導讀：如果說哥白尼革命從根本上改變了我們對自己在宇宙中所處位置的思考，這種說法并不夸大。在古代，人們認為地球是太陽系和宇宙的中心，而現在，我們知道我們只不過是圍繞太陽運轉的多個行星中的一個。

 

 

如果說哥白尼革命從根本上改變了我們對自己在宇宙中所處位置的思考，這種說法并不夸大。在古代，人們認為地球是太陽系和宇宙的中心，而現在，我們知道我們只不過是圍繞太陽運轉的多個行星中的一個。

但這種觀點的轉變并不是一夜之間發生的。相反，新理論和科學觀察歷經一個世紀的時間，利用簡單的數學和基本的儀器，才揭示出我們在宇宙中的真實位置。

我們可以通過閱讀對此做出貢獻的天文學家的筆記，來深入了解這一深遠的轉變是如何發生的。這些筆記為我們提供了引領哥白尼革命的那些辛苦勞動、創新見解和過人才能的線索。

 

游星

想象你是一位來自古代的天文學家，在沒有望遠鏡的幫助下探索夜空。最初，行星與其它星星并沒有多大區別。它們只是比大多數的星星更加明亮，而且不怎么閃爍，其它方面沒什么不同。

在古代，區分行星和其它星星的真正方式是它們在天空中的運動。夜復一夜，相對于星星來說，行星是逐漸移動的。實際上“行星”一詞起源于古希臘的“游星”（wandering star）

行星的運動方式并不簡單。當它們穿過天空的時候，有時候加速有時候減速。甚至，有時候會出現暫時的反向運動，天文學中稱為“逆行”。這又怎么解釋呢？

 

托勒密的本輪

古希臘的天文學家建立了太陽系的地心說模型，托勒密是地心說的集大成者。上面所述的這個模型，來自托勒密的《天文學大成》（公元二世紀時普托勒密作的天文學數學名著）的阿拉伯版本。

 

 

托勒密利用兩個疊加的圓周運動來解釋行星運動，行星在一個小的圓上運動，稱為本輪 (epicenter，)，而本輪的中心循著均輪（deferent）的大圓繞地球運行。此外，每個行星的均輪可能偏離地球的位置，因為地球并不在均輪的中心上，而是略偏于中心的一次，圍繞均輪的穩定（角）運動可以用托勒密體系天體運行軌道的等分來定義，而不是用地球的位置或均輪的中心點，等分點位于均輪中心的另一側。明白了嗎？

這理解起來有點復雜。托勒密模型最大的貢獻是，它預測了行星在夜空中的位置，與實際相差很小。這個模型成為一千多年的時間里，解釋行星運動的主要方法。

 

哥白尼的轉變

在1543年，哥白尼去世那一年，隨著《天體運行論》（對宇宙認識的革命）的出版，與他同名的革命才正式開始。哥白尼模型認為太陽是太陽系的中心，行星圍繞太陽，而不是地球運轉。

 

 

也許是哥白尼模型最精彩的部分是解釋了行星視運動的規律。像火星這樣的行星逆行只是一種錯覺，因為它們都在軌道上繞太陽運行，當火星運行的軌道方向與地球不同時，在地球上觀看火星，就會產生火星在倒退行進的視覺效果。

 

托勒密體系

最初的哥白尼模型是建立在托勒密體系基礎上的。哥白尼學說中行星圍繞太陽運轉，依然利用疊加的圓周運動來描述。哥白尼對使用等分點持蔑視態度，因此他放棄了這一概念，取而代之的是小本輪。

天文學和天文史學家歐文 金格里奇和他的同事利用那個時代的托勒密和哥白尼模型計算了行星坐標，發現兩個模型都有類似錯誤。在某些情況下，火星位置的誤差是2度或更多（遠大于月球直徑）

由于16世紀的天文學家沒有望遠鏡、牛頓物理學和統計學，因此，哥白尼模型和優于托勒密模型，對他們來說并不明顯，即使哥白尼正確地指出了太陽是太陽系的中心。

 

 

到了伽利略時代

從1609年開始，伽利略用剛剛發明的望遠鏡觀察太陽，月亮和行星。他看到了月球上的山脈和環形山，首次揭示了行星存在衛星。伽利略通過強大的觀測證據，強有力地支持了行星圍繞太陽運轉這一事實。

伽利略對金星的觀測特別引人注目。根據托勒密模型，金星始終處于地球和太陽之間，因此我們應該看到金星的暗面。但是伽利略能夠觀察到金星白晝面，推斷出金星是在太陽與地球之間的軌道上繞太陽旋轉。

 

 

開普勒與金星的戰斗

托勒密的圓周運動和哥白尼模型導致了更大的誤差，尤其是火星，其預測位置有幾度的誤差。開普勒花費了多年時間來了解火星的運動，他用一個最巧妙的方式破解了這個問題。

行星繞太陽運轉時重復同樣的路徑，因此它們在完成每一個軌道周期后，都返回到同一位置。例如，火星每687天返回到軌道上的同一位置。

開普勒知道一顆行星出現在太空中同一位置的日期，他可以利用地球的不同位置，沿著地球軌道對行星的位置做出三角測量，如上圖所示。開普勒，利用天文學家第谷 布拉赫用肉眼觀測的信息，能夠勾畫出行星繞太陽運轉的橢圓形路徑。

 

 

這讓開普勒創立了行星運動的三大規律，以及遠比之前更高的精度預測行星的位置。他為17世紀晚期牛頓物理學的建立打下了基礎，這一非凡的科學隨后出現。

 

“英文原文”

Copernicus' revolution and Galileo's vision: Our changing view of the universe in pictures

It's not a stretch to say the Copernican revolution fundamentally changed the way we think about our place in the universe. In antiquity people believed the Earth was the centre of the solar system and the universe, whereas now we know we are on just one of many planets orbiting the sun.

But this shift in view didn't happen overnight. Rather, it took almost a century of new theory and careful observations, often using simple mathematics and rudimentary instruments, to reveal our true position in the heavens.

We can gain insights into how this profound shift unfolded by looking at the actual notes left by the astronomers who contributed to it. These notes give us a clue to the labour, insights and genius that drove the Copernican revolution.

Wandering stars

Imagine you're an astronomer from antiquity, exploring the night sky without the aid of a telescope. At first the planets don't really distinguish themselves from the stars. They're a bit brighter than most stars and twinkle less, but otherwise look like stars.

In antiquity, what really distinguished planets from stars was their motion through the sky. From night to night, the planets gradually moved with respect to the stars. Indeed "planet" is derived from the Ancient Greek for "wandering star".

And planetary motion isn't simple. Planets appear to speed up and slow down as they cross the sky. Planets even temporarily reverse direction, exhibiting "retrograde motion". How can this be explained?

Ptolemy epicycles

Ancient Greek astronomers produced geocentric (Earth-centred) models of the solar system, which reached their pinnacle with the work of Ptolemy. This model, from an Arabic copy of Ptolemy's Almagest, is illustrated above.

Ptolemy explained planetary motion using the superposition of two circular motions, a large "deferent" circle combined with a smaller "epicycle" circle.

Furthermore, each planet's deferent could be offset from the position of the Earth and the steady (angular) motion around the deferent could be defined using a position know as an equant, rather than the position of the Earth or the centre of the deferent. Got that?

It is rather complex. But, to his credit, Ptolemy's model predicted the positions of planets in the night sky with an accuracy of a few degrees (sometimes better). And it thus became the primary means of explaining planetary motion for over a millennium.

Copernicus' shift

In 1543, the year of his death, Nicolaus Copernicus started his eponymous revolution with the publication of De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). Copernicus' model for the solar system is heliocentric, with the planets circling the sun rather than Earth.

Perhaps the most elegant piece of the Copernican model is its natural explanation of the changing apparent motion of the planets. The retrograde motion of planets such as Mars is merely an illusion, caused by the Earth "overtaking" Mars as they both orbit the sun.

Ptolemaic baggage

Unfortunately, the original Copernican model was loaded the Ptolemaic baggage. The Copernican planets still travelled around the solar system using motions described by the superposition of circular motions. Copernicus disposed of the equant, which he despised, but replaced it with the mathematically equivalent epicyclet.

Astronomer-historian Owen Gingerich and his colleagues calculated planetary coordinates using Ptolemaic and Copernican models of the era, and found that both had comparable errors. In some cases the position of Mars is in error by 2 degrees or more (far larger than the diameter of the moon). Furthermore, the original Copernican model was no simpler than the earlier Ptolemaic model.

As 16th Century astronomers did not have access to telescopes, Newtonian physics, and statistics, it wasn't obvious to them that the Copernican model was superior to the Ptolemaic model, even though it correctly placed the sun in the centre of the solar system.

Along comes Galileo

From 1609, Galileo Galilei used the recently invented telescope to observe the sun, moon and planets. He saw the mountains and craters of the moon, and for the first time revealed the planets to be worlds in their own right. Galileo also provided strong observational evidence that planets orbited the sun.

Galileo's observations of Venus were particularly compelling. In Ptolemaic models, Venus remains between the Earth and the sun at all times, so we should mostly view the night side of Venus. But Galileo was able to observe the day-lit side of Venus, indicating that Venus can be on the opposite side of the sun from the Earth.

Kepler's war with Mars

The circular motions of Ptolemaic and Copernican models resulted in large errors, particularly for Mars, whose predicted position could be in error by several degrees. Johannes Kepler devoted years of his life to understanding the motion of Mars, and he cracked this problem with a most ingenious weapon.

Planets (approximately) repeat the same path as they orbit the sun, so they return to the same position in space once every orbital period. For example, Mars returns to the same position in its orbit every 687 days.

As Kepler knew the dates when a planet would be at the same position in space, he could use the different positions of the Earth along its own orbit to triangulate the planets' positions, as illustrated above. Kepler, using astronomer Tycho Brahe's pre-telescopic observations, was able to trace out the elliptical paths of the planets as they orbited the?sun.

This allowed Kepler to formulate his three laws of planetary motion and predict planetary positions with far greater precision than previously possible. He thus laid the groundwork for the Newtonian physics of the late 17th century, and the remarkable science that followed.

 

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哥白尼革命和伽利略的視野：改變我們的宇宙觀

圖文簡介

人們曾認為地球是太陽系和宇宙的中心，現在我們知道我們只不過是圍繞太陽運轉的多個行星中的一個。