轉軌道相對於地球的軌道傾斜了大約5度。
What that means is that, at new Moon, the Moon can be as much as 5° away from the Sun, passing "above" or "below" the Sun in the sky, thereby missing it, from our perspective.
這意味著，在朔月的時候，月亮最多與離太陽有5度夾角，會在天空中從太陽「上方」或「下方」經過，因此從我們的角度來看，月亮就沒有完全遮住太陽。
But sometimes the Moon is in the right place at the right time, and at new Moon, it lies perfectly in line between the Sun and the Earth.
但有時月亮會正好在正確的時間出現在正確的位置，而在朔月時，它正好位於太陽和地球之間。
And when that happens, we get a solar eclipse.
這時，我們就會看到日食。
This geometry happens at least twice per year, and sometimes as much as five times per year.
這種情況每年至少發生兩次，有時甚至多達五次。
What's happening physically in space is that the Moon is casting a long shadow.
在太空中實際發生的情況是，月亮投下了一道長長的陰影。
Usually that shadow misses the Earth, but during an eclipse the Moon's shadow falls on the Earth's surface.
通常這道陰影會錯過地球，但日食時陰影正好落在地球表面。
In fact, there are two shadows from the Moon, one inside the other.
實際上，月球有兩個陰影，一個套著另一個。
One is a narrow cone, tapering to a point away from the Moon.
一個是短短的圓錐形，尖角朝向遠離月球的方向。
If you're anywhere physically inside this cone, the Moon appears big enough to completely block the Sun.
如果你處於這個圓錐內部，月球就會顯得足夠大，以至於完全遮住太陽。
That means this shadow is very dark, and we call it the umbra (which is Latin for — you guessed it — "shadow").
這也意味著陰影非常暗，我們稱之為本影（umbra）（猜猜這個英文單詞的拉丁文意思？對啦，就是『陰影』）。
Outside of this deep umbral shadow is a wider conical region where, if you're in it, the Sun is only partially blocked; you can still see some of the Sun past the Moon.
在這個本影範圍之外，有一個更寬的錐形區域，如果你身處其中，太陽只會被部分被遮擋；你仍然可以看到一部分沒被月球遮擋的太陽。
You're getting less light, and so you're technically shadowed, but it's not quite as dark as the umbra.
照在你身上的陽光減少，所以嚴格上說來你處於陰影中，但並不像本影那麼黑。
This region is called the "penumbra" ; "pen" in this case for Latin meaning "almost," or "nearly."
這個區域被稱為半影（penumbra）；這裡英文中的「pen」的拉丁文意思是「大致」或「幾乎」。
When the umbra touches the Earth, we get a total solar eclipse.
當本影接觸地球時，我們會看到日全食。
But what does that look like from the ground?
但是從地面上看是什麼樣子呢？
You don't get a total eclipse right away.
你不會馬上看到日全食。
First, the edge of the Moon slips in front of the Sun, and we see a little dip in the Sun's limb, its edge as seen from Earth (that's the start of the penumbra sweeping over you).
首先，隨著月球的邊緣進入太陽正面，我們會看到太陽缺了一點點（這表示半影開始掃過你）。
As the Moon slowly moves, that dip grows, becoming a bite.
隨著月亮緩緩移動，這個小凹陷慢慢變大，變成一個缺口。
The Sun becomes a thick crescent, then a thin one.
而太陽變成一個寬寬的鐮刀狀，然後變窄。
As the Sun becomes an ever-thinner crescent, the sky begins to darken.
當太陽逐漸變窄時，天空開始暗下來。
Then, finally, the Moon's black disk completely covers the Sun — the umbra sweeps over your location.
最後，當本影掃過你所在的位置時，黑暗的月球完全遮住了太陽。
And at that moment, totality begins.
此時，日全食就開始了。
You might think that this just means the sky gets dark, and it's like night outside for a while.
你可能認為這只是意味著天空變暗了，確實有一段時間會像夜晚一樣。
But a total eclipse is far more than that.
但日全食遠遠不止這些。
And that's because of the Sun's corona.
這是因為太陽有日冕。
As I'll cover in more detail in a future episode, the corona is the sun's atmosphere, an ethereally thin envelope of gas that stretches from the Sun's surface into space for millions of kilometers.
日冕是太陽的大氣層，我將在以後的章節中更詳細地介紹，日冕是一層空氣般稀薄的氣體，從太陽表面延伸到幾百千米外的太空中。
It's really faint, and therefore usually completely overwhelmed by the intensely bright light from the Sun.
它很暗淡，所以一般會被高強度的太陽光掩蓋。
But when the Moon blocks the Sun's face, the corona becomes visible.
但當月球遮蔽太陽表面時，日冕就顯現出來。
It surrounds the Sun, filaments and tendrils extending into the sky, an incredibly beautiful sight.
它包裹著太陽，呈絲狀環繞捲曲，在天空伸展，是一個令人難以置信的美麗景象。
I know many people who have said it's the most spectacular thing they have ever seen.
我知道很多人都說過這是他們見過的最壯觀的景象。
And there's more.
但還不止如此。
The Moon's edge isn't smooth — there are craters and other depressions.
月亮的輪廓並不平滑——上面有隕石坑和其它凹陷。
Craters right at the Moon's edge allow sunlight to stream past.
正好在月球輪廓上的隕石坑會讓陽光通過。
We see these as bright patches around the eclipsed Sun, which are called Baily's Beads — because they were first described by English astronomer Francis Baily in 1836!
看起來就是暗淡的太陽邊緣的亮斑，這被稱為貝利珠——因為這個現象是由一個英國天文學家弗拉西斯·貝利在1836年率先描述的。
Because the Moon and Sun are very nearly the same apparent size, totality is brief.
由於月球和太陽的表面大小非常接近，所以全食很短暫。
The longest it can last is only about seven or eight minutes.
它最長只能持續七到八分鐘。
That's how long it takes the umbra to move over one spot on the Earth.
這就是本影掃過地球上某一點所需要的時間。
When totality ends, and the Moon starts to move off of the Sun's face, for a moment just a single spot of the Sun is unblocked, glowing fiercely on one side of the Moon.
當全食結束，月亮開始從太陽表面移開的時候，有那麼一會兒，太陽只有一小部分露出來，在月亮的一側發出耀眼的光芒。
Sometimes you can get a circle of light around the Moon's surface, and together with the bright spot it looks like a celestial wedding ring.
有時你會看到月亮周邊有一圈光線，再加上貝利珠的光斑，它看起來就像天上的一枚結婚戒指。
In fact, this is called the Diamond Ring effect.
事實上，這就是所謂的鑽石環效應。
Then, inexorably, the Moon pulls away from the Sun, and the order of events is reversed.
然後，不可避免地，月亮遠離太陽，以上效應以相反的順序再次出現。
The umbra is gone, but you're still in the penumbral shadow.
本影已經移開，但你仍處在半影中。
The Sun shows a thin crescent, then a thick one, then a dip in its side. . . and then it's all over.
太陽露出一線，然後逐漸變寬，再到僅有一個小缺口……最後結束。
The umbral shadow of the Moon is pretty small where it hits the Earth, so a total eclipse is a local event.
月亮的本影籠罩地球的部分很小，所以日全食是地區性的現象。
If you're too far north and south, you don't get a total eclipse, you only get a partial one.
如果你偏南或者偏北太遠，就看不到日全食，日能看到日偏食。
Which is still cool, but lacks the mystique of a total eclipse.
不過這仍然很酷，但缺少了完整日全食那種神秘感。
Remember too that the Moon's orbit around the Earth is an ellipse.
同樣要記住的是，月亮繞地球的公轉軌道是個橢圓。
That means sometimes it's closer to the Earth, and sometimes farther.
這意味著它有時離地球更近，有時離地球更遠。
If a solar eclipse happens when the Moon is at the far end of its orbit, it can actually be smaller than the Sun in the sky.
如果日食發生在月球處於其軌道遠端時，它實際上可能比天空中的太陽還要小一些。
It doesn't block the entire face of the Sun, and it leaves a ring of light around the black circle of the Moon.
它不再能完全遮擋太陽，而會在月亮的黑色圓圈周圍留下一個亮環。
This technical name for this shape is annulus, so this event is called an annular eclipse.
嚴格來說這個形狀的名稱是日環。所以這一現象就被稱為日環食。
A lot of people think if you look at a total solar eclipse you can go permanently and completely blind.
很多人以為如果你直視完整的日食過程，你可能會永久失明。
That's really not true.
但這並不完全是真的。
But, some parts of eclipse-watching are more dangerous than others.
但是，觀賞日食的某些階段會比其他部分更危險。
I mean, obviously it's not a good idea to stand there and stare at the sun.
我的意思是，顯然站在那裡盯著太陽不是個好主意。
Looking at even the uneclipsed Sun for more than a moment is painful, and that pain is the result of the damage that solar radiation is doing to your retinas.
即使是看著不處於日食中的太陽超過一分鐘也很痛苦，這種痛苦是由於太陽輻射對視網膜造成的傷害造成的。
So I don't recommend it - Duh.
所以我不會建議你這麼做的。
But when viewing an eclipse, the real concern is right after totality ends.
其實觀看日出真正需要擔心的是日全食剛結束的時候。
During totality it's dark, so your pupils have dilated to let more light in.
日全食時天空完全黑暗，你的瞳孔會擴張來讓更多的光線通過。
But then there's the flash of sunlight when the Moon moves off, and that's intense enough to damage your retinas.
但是當月亮移開的時候，會有一道強烈的陽光射出，其強度足以損害你的視網膜。
That's why astronomers recommend extreme caution when viewing an eclipse; because that flash can catch you by surprise.
這就是為什麼天文學家建議觀測日食時要格外小心的原因；因為那道突然出現的強光可能會讓你措手不及。
When viewing the Sun, don't just stand there and stare at it; you should always have eye protection.
看太陽的時候，不要只是站在那裡盯著太陽看；你應該時刻保護眼睛。
And make sure you have safety-approved filters; don't try the the home-made tricks you might have heard of. . . like looking through an old CD or DVD, or using old-style camera film as a filter.
要確保你有經過安全認證的濾鏡；不要嘗試那些道聽途說的自製小工具……比如透過一張舊的CD或DVD看，或者使用老式的相機膠片當濾鏡。
These can let through too much infrared and ultraviolet light, and again can dilate your pupils, actually making things worse.
過多的紅外線和紫外線都能穿過他們，同樣會使瞳孔擴散。實際上讓情況更糟糕。
Lots of companies make inexpensive filters that are great for Sun-spotting; we have links in dooblydoo for more information on eye safety.
許多公司都會針對太陽觀測生產價廉物美的濾鏡；在視頻下方的簡介欄裡有更多關於用眼安全的信息連結。
Now, you don't have to worry about hurting your eyes at all when viewing a lunar eclipse.
現在，觀察月食你就完全不用擔心損傷眼睛了。
Because, in that case, it's the Earth that blocks the Sun, and the Earth's shadow falls on the Moon.
因為這種情況下，是地球擋住了太陽，而地球的影子投射在月亮上。
So go nuts.
所以想怎麼看就怎麼看吧。
But one big difference between the two kinds of eclipses is who can see them.
但日食月食最大的不同在於誰能看到它們。
A solar eclipse is localized to one spot on the Earth, or really a swath along the ground as the Moon's umbral shadow sweeps across the Earth's surface.
日食是區域性現象，僅限於地球上某一特定地點，或者說是月亮本影掃過地球表面的那片區域。
But a lunar eclipse is when the Moon moves into Earth's shadow, so anyone on Earth facing the Moon can see a lunar eclipse.
但是月食是在月亮進入地球陰影時發生，所以地球上任何一個面對月亮的人都可以看到月食。
This is why I've seen dozens of lunar eclipses but never a total solar one.
這就是為什麼我見過了成打的月食，卻還沒見過日全食的原因。
I've never been at the right place at the right time.
我從來沒有在對的時間出現在對的地方。
Not that I'm bitter.
我沒有在抱怨的意思。
The Earth has umbral and penumbral shadows, too.
地球也有本影和半影。
When the Moon first enters the Earth's penumbra, the dimming is so slight you hardly notice it.
當月亮剛進入地球的半影時，亮度只有輕微的減弱，以至於你很難注意到。
But as the Moon moves deeper into the penumbra, it starts to darken.
不過隨著月亮更加深入半影區域，它就越來越暗。
Sometimes it changes color, turning a deep orange or blood red.
有時還會變色，變成深橘色或者血紅色。
That's because the Earth is starting to block the sunlight heading toward the moon, and the only light that gets through is coming through the thickest part of our atmosphere.
這是因為地球開始擋住照向月球的陽光，而唯一能通過的光需要穿過厚厚的地球大氣層。
This blocks blue and green light, leaving only red to come through.
這樣就阻擋了藍光和綠光，只留下紅光。
That's why the Moon and Sun look red to us when they're on the horizon, rising and setting, too.
這也是為什麼月亮和太陽在地平線上或升起和落下時看起來有點紅的原因。
When you look upon the red eclipsed Moon, you're seeing the light from all the sunrises and sunsets in the world hitting the Moon and reflecting back to us.
當你看著月食中的紅月亮時，你所見到的光線就是所有日出日落照射向月亮後再反射回來的光線。
Finally, the Moon starts to enter the Earth's umbra, and the real eclipse begins.
最後，月亮開始進入地球的本影，真正的月食就開始了。
At first it looks like a bite is taken out of it - that curving arc is the shadow of the edge of the Earth!
起先它看起來像是被咬了一口——那弧線的缺口是地球陰影的輪廓！
The Moon moves deeper and deeper into the shadow until it's completely darkened.
月亮越來越深入本影，直到完全變暗。
The Earth is bigger than the Moon, so the Earth's umbra is much wider; while a solar eclipse is over in minutes, a total lunar eclipse can last nearly two hours.
地球比月球大，所以地球的本影要寬得多；雖然日全食在幾分鐘內就結束了，但月全食可以持續近兩個小時。
I once saw a lunar eclipse so deep that it took me a minute to find the Moon in the sky!
有一次我觀察月食時，月亮已經深入本影，以至於我花了一分鐘才找出天上的月亮。
There's not a lot of new science you can do with a lunar eclipse.
月食並沒有包含更多新的知識。
But if you know a little geometry, you can use the size and shape of the Earth's shadow on the Moon to get the relative sizes of the Earth and Moon.
不過如果你了解一些幾何學，你就可以通過地球投射在月球上的陰影的大小和形狀來算出地球相對於月球的大小。
Ancient Greeks did just this, and got a number that wasn't too far off.
古希臘人就是這麼做的，他們得出了一個跟真實情況差不太多的數字。
They also knew how big the Earth was using other methods, and so they had a decent estimate for the size of the Moon. . . nearly 2000 years before the invention of the telescope!
他們還曾經通過其它方法得知了地球的大小，所以他們也對月球尺寸做出了基本正確的估計……這可是在天文望遠鏡被發明的2000年之前！
They also knew the shape of the Earth's shadow was always a circle, which only makes sense if the Earth were a sphere.
他們還知道，地球的陰影總是圓形，這當然只有在地球是一個球體的情況下才成立。
If the Earth were flat, it would sometimes cast a thin shadow, but it never does.
如果地球是扁平的，陰影有時候就會是一條細線，但這從來沒有發生過。
Pretty clever, those ancient Greeks.
那些古希臘人真聰明。
One final note.
最後一點。
Because of tides from the Earth — which we'll learn more about in detail in a later episode — the Moon is slowly moving away from the Earth, by about four centimeters a year.
由於地球的潮汐——我們將在後面的章節中詳細了解——月球正緩慢地遠離地球，速度大約是每年4釐米。
As it recedes, it's slowly getting smaller in the sky.
天空中的月亮正在慢慢變小。
This means that, eventually, it will be too far away to completely cover the Sun, and we won't get any more total eclipses.
這意味著，最終月亮會離得太遠，以至於無法完全遮住太陽，我們將再也看不到日全食。
Doing the rough math, that will be in about a billion years.
粗略計算的話，這會在十億年後發生。
Better watch eclipses while you can.
趁著還能看到日食，得抓緊時間看了。
Today you learned that a solar eclipse is when the Moon blocks the Sun so its shadow falls on the Earth, and a lunar eclipse is when the Earth's shadow falls on the Moon.
今天你學到了日食發生在月亮擋住太陽、在地球上投下陰影之時，而月食是地球的陰影籠罩月球時發生。
We don't get them every two weeks because the Moon's orbit is tilted.
由於月球軌道是傾斜的，我們不會每兩周經歷一次日食或月食。
And if you're clever, you can use lunar eclipses to figure out how big the Earth and Moon are.
另外，如果你夠聰明的話，就可以用月食來算出地球和月球的大小。
This episode is brought to you by Squarespace.
本集由 Squarespace 為您呈現。
The latest version of their platform, Squarespace 7, has a completely redesigned interface, integrations with Getty Images and Google Apps, new templates, and a new feature called Cover Pages.
他們最新的平臺 Squarespace 7經過了全新交互界面設計，加入了 Getty 圖片社、谷歌應用、新模板和封面頁的新功能。
Try Squarespace at squarespace. com, and enter the code Crash Course at checkout for a special offer.
你可以在 squarespace.com 試用，付款時輸入優惠碼 Crash Course 可享受特價。
Squarespace. Start Here. Go Anywhere.
Squarespace，從此始，任你行。
Crash Course Astronomy is produced in association with PBS Digital Studios.
Crash Course 是與 PBS 數字工作室聯合製作的。
Head on over to their channel and discover more awesome videos.
你們可以去看看，還有更多精彩的視頻。
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, edited by Nicole Sweeney, 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。