天文学 1610

《星际信使》（Sidereus Nuncius）

伽利略·伽利莱

伽利略把望远镜对准夜空，看见了满是山脉的月球，和四颗绕着木星运转的卫星。

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In depth · the introduction

1610 年的冬天，一个人用一支自制的玻璃筒，看见了从未有人眼见过的东西——而那完美、不变的天空，就此崩塌。

核心想法

伽利略造了最早的几架望远镜之一，并做了一件事后看来理所当然、在当时却是全新的事：他把它对准了夜空。他的发现，写进一本名叫《星际信使》的薄薄小书里，击碎了古人那幅完美无瑕的宇宙图景。

本该是光滑天球的月亮，原来是一个崎岖的世界，像地球一样布满山脉与峡谷。银河那片朦胧的微光，化作了无数颗各自独立的恒星。而在木星这颗行星周围，有四个细小的光点正悄悄绕行——那是它自己的卫星。

它是如何诞生的

望远镜是 1608 年的荷兰发明，一件用来看远处船只和尖塔的新奇玩意儿。在帕多瓦当教授的伽利略听说了它，自己琢磨出了镜片，还磨出了好得多的一架。1609 年秋天，他开始用它扫视天空。

1610 年 1 月 7 日，他把它对准明亮的木星，注意到旁边有三颗排成一线的小「星」。他观察了好几个星期。它们一夜一夜地移动，又来了第四颗，它们躲到行星背后、再转出来——却始终不曾走开。它们只能是卫星，绕着木星转。他赶忙把这个发现付印，并把这几颗新卫星献给他的赞助人美第奇家族，称之为「美第奇星」。（一位德国天文学家西门·马里乌斯声称自己也看到了它们；流传下来的名字——木卫一、木卫二、木卫三、木卫四——正是他取的。）

它为何重要

两千年来，人们一直被教导：天上的一切都绕着地球转，而天空是完美、不变的。伽利略这本小书，一举推翻了这两点。多山的月亮并不完美；而绕着木星转的四颗卫星，则证明并非万物都绕地球转。他并没有证明地球在动——那是后来的事——但他表明了那些古老的笃信是错的，而且他靠的不是辩论，而是观看。近代的观测科学，正始于此。

一个可以想象的画面

想象你在田野的另一头，正好从侧面看一座旋转木马。你看不见那个圆——骑手们只是向左滑、减速、停住，再向右滑回来，如此反复。从地球看木星的卫星，正是这副样子：它们的圆形轨道，从侧面看去，就成了沿一条线的来回挪动。靠中心、又快的那颗（木卫一）来回飞窜；远在外侧、又慢的那颗（木卫四）则懒洋洋地漂移。看得够久，唯一诚实的解释就是：它们在绕圈。

它的位置

半个世纪以前，哥白尼（1543，见本馆）已斗胆把太阳放到了中心；而就在前一年，开普勒（1609）把行星的轨道弯成了椭圆。伽利略给出了二人都没有的东西：来自一件新仪器的、亲眼所见的证据。他发现的木星卫星，成了「哥白尼图景可能是真的」最有力的论据之一——而这条线索一路通向牛顿（1687），是牛顿的引力解释了行星何以能拥有卫星。伽利略自己的胆识，后来让他付出了代价：教会在 1633 年审判了他。

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