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Western University
Astronomy 2021A/B

3 • The atomists held that both Earth and the heavens were made from an infinite number of indivisible atoms of each of the four elements. • The Aristotelians (after Aristotle) held that the four elements—not necessarily made from atoms—were confined to the realm of Earth, while the heavens were made of a distinct fifth element, often called the aether (or ether). The atomist doctrine was developed largely by Democritus and his views show how the idea led almost inevitably to belief in extraterrestrial life. Democritus argued that both Earth and the heavens had been created by the random motions of infinite atoms. Because this idea held that the number of atoms was infinite, it was natural to assume that the same processes that created our world could also have created others. This philosophy on life beyond Earth was clearly described in the following quotation from a later atomist, Epicurus (341–270 B.C.): “There are infinite worlds both like and unlike this world of ours ... we must believe that in all worlds there are living creatures and plants.” Aristotle had a different view. He believed that each of the four elements had its own natural motion and place. For example, he believed that the element earth moved naturally toward the center of the universe, an idea that offered an explanation for the Greek assumption that Earth resided in a central place. The element fire, he claimed, naturally rose away from the center, which explained why flames jut upward into the sky. Aristotle rejected the atomist idea of many worlds. If there was more than one world, there would be more than one natural place for the elements to go, which would be a logical contradiction. Aristotle concluded: “The world must be unique.... There cannot be several worlds.” The contradiction between the Aristotelian notion of a single world and the atomist notion of many worlds became a subject of great concern to Christian theologians. Moreover, because the atomist view held that our world came into existence through random motions of atoms, and hence without the need for any intelligent Creator, atomism became associated with atheism. COPERNICUS: In 1543, Nicholas Copernicus published De Revolutioninus Orbium Coelestium (“Concerning the Revolutions of the Heavenly Spheres”). In his book, Copernicus revived Aristarchus’s Sun-centered solar system. By the time of Copernicus’s birth tables of planetary motion based on the Ptolemaic model had become inaccurate. Copernicus adopted Aristarchus’s Sun-centered idea, probably because he was drawn to its simple explanation for the apparent retrograde motion of the planets. The success of his model in providing a geometric layout for the solar system further convinced him. High-ranking officials of the Church urged him to publish a book. Copernicus saw the first printed copy of his book on the same day that he died. The model gained relatively few converts over the next 50 years, and for a good reason: It didn’t work. The primary problem was that he kept the ancient belief that heavenly motion must occur in perfect circles. TYCHO: In the late sixteenth century, Danish nobleman Tycho Brahe, set about correcting this problem. He built large naked-eye observatories that worked much like giant protractors, and over a period of three decades he used them to measure planetary positions to within 1 minute of arc (of 1°)—which is less than the thickness of a fingernail held at arm’s length. KEPLER: In 1600, Tycho hired a young German astronomer named Johannes Kepler. Like Copernicus, he believed that planetary orbits should be perfect circles, so he worked diligently to match circular motions to Tycho’s data. After years of effort, he found a set of circular orbits that matched most of Tycho’s observations quite well. However, small discrepancies, such as being 8 arc minutes off Tycho’s positing of Mars, finally led Kepler to abandon the idea of circular orbit. He found that planetary orbits take the shapes of the special types of ovals known as ellipses. He then constructed his laws of planetary motion: • Kepler’s first law: The orbit of each planet about the Sun is an ellipse with the Sun at one focus. In essence, this law a planet’s distance from the Sun varies during its orbit. It is closest at the point called perihelion (from the Greek for “near the Sun”) and farthest at the point called aphelion (from the Greek for “away from the Sun”). The average of a planet’s perihelion
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