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  • Pluto: Planet or not? Before we can answer this question we need

    冥王星究竟是不是行星呢? 回答這個問題之前,我們要

  • to know what the word planet is for, and that takes us back to the ancient greeks who called


  • Mercury, Venus, Mars, Jupiter, Saturn, the Moon and sun planets. Basically if it moved

    水星、金星、火星、木星、土星、月球和太陽作「行星」。 基本上,當一個天體在天上移動,

  • across the sky and was bright, it was a planet. This is a terrible start for the word because,

    而且外表光亮,那就是一個行星。 這個定義非常模糊,因爲

  • 1) it excludes Earth from the list and 2) it groups wildly different things together.

    第一,行星此詞的定義不包括地球在內, 第二,這個字包含很多不同種類的天體。

  • But the greeks couldn't know how different the Moon was from Saturn, because the best


  • technology they had to observe the Universe was sadly limited.


  • It would take several thousand years until the industrious Dutch made the first telescopes


  • and astronomy became much more interesting. Astronomers could now confidently rearrange

    天文學因而變得有趣。 現在的天文學家能有信心地重新整理

  • the solar system -- an elegant scientific advance that no one could possibly object


  • to -- and reclassify its parts, dropping the Sun and moon from the list of planets and

    這套系統重新把天體分類, 把太陽和月球從行星的定義中除下,

  • adding Earth. Now, if it orbited the Sun, it was a planet.

    並加上地球。現在的定義是, 當天體圍着太陽公轉,就是行星。

  • As time went on and telescopes got better and better each new century brought with it


  • the discovery of a new planet. Which brings us to this familiar solar system:


  • nine planets orbiting one star. And looking at this model makes people wonder,

    九個行星圍繞着一個恆星。 看到這個模型,有些人會想,

  • why do astronomers want to ditch Pluto? The problem is pictures like this in textbooks

    爲什麼天文學家都想拋棄冥王星不讓它當行星? 問題是,課本上像這樣的圖片

  • are lies. Well, not lies exactly, but unhelpful. They give the impression that the planets

    都是謊言,也不完全是,只是不太有用。 這些圖片令人覺得那些行星的

  • are similar-ash in size and evenly-ish spaced, but the reality couldn't be more different.


  • Here, dear Terrans, is our home planet Earth, and this is Jupiter next to it at the correct


  • scale -- rather bigger than you probably thought. If we take this diagram and adjust for the

    也許比你想象的更大。 如果我們像這幅圖片調整至

  • correct sizes of the planets it looks like this. Unless you're watching the video in

    所有行星都符合比例,就好像這樣。 如果你不是用全螢幕高清模式

  • fullscreen HD mode, you might not even be able to see Pluto.


  • So size differences are vast, and Pluto is the smallest by far. But it's not just small

    它們的大小相差很大,冥王星是暫時最小的。 冥王星也不只是小於

  • for a planet, it's also smaller than seven moons: Triton, Europa, our own Moon, Io, Callisto,

    其他行星,甚至小於七個衛星: 海衛一、木衛二、我們的月球、木衛一、木衛四、

  • Titan, and Ganymede. Even if you show the correct relative sizes

    土衛六和木衛三。 即使你顯示了正確的比例,

  • the distances are still a problem. Think about it, if Jupiter was this close to Earth it


  • wouldn't look like a dot in the night's sky but would be rather overwhelming -- so it


  • must be really far away, which makes drawing it to scale rather a challenge. If you want


  • the length of a piece of paper to represent the distance from Mercury to Pluto, then giant


  • Jupiter would be the size of a dust mite on that page, and Pluto a bacterium.

    木星的大小就相當於一隻塵蟎, 而冥王星就是一個細菌。

  • But excluding Pluto from the plant club just for being tiny and far away isn't reason enough

    但只因爲冥王星的細小和距離的長遠 似乎不至於把它逐出行星行列,

  • and quickly brings out the Pluto defenders. In order to understand what Pluto really is,

    而又產生了很多冥王星保衛者。 如果要知道冥王星到底是什麼,

  • we need to first discuss a planet you've never heard of: Ceres.


  • Back in the 1801, astronomers found a new planet in the huge gap between Mars and Jupiter

    1801年,天文學家在火星和木星的 大空隙中發現了一顆新行星。

  • -- it was a small planet, but they loved it anyway and named it Ceres.

    它非常細小,但天文學家們非常愛它, 於是起了「穀神星」這個名字。

  • The next year astronomers found another small planet in the same area and named it Pallas.

    翌年,天文學家在同一個區域中 發現了令一個行星,叫「智神星」。

  • A few years later they found a third one, Juno, and then, funnily enough, a fourth one,

    幾年後,他們發現了第三個——「婚神星」, 然後又發現了第四個——

  • Vesta. And for a several decades children learned the 11 planets of the solar system.

    「灶神星」。接着的幾十年, 小孩們都在學太陽系的11個行星。

  • But, astronomers kept finding more and more of these objects and became increasingly uncomfortable


  • calling them planets because they were much more like each other than planets the on either

    叫他們作行星,因爲這些 不太像行星的天體都非常相似,

  • side, so a new category was born: asteroids in the asteroid belt -- and the tiny planets

    所以新的天體分類由此誕生:小行星。 他們在小行星帶中,而這些「行星」

  • were relabeled which is why you've never heard of them. And it was a good decision too, as

    被天文學家重新分類,所以你便從來沒聽過 這些「行星」。這是一個好的決定,因爲

  • astronomers have now found hundreds of thousands of asteroids, which would be a lot for a kid


  • to memorize if they were all still planets. Back to Pluto. It was discovered in 1930 making

    可憐的小孩還要背熟它們的名字。 說回冥王星,冥王星在1930年被發現,

  • it the 9th planet. First estimates put Pluto about the size of Neptune, but with more observations

    成爲第九個行星。最初天文學家估計冥王星的 體積於海王星差不多,但經過更多觀察,

  • that was revised down, and down and down. While Pluto shrank astronomers started to

    冥王星的體積逐漸縮小,縮小,再縮小。 冥王星「正在縮小」之際,天文學家開始

  • find other, similar objects orbiting in the same zone.


  • Sound familiar? While school kids kept memorizing the nine


  • planets, some astronomers grew uneasy about including Pluto because the size estimates

    行星的時候,某些天文學家反對 把冥王星稱爲行星,因爲冥王星的估計體積

  • continued to shrink, they learned that Pluto is made mostly ice, and they continued to


  • find lots and lots of icy objects at the edge of the solar system just like Pluto.

    繼續發現大量這種像冥王星的 冰球在太陽系的邊緣漂浮。

  • This problem could be ignored as long as no one found an ice ball bigger than Pluto, which

    如果沒有人能發現比冥王星大的冰球, 可以擱置這個問題。

  • is exactly what happened in 2006 with the discovery of Eris. Once again, astronomers


  • recategorized the solar system and grouped these distant objects, including Pluto, into

    把太陽系重新分類,並將這些 包括冥王星的遙遠星體組合成

  • a new area called Kuiper belt. And that's the story of Pluto -- a miscategorized

    一個新的區域,「柯伊伯帶」。 這就是冥王星的故事——一個被編錯分類的天體,

  • planted that finally found its home -- just like Ceres. But this story is really less

    終於找回自己的家,像穀神星。 而這個故事的重點似乎不是關於

  • about Pluto than it is about realizing the word 'planet' isn't very helpful.


  • The first four planets are nothing at all like the next four, so it's even a little

    首四個行星於末四個行星分別很大, 所以把他們組成一組非常奇怪,

  • weird to group these eight together which is why they often aren't and are separated


  • into terrestrial planets and gas giants. And now that we have telescopes that can see

    類地行星和氣體巨行星。 現在我們有這些能看到

  • planets around stars not our own, and we've found rogue planets drifting in empty space


  • and brown dwarfs -- objects that blur the very line between planet and star -- the word


  • planet becomes even less clear. So as we increase our knowledge of the Universe

    「行星」這個詞的定義亦開始模糊。 隨着我們對宇宙的認識增加,

  • the category of 'planet' will probably continue to evolve, or possibly, fall out of favor


  • entirely. But, for the time being the best way to categorize


  • the stuff in our solar system is into one star, eight planets, four terrestrial, four

    最佳方法是:一顆恆星、八顆行星、 四顆類地行星、四顆

  • gas giants, the asteroid belt, and the distant Kuiper belt, home to Pluto


Pluto: Planet or not? Before we can answer this question we need

冥王星究竟是不是行星呢? 回答這個問題之前,我們要


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