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一篇自己寫的關於古代希臘天文學的文章
作者: 旅行者
The Greek Heavenly Thoughts
The Greek was the first recognize the distinction between the natural and the supernatural. In other words , they found the nature, since then, the cosmology(宇宙學) could never be maintain a system of myth.
Gradually, the Greek believed themselves to live in a cosmos, and considered the cosmos to be an entirely natural place. Besides they saw an order in the cosmos. The order, to them, not only means an arrange, but also a good order, and also a beautiful, elegant order. With an optimism(樂觀主義), they believed the order, and the cosmos were comprehensible(可知的), and furthermore, could be described in words and numbers. When the Greek formed these crucial ideas, some earliest philosophers and scientists were important and influential. The Milesians(米利都學派得的哲學家), including Thales(泰勒斯), Anaximander(阿納克西曼德), described the cosmos in entirely natural terms for the first time. Heraclitus(赫拉克立特) insisted that the cosmos worked according to a logos(理性), which mean 「word」, 「account」, 「measure」, and 「proportion」, and if we grasp this logos, the cosmos could be understood and described well.
After the Greek accepted the idea that the cosmos was a natural place and comprehensible, some pre-Socratics(前蘇格拉底時代德哲學家) overcame some significant conceptual(觀念上的) hurdles to achieve a more sophisticated cosmology. There was the move from a hemispherical(半球形) universe to a spherical(球形) one, and a earth supported by water to supported by air, and then to one which need no support. In the earlier Greek cosmologies, objects were thought to drop in parallel straight lines from the top of the cosmos to the bottom, this led to the problem why the earth doesn』t fall to the bottom of the cosmos, so in this sort of the cosmos, something is required to support the earth. Then Aristotle placed the earth at the center of the cosmos, there was no the problem of the earth dropping. He believed that objects moved to the center of the cosmos, that is the earth itself.
The idea that the earth was central and stable dominated Greek astronomy and cosmology. In Greek minds, it can explain many phenomena. Firstly, the experience told Greek, if one was in rapid motion, he certainly knew about it, so they thought, if the earth has a daily rotation from west to east, there should be constant wind from east to west; if the earth is in motion around the sun, all objects will sweep off the face of the earth. Secondly, because they had no knowledge of gravity, they believe if the earth in rapid motion, it will disintegrate(解體). Thirdly, if the earth takes up different positions relative to the stars(恆星), there should be a stellar parallax(恆星視差量), one can detect slight changes in the apparent positions of the stars from the earth.
When the Greek physics, astronomy, philosophy and common sense all seemed to indicate an immobile earth, there was an important consequence, all of the motions of the heavens were real motions to the earth, not apparent ones due to the motion of the earth.
At the outset, Greek observational astronomy was rather divorced(背離) from philosophical speculation about the nature of the cosmos. Some observed the heavens and took careful notes of what they saw, some produced cosmological models based on general philosophical considerations. But no one gave a cosmological model that were anywhere near explaining, in a precise manner, the phenomena that had been record. The Pythagoreans(畢達哥拉斯學派的哲學家) made the first attempt. They believed in the center of the cosmos was a fire, not the sun, the fire was shield from the earth by a body known as the counter earth. The model was vague and speculative, and the earth moves around the fire, and between the fire and earth, there was the counter earth. Plato(柏拉圖) produced a slightly better model, had central earth, with the sun, the moon, the five planets and the stars orbiting around it. Then it was a basic assumption of Greek astronomy and cosmology, if some motions seems not circular or regular, almost all Greek philosophers and scientists would consider these as some combination of several circular and regular motions. Neither of two models could account for two important phenomena relating to the point at which the sun sets. The sun does not always set due west(正西方). Where the sun sets during a year changes from a maximum of 23.5º north of west to a maximum of 23.5°north of west. Solstices(至日，包括冬至、夏至) occur at the maximum points, while equinoxes(分日，包括春分、秋分) occur at the due west. Later Plato gave the first model that could give a reasonable account for these phenomena. The model was one of the most important Greek cosmological models. Plato made the first serious attempt to unite astronomical phenomena and cosmological traditions. He insisted that the sun, the moon, have a combination of two regular circular motions, that』s to say, the sun and the moon have a second motion in addition the daily one, offset(偏移) at an angle to it. As to the five planets, which are wandering in the heaven apparently, Plato gave a similar but more complex explanation. The model could give a reasonable explanation approximately, but couldn』t explain the planets exceptive motions. They will progress against the background of the fixed stars over a year or two, or even longer, then they will halt suddenly, and go in the other direction for a while, stop again, after that, progress in the normal direction. Facing a series strange exceptions and motions of the planets, Eudoxus(人名，難以翻譯，重要的哲學家、科學家) thought regressively(回歸，追本溯源). He was a pupil of Pythagorean, and had a close relationship with Plato. About planets, Plato』s model had two regular circular movements for each planet, Eudoxus gave four circular movements for each planet. In this model, the first sphere (環), generate a daily motion; the second generate the motion along the ecliptic(黃道), the other two produce a pattern like the figure 8 laid on its sides(行星出現在黃道正負8度範圍內). Besides, Eudoxus had a record of planets』 motion, and made a good use of mathematics, so compared with Plato』s model, his is more sophisticated.
Eudoxus』 model was excellent model for a long time, but there were some inherent difficulties. Because the distance between the earth and each one of five planets that we can see with naked eyes, always changes, it is not difficult to find that all planets』 size are varying apparently. Eudoxus can』t cope with the phenomenon, in his model, the distances between the earth and the planets are constant, that means the size of planets can』t vary. Account for these problems, Ptolemy(托勒密) produced a whole new system which was to last for nearly 1500 years. The system was based on combination of regular circular motions too. He gave up concentric(同心圓) sphere model in favor of a system based on a device, the epicycle(本輪，天文模型的一種). The epicycle is a combination of two regular circular motions, but not around the same center. The center of smaller circle moves around the larger circle. The two circles can produced many complex patterns, because these patterns, Ptolemy was able to explain most problems that beset the concentric sphere model, including the variation of apparent size, the brightness, the velocity(速度) of the planets. Besides the epicycle, Ptolemy made another subtle change, the earth is offset from the center of the sun』s orbit. So the model can account for that seasons have slightly different lengths. Now before the telescope was invented and new phenomena that can』t be found with naked eyes were detected, the model is the best in the ancient times.
After talking about some important Greek cosmological models, we have to mention an exceptive astronomer, Aristarchus(阿里斯塔克斯). He is the first one who have a revolutionary thinking that the earth is orbiting around the sun. But sadly, we don』t know how and why he can be aware this without any idea about gravity and the phenomena which only can be detected with telescope, only we know he is a follower of Aristotle. |
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