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Previous Models of the Solar System
Ancient Times
The Greek Thales, who had successfully predicted the sun’s eclipse in 585 B.C., suggested that the sky was a huge bubble, inside of which the cylindrical Earth floating on the surface of water. The stars and planets were embedded in the bubble. His theory didn’t satisfy the scientific community of his time, which wanted to know how the Earth was supported.His contemporary, Anaximander, had an answer. It was unnecessary, he said, for the Earth to have supports. It keeps its pace because it is “of equal distance from everything.” He differed from Thales in thinking the world was a flat plate.
The mathematician and cultist Pythagoras taught the Earth was a star, and that all stars were possible earths.
Philolaus, his disciple, correctly guessed that the Earth’s shape was round, like a ball. He then went further and removed the planet from the center of the universe, putting it instead on a revolving disc like those of the sun, moon, and planets. All of these rotated around the “hearth of the universe.” Since, Philolaus thought, the Earth was so heavy when compared to the planets, it needed a counterweight, Antichton, which rotated directly opposite the Earth, behind the central fire, and so was never seen.
Despite Philolaus’ missteps, his rejection of geocentrism, combined with many other discoveries (such as Empedocles' 440 B.C. discovery of the Earth’s atmosphere) meant that some steps towards an accurate picture of the solar system were being made. Further advances were retarded, however, by Socrates and later by Plato. Both placed the Earth in the center of the universe, and replaced observations with geometry. They accepted the Earth as round, but only because the circle was a perfect figure, the only possible creation of a perfect God. For the same reason they thought that the planets must orbit in perfect circles as well.
This was known to be manifestly untrue, even in Greek times. In their wanderings, the planets seem to loop around, moving back east before continuing west. It was necessary to invent elaborate theoretical justification for what wouldn’t fit into a dogmatic theory. (The Greeks didn’t have a monopoly on this myopia, however. A wandering Muslim merchant stopped a Hindu Brahmin who was lecturing on the structure of the universe. It was, said the Brahmin, supported on the back of an elephant. What supported the elephant, asked the merchant. A turtle, answered the Brahmin. What supported the turtle, asked the merchant. Another turtle, answered the Brahmin. What supported that turtle, asked the merchant. Suppose we stop asking questions, suggested the Brahmin.)Heraclides suggested that Venus and Mercury rotated around the sun, which rotated in turn around the Earth.
Aristarchus, in 250 B.C., was the first man to put the Earth in rotation around the sun. He also put the moon in rotation around the Earth, explaining its phases. He persisted, however, in believing that the planets’ structure was made up of a divine element missing on Earth.
In 150 B.C., Hipparchus put the Earth back in the center of the universe. He explained the planets’ rotational eccentricities by “epicycles,” or smaller circles in the larger circular orbit.
In 100 B.C., the Epicureans taught that all planets were made of inanimate matter. No one paid any attention.
Eratosthenes correctly measured the Earth within 402 kilometers of its true diameter and found it half the size that Aristotle claimed it was.
Ptolemy, in A.D. 140, compiled the work of his predecessors, including Hipparchus, the Father of Astronomy, and edited a geocentric work that was accepted by the practically-minding Romans, ignored by the unfortunate inhabitants of the Dark Ages, and made an article of faith for inhabitants of the Middle Ages. Ptolemy’s work put the study of astronomy on hold for 1400 years.
Modern Times
More than a millennium later, a Polish astronomer, Copernicus, came across the works of Ptolemy and Aristarchus that had been brought to Europe by the invading Moors. He compared the two theories and synthesized them into a theory of his own, where Ptolemy’s eighty epicycles were reduced by half. He then simplified it further, developing it into the solar system we know today, with the Sun surrounded by Mercury, Venus, the Earth, Mars, Jupiter, and Saturn. He justly feared the Church, which included all scientists of the day, and which espoused the Ptolemaic model, so he had the work published in 1543 on the day before his death, when it was too late to excommunicate him.Many astronomers welcomed the Copernican model of the solar system. The great Dane, Tycho Brahe, however, was loath to dethrone the Earth from the center of the universe. From his observatory on the Baltic Sea, he accumulated a life’s work of accurate information of the planetary progressions. He proposed a world where the Moon and the Sun rotated around the Earth independently. The planets rotated around the Sun. Brahe’s dying wish was that his German pupil Johannes Kepler would devote himself towards proving the Brahian system true.
Kepler labored for eight years, finally bringing himself to prove his master’s system false. He returned to the Copernican theory of the solar system, but found it buggy as well. Instead of traveling in circles, Kepler proved, planets orbit in ellipses. He also found time to write the three laws of planetary motions, which explain the path that the planets.Another student of Brahe, the Italian Giordano Bruno, took his master’s discovery of the elliptical orbits of comets to prove that the crystal sphere encasing the universe did not exist at all. He taught that the stars were at different distances from each other, surrounded in limitless space. Less cautious than Copernicus, he was tried for heresy and burned at the stake.
Another Italian, Galileo Galilei, discovered Jupiter’s four major moons through a homemade telescope. He observed that they orbited around Jupiter just as Copernicus claimed the planets orbited the sun. He was the man who finally taught that the planets were worlds just as ours, not slivers of some fifth element nor gears in a clockwork universe. He recanted his theory under threat of torture, and was confined to house arrest, dying in 1642, the year Isaac Newton was born.
Newton’s universal law of gravitation was his most important contribution to the study of the solar system. With it he showed why the planets follow the orbits described by Kepler. For the first time astronomers were able to predit the positions of the planets and foretell eclipses, even centuries in the future. With Kepler and Newton, modern astronomy may truly be said to have arrived.