As far as Kepler was concerned, elliptical orbits were merely an ad hoc hypothesis, and a rather repugnant one at that, because ellipses were clearly less perfect than circles. Having discovered almost by accident that elliptical orbits fit the observations well, he could not reconcile them with his idea that the planets were made to orbit the sun by magnetic forces. An explanation was provided only much later, in 1687, when Sir Isaac Newton published his Philosophiae Naturalis Principia Mathematica, probably the most important single work ever published in the physical sciences. In it Newton not only put forward a theory of how bodies move in space and time, but he also developed the complicated mathematics needed to analyze those motions. In addition, Newton postulated a law of universal gravitation according to which each body in the universe was attracted toward every other body by a force that was stronger the more massive the bodies and the closer they were to each other. It was this same force that caused objects to fall to the ground. (The story that Newton was inspired by an apple hitting his head is almost certainly apocryphal. All Newton himself ever said was that the idea of gravity came to him as he sat “in a contemplative mood” and “was occasioned by the fall of an apple.”) Newton went on to show that, according to his law, gravity causes the moon to move in an elliptical orbit around the earth and causes the earth and the planets to follow elliptical paths around the sun.
The Copernican model got rid of Ptolemy’s celestial spheres, and with them, the idea that the universe had a natural boundary. Since “fixed stars” did not appear to change their positions apart from a rotation across the sky caused by the earth spinning on its axis, it became natural to suppose that the fixed stars were objects like our sun but very much farther away.
ad hoc 特別的
repugnant 使人極度反感的
apocryphal 偽的
contemplative 冥想的
就開普勒而言，橢圓軌道僅僅是想當然的，並且是相當討厭的假設，因為橢圓明顯地不如圓那麼完美。雖然他幾乎是偶然地發現橢圓軌道能很好地和觀測相符合，但卻不能把它和他的行星繞太陽運動是由於磁力引起的另一思想相互調和起來。對這一切提供解釋是晚得多的事，那是由於1687年伊薩克·牛頓爵士出版了他的《數學的自然哲學原理》，這部也許是有史以來物理科學上最重要的單獨的著作。在這本中，牛頓不但提出物體如何在空間和時間中運動的理論，並且發展了為分析這些運動所需的複雜的數學。此外，牛頓提出了萬有引力定律，根據這定律，宇宙中的任一物體都被另外物體所吸引，物體質量越大，相互距離越近，則相互之間的吸引力越大。這也就是使物體落到地面上的力。（由於一個蘋果落到牛頓的頭上而使他得到靈感的故事，幾乎肯定是不足憑信的。所有牛頓自己說過的只是，當他陷入沉思之時，一顆蘋果的落下使他得到了萬有引力的思想。）牛頓繼而指出，根據他的定律，引力使月亮沿著橢圓軌道繞著地球運行，而地球和其他行星沿著橢圓軌道繞著太陽公轉。
哥白尼的模型擺脫了托勒密的天球，以及與其相關的宇宙存在著自然邊界的觀念。“固定恆星”除了由於地球繞著自身的軸自轉引起的穿越天空的轉動外，不改變它們的位置，很自然會使人設想到固定恆星是和我們的太陽類似的物體，只是比太陽離開我們遠得多了。
閱讀更多 讀書看劇記單詞 的文章
