我們對「月亮」的定義被打破（內含中文視頻）

視頻版權：MinuteEarth；

字幕翻譯：天文志願字幕組（有錯誤的地方望各位指出） 翻譯：@Juliet z 審核：@WaydeNichol

解說詞

（字幕翻譯：天文志願字幕組，若有錯誤，歡迎指出）N【視頻版權：MinuteEarth】

嗨，我是MinuteEarth的KateNHi, this is Kate from MinuteEarth.

直到最近，人們都相信我們在天空中看到的一切NUntil relatively recently, humans believed everything we could see in the sky was orbiting

都在圍繞地球旋轉。Naround the earth.

當然，月球是唯一一個實際上圍繞地球轉的天體，N並且地球本身都圍繞著一顆恆星NOf course, the Moon was the only thing actually orbiting the earth, and the earth itself was

旋轉。Norbiting a star.

但是還有其他天體繞著這顆恆星運行並且也有天體圍繞它們旋轉，NBut there were other objects orbiting that star that also had things orbiting them, so

因此，我們決定把所有的次級衛星都成為「月球」Nwe decided to call all of those secondary satellites 「moons」, too.

如今當人們提到「月球」，本質上是指一顆恆星的衛星NNowadays when people say 「moon」, they essentially mean a natural satellite of a

它似乎是我們用以辨別月球的Nsatellite of a star, which seems to capture our collective intuition for what distinguishes

集體直覺Na moon.

然而，這有一個問題：太空岩石形狀大小各異，運行方式更是不同，NHowever, there』s a problem: space rocks come in all sorts of shapes and sizes and

其中的許多根本不符合我們對月球的Nbehave in many ways, many of which don』t at all fit into our traditional idea of a

傳統看法。Nmoon.

這時我們可以放棄傳統看法並說：N「好吧，這些所有都是衛星」——但是在這個視頻中NWe could at this point give up and say 「ok, all these things are moons」 – but in this

我們將會捍衛「月球」是一個特殊的稱呼並且我們不應該Nvideo we』re going to defend the idea that 「moon」 is a special title and we shouldn』t

過於寬泛地授予這個稱呼。Naward it too broadly.

首先，通過技術的進步，我們最終將越來越小的天體稱為NTo start with, through improvements in technology, we』ve ended up calling smaller and smaller

衛星Nthings moons.

我們已經發現了超過60顆圍繞土星旋轉的小衛星，NWe』ve already identified more than 60 increasingly small moons orbiting Saturn alone, some of

其中一些基本上是隱藏在土星環中公里寬的岩石Nwhich are essentially kilometer-wide rocks hiding in its rings.

我們不可避免地不斷發現更小的圍繞行星運行的單獨的岩石塊。NIt』s inevitable that we』ll keep identifying ever smaller individual chunks orbiting planets.

因此，如果我們設定低於月球尺寸的下線，我們是否願意考慮NSo if we don』t set some lower limit on moon sizes, are we willing to think of each of

土星環上的數十億微小的岩石和塵埃粒子，Nthe billions of tiny rocks and dust particles in Saturn』s rings – and every last speck

以及環繞地球的每一粒塵埃作為衛星？Nof dust orbiting the earth – as moons?

在另一個極端，一些衛星太大而不能認為是衛星：冥王星和NAt the other extreme, some 「moons」 are too big to be considered moons: Pluto and

和他的衛星Charon的大小太過於接近以至於說它們彼此環繞在Nits maybe-moon Charon are close enough in size that it』s really fairer to say they

軌道上Nboth orbit each other.

實際上，從專業的角度來說，N在任何時候任何其他的軌道上，它們都圍繞著NIn fact, technically, any time anything orbits any other thing, they』re actually both orbiting

它們公共的中心軌道運行，這就是重心。Ntheir common center of mass, called the barycenter.

這一點取決於兩個天體的相對質量——如果一個天體NWhere that point is depends on the relative masses of the two objects – if one is way

比另一個天體大，比如地球，N那麼這個重心就會靠近或者甚至在這個天體內部，Nbigger than the other, like the earth, the barycenter will be close to or even inside

所以說小一點的天體繞著大一點的天體運行是正確的。Nit so it feels right to say the smaller one is orbiting around the bigger one.

但是當它們幾乎是相同大小的時候就像二叉小行90 Antiope，N重心就會是NBut when they』re almost the same size like with the binary asteroid 90 Antiope, the barycenter

兩個天體的中間，那麼稱呼它們中一個或另一個（或者兩個）為衛星Nwill be almost halfway between, and it really doesn』t make sense to call one or the other

都是沒有意義的。N(or both) of them moons.

因此，在尺寸比率的連續範圍中——N也就是「誰是真正繞著誰運行，怎樣運行」——NSo where in the continuum of size ratios – aka 「who』s really orbiting who, and by how

我們應該什麼地方停止稱呼一些天體為衛星呢？Nmuch」 – Where should we stop calling something a moon?

也許大小的比例不能準確地反映出你對衛星判斷的直覺NMaybe the ratio of sizes doesn』t accurately capture your intuition about what a moon is

—也許絕對的大小會更好，或者是候選衛星的圓度，或者N– maybe absolute size would be better, or the roundness of the candidate moon, or whether

它是否能夠從它行星的表面上被看到，N或者它是否有一個常規的橢圓軌道。Nit can be seen from the surface of its planet, or whether it has a regular elliptical orbit.

或者也許根本不存在一個對於月亮的定義，使它能夠既包含NOr maybe there』s simply no one definition for 「moon」 that captures both the complexity

一個東西圍繞另一個東西轉的複雜原理，又能讓我們撥雲見霧一般Nof the different ways stuff can orbit other stuff and our nebulous, intuitive 「I know

憑直覺「看到它就知道它是什麼」Nit when I see it」 idea of what makes a moon.

與衛星不同的是，太空中還有許多其他的天體，比如在一顆恆星死亡後NUnlike moons, there are many other objects in space, like the different things a star

產生的天體，那實際上都是很自然地產生的，Ncan become when it dies, that are actually delineated quite naturally, and you should

你應該到MinutePhysics去了解更多關於它們的信息！Ncome over to MinutePhysics to find out more about them!

以上是「如果月球上有月亮，那它還是月亮嗎？」這個話題NEnough of this 「if there』s a moon around a moon, is it still a moon?」

——歡迎再次收看MinutePhysics，我們將會談談黑洞、白矮星N– come over to MinutePhysics, we』re going to talk about black holes and white dwarfs

和中子星！N翻譯提供者：@Juliet zN審核：WaydeNicholNand neutron stars!N（觀看更多中文天文視頻歡迎關注weibo天文在線）N【也歡迎您加入我們字幕組，私信koukou零度星系（南充）】

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http://www.imo.net/resources/calendar/

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