哥白尼革命和伽利略的视野：改变我们的宇宙观
导读：如果说哥白尼革命从根本上改变了我们对自己在宇宙中所处位置的思考，这种说法并不夸大。在古代，人们认为地球是太阳系和宇宙的中心，而现在，我们知道我们只不过是围绕太阳运转的多个行星中的一个。
如果说哥白尼革命从根本上改变了我们对自己在宇宙中所处位置的思考，这种说法并不夸大。在古代，人们认为地球是太阳系和宇宙的中心，而现在，我们知道我们只不过是围绕太阳运转的多个行星中的一个。
但这种观点的转变并不是一夜之间发生的。相反，新理论和科学观察历经一个世纪的时间，利用简单的数学和基本的仪器，才揭示出我们在宇宙中的真实位置。
我们可以通过阅读对此做出贡献的天文学家的笔记，来深入了解这一深远的转变是如何发生的。这些笔记为我们提供了引领哥白尼革命的那些辛苦劳动、创新见解和过人才能的线索。
游星
想象你是一位来自古代的天文学家，在没有望远镜的帮助下探索夜空。最初，行星与其它星星并没有多大区别。它们只是比大多数的星星更加明亮，而且不怎么闪烁，其它方面没什么不同。
在古代，区分行星和其它星星的真正方式是它们在天空中的运动。夜复一夜，相对于星星来说，行星是逐渐移动的。实际上“行星”一词起源于古希腊的“游星”（wandering star）
行星的运动方式并不简单。当它们穿过天空的时候，有时候加速有时候减速。甚至，有时候会出现暂时的反向运动，天文学中称为“逆行”。这又怎么解释呢？
托勒密的本轮
古希腊的天文学家建立了太阳系的地心说模型，托勒密是地心说的集大成者。上面所述的这个模型，来自托勒密的《天文学大成》（公元二世纪时普托勒密作的天文学数学名著）的阿拉伯版本。
托勒密利用两个叠加的圆周运动来解释行星运动，行星在一个小的圆上运动，称为本轮 (epicenter，)，而本轮的中心循著均轮（deferent）的大圆绕地球运行。此外，每个行星的均轮可能偏离地球的位置，因为地球并不在均轮的中心上，而是略偏于中心的一次，围绕均轮的稳定（角）运动可以用托勒密体系天体运行轨道的等分来定义，而不是用地球的位置或均轮的中心点，等分点位于均轮中心的另一侧。明白了吗？
这理解起来有点复杂。托勒密模型最大的贡献是，它预测了行星在夜空中的位置，与实际相差很小。这个模型成为一千多年的时间里，解释行星运动的主要方法。
哥白尼的转变
在1543年，哥白尼去世那一年，随着《天体运行论》（对宇宙认识的革命）的出版，与他同名的革命才正式开始。哥白尼模型认为太阳是太阳系的中心，行星围绕太阳，而不是地球运转。
也许是哥白尼模型最精彩的部分是解释了行星视运动的规律。像火星这样的行星逆行只是一种错觉，因为它们都在轨道上绕太阳运行，当火星运行的轨道方向与地球不同时，在地球上观看火星，就会产生火星在倒退行进的视觉效果。
托勒密体系
最初的哥白尼模型是建立在托勒密体系基础上的。哥白尼学说中行星围绕太阳运转，依然利用叠加的圆周运动来描述。哥白尼对使用等分点持蔑视态度，因此他放弃了这一概念，取而代之的是小本轮。
天文学和天文史学家欧文 金格里奇和他的同事利用那个时代的托勒密和哥白尼模型计算了行星坐标，发现两个模型都有类似错误。在某些情况下，火星位置的误差是2度或更多（远大于月球直径）
由于16世纪的天文学家没有望远镜、牛顿物理学和统计学，因此，哥白尼模型和优于托勒密模型，对他们来说并不明显，即使哥白尼正确地指出了太阳是太阳系的中心。
到了伽利略时代
从1609年开始，伽利略用刚刚发明的望远镜观察太阳，月亮和行星。他看到了月球上的山脉和环形山，首次揭示了行星存在卫星。伽利略通过强大的观测证据，强有力地支持了行星围绕太阳运转这一事实。
伽利略对金星的观测特别引人注目。根据托勒密模型，金星始终处于地球和太阳之间，因此我们应该看到金星的暗面。但是伽利略能够观察到金星白昼面，推断出金星是在太阳与地球之间的轨道上绕太阳旋转。
开普勒与金星的战斗
托勒密的圆周运动和哥白尼模型导致了更大的误差，尤其是火星，其预测位置有几度的误差。开普勒花费了多年时间来了解火星的运动，他用一个最巧妙的方式破解了这个问题。
行星绕太阳运转时重复同样的路径，因此它们在完成每一个轨道周期后，都返回到同一位置。例如，火星每687天返回到轨道上的同一位置。
开普勒知道一颗行星出现在太空中同一位置的日期，他可以利用地球的不同位置，沿着地球轨道对行星的位置做出三角测量，如上图所示。开普勒，利用天文学家第谷 布拉赫用肉眼观测的信息，能够勾画出行星绕太阳运转的椭圆形路径。
这让开普勒创立了行星运动的三大规律，以及远比之前更高的精度预测行星的位置。他为17世纪晚期牛顿物理学的建立打下了基础，这一非凡的科学随后出现。
“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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