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| February 2008 |
| Billie Silvey |
| Human beings have always looked into the sky and marveled at its beauty. They’ve also speculated about its nature, and tried to impose system and order on it. |
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| A Brief History of Cosmology |
| Aristotle |
| The Greek philosopher Aristotle was an early speculator. His theory was based on the common-sense observation that the sun, moon and stars seemed to move around the earth. This was satisfying to humans, who liked to put themselves in the middle of things. Aristotle developed the theory of eight crystal spheres on which the sun, moon, planets and stars rotated in perfect, unchanging paths. Their circular perfection was in contrast with the earth, where all changed and decayed, motion was in straight lines, and the earth itself stood still. |
| Ptolemy |
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| Claudius Ptolemaeus in the 2nd century A.D. agreed with Aristotle that the universe orbits about a stationary Earth, but he refined the theory by suggesting that the planets move in circular epicycles with a center that moved in a larger circular orbit around a point near the earth. The Ptolemaic system was the longest-lasting of the universe models. |
| Copernicus |
| Ptolemaeus’ theory reigned supreme for some 13 centuries, when the Council of Trent in 1543 asked a mathematician, Nicholas Copernicus, to address a difficulty with the calendar, the difference between solar and lunar months, which caused days to be lost and made it particularly difficult to determine Easter. Copernicus ended up questioning the entire Aristotelian concept of an earth-centered universe, suggesting instead that the earth, as well as the other planets, revolved around the sun. The Copernican theory enabled accurate and repeatable observation, the basis of science. |
| Galileo |
| Galileo Galilei, who became professor of mathematics at Padua in 1591, decided that science should be based on regular occurences, which could be repeatedly observed and reduced to mathematics. He determined that falling objects accelerated as they fell. In 1610, he identified the three moons of Jupiter, positing that, if they circled Jupiter while Jupiter circled the sun, our moon could do the same. He discovered sunspots. And in 1624, he argued that the tides were due to the earth’s movement. Finally, in 1632, his Dialogues on the Two Chief Systems of the World attacked the opponents of the Copernican system and led to his house arrest, which continued until his death in 1642. The book was in the Index of Prohibited Books until 1835. |
| Johannes Kepler studied planetary motion in detail, discovering that Mars’ path around the sun was not circular, but elliptical. It speeded up when it was closer to the sun and slowed down when it was farther away. Kepler discovered geometrical laws which applied to planetary orbit: the duration of a planetary orbit equals its distance from the sun cubed. |
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| Kepler |
| Descartes |
| In 1631, the frenchman Rene Descartes, in The Discourse on Method, said to doubt everything, accept as probable what was considered certain, and reject everything else. The only certainty was thought or critical doubt. The simplest solution should be examined before the more complex, straight lines before curves. He imagined a problem solved and studied the consequences of the solution to determine whether his solution was right or wrong. He described a universe that wasn’t a vacuum and didn’t need attraction to function, postulating that three kinds of matter--solids, light and ether--filled space. The spinning of the planets in a vortex created gravity. “The world is a machine,” he concluded, using graphing to determine position. |
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| Isaac Newton, an Englishman, discovered how the universe worked in two years, but didn't publish his findings until 20 years later, in 1687, in Principia Mathematica. By limiting his search to how, not why, he developed calculus to measure the changing forces involved in planetary dynamics. Differential calculus measured the difference caused by change. Integral calculus studied how the rates of change varied due to mutual attraction. |
| Newton |
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| Einstein |
| Friedmann |
| Modern scientific cosmology began in Germany in 1917 with Albert Einstein’s general theory of relativity, which enabled the study of very distant objects. Earlier physicists assumed the universe to be static and unchanging. General relativity consists of a set of equations that must be solved from the distribution of mass-energy and momentum throughout the universe. Einstein postulated matter without motion and that matter was uniformly distributed in a uniformly curved spherical space. |
| The Russian, Alexander Friedmann, pointed out in 1922 that the universe expands then recollapses. The curvature is negative but unbounded, so theoretically the universe can expand indefinitely. That paved the way for the Big Bang theory of origins. |