天文學 1610

《星際信使》（Sidereus Nuncius）

伽利略·伽利萊

伽利略把望遠鏡對準夜空，看見了滿是山脈的月球，和四顆繞著木星運轉的衛星。

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In depth · the introduction

1610 年的冬天，一個人用一支自製的玻璃筒，看見了從未有人眼見過的東西——而那完美、不變的天空，就此崩塌。

核心想法

伽利略造了最早的幾架望遠鏡之一，並做了一件事後看來理所當然、在當時卻是全新的事：他把它對準了夜空。他的發現，寫進一本名叫《星際信使》的薄薄小書裡，擊碎了古人那幅完美無瑕的宇宙圖景。

本該是光滑天球的月亮，原來是一個崎嶇的世界，像地球一樣布滿山脈與峽谷。銀河那片朦朧的微光，化作了無數顆各自獨立的恆星。而在木星這顆行星周圍，有四個細小的光點正悄悄繞行——那是它自己的衛星。

它是如何誕生的

望遠鏡是 1608 年的荷蘭發明，一件用來看遠處船隻和尖塔的新奇玩意兒。在帕多瓦當教授的伽利略聽說了它，自己琢磨出了鏡片，還磨出了好得多的一架。1609 年秋天，他開始用它掃視天空。

1610 年 1 月 7 日，他把它對準明亮的木星，注意到旁邊有三顆排成一線的小「星」。他觀察了好幾個星期。它們一夜一夜地移動，又來了第四顆，它們躲到行星背後、再轉出來——卻始終不曾走開。它們只能是衛星，繞著木星轉。他趕忙把這個發現付印，並把這幾顆新衛星獻給他的贊助人美第奇家族，稱之為「美第奇星」。（一位德國天文學家西門·馬里烏斯聲稱自己也看到了它們；流傳下來的名字——木衛一、木衛二、木衛三、木衛四——正是他取的。）

它為何重要

兩千年來，人們一直被教導：天上的一切都繞著地球轉，而天空是完美、不變的。伽利略這本小書，一舉推翻了這兩點。多山的月亮並不完美；而繞著木星轉的四顆衛星，則證明並非萬物都繞地球轉。他並沒有證明地球在動——那是後來的事——但他表明了那些古老的篤信是錯的，而且他靠的不是辯論，而是觀看。近代的觀測科學，正始於此。

一個可以想像的畫面

想像你在田野的另一頭，正好從側面看一座旋轉木馬。你看不見那個圓——騎手們只是向左滑、減速、停住，再向右滑回來，如此反覆。從地球看木星的衛星，正是這副樣子：它們的圓形軌道，從側面看去，就成了沿一條線的來回挪動。靠中心、又快的那顆（木衛一）來回飛竄；遠在外側、又慢的那顆（木衛四）則懶洋洋地漂移。看得夠久，唯一誠實的解釋就是：它們在繞圈。

它的位置

半個世紀以前，哥白尼（1543，見本館）已斗膽把太陽放到了中心；而就在前一年，克卜勒（1609）把行星的軌道彎成了橢圓。伽利略給出了二人都沒有的東西：來自一件新儀器的、親眼所見的證據。他發現的木星衛星，成了「哥白尼圖景可能是真的」最有力的論據之一——而這條線索一路通向牛頓（1687），是牛頓的引力解釋了行星何以能擁有衛星。伽利略自己的膽識，後來讓他付出了代價：教會在 1633 年審判了他。

The original document

Original source text

Galileo Galilei · Sidereus Nuncius · Venice: Thomas Baglioni, March 1610 · trans. E. S. Carlos (1880)

The title and the dedication

A short quarto pamphlet, dedicated to Cosimo II de' Medici, Grand Duke of Tuscany. Its title — Sidereus Nuncius — has been read both as 'The Starry Messenger' (one who carries news of the stars) and 'The Starry Message'; Galileo announces "great and very wonderful sights" revealed by a new instrument, and promises news of the Moon, the fixed stars, the Milky Way, and four planets never seen before.

The instrument

Galileo describes how, on hearing that a Dutchman had made a glass that brought distant things near, he worked out the optics for himself and built a telescope — first magnifying about eight times, then, after grinding better lenses, roughly twenty times — good enough to turn on the heavens.

The Moon

From the ragged line dividing light from dark, and from bright points that catch the dawn while still surrounded by shadow, Galileo argues the Moon is no polished sphere but a world like ours, and even estimates its mountains at more than four miles high:

I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical … it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys.

The stars and the Milky Way

The telescope multiplies the stars: Galileo charts dozens of new ones around Orion's belt and in the Pleiades, and resolves the cloudy patches and the Milky Way itself into separate suns.

the Galaxy is nothing else but a mass of innumerable stars planted together in clusters.

The Medicean Stars — Jupiter's moons

On the night of 7 January 1610, Galileo turned the glass on Jupiter and noticed what he had never seen before:

… three little stars, small but very bright, were near the planet.

Night after night the little stars changed their places along a straight line through Jupiter — and a fourth appeared. They were sometimes east of the planet, sometimes west, sometimes hidden altogether; yet they never left it. Galileo concluded that four bodies revolve about Jupiter as the Moon revolves about the Earth, and named them the Medicean Stars in honour of Cosimo and his brothers.

[ … ]

Galileo Galilei · Padua · March 1610