Models of the Universe
by Anna Snyder
As Plato’s most famous student, Aristotle had many of his ideas. Aristotle believed that everything in the universe with the exception of Earth was perfect and without flaw. His theory of the universe was based on the assumption that the Earth laid at rest in the center of it. The rest of the sky revolved around the Earth once during the course of a day and night. The other Heavenly bodies revolved on crystal spheres of their own.
In the second century A.D., Ptolemy said Earth was a solid body around which the Sun, Moon, planets, and stars revolved. His system said that most heavenly bodies were carried about the Earth on their own rotating circles. This was known as the Ptolemaic, or geocentric (earth-centered) theory.
In about 260 B.C., Aristarchus of Samos suggested that the Earth revolves around the Sun. For a hundred years or so, astronomers fought with the idea before ultimately discarding it. In the 1500’s, once again, Copernicus suggested that Earth revolved around the Sun. Copernicus observed that the further a planet was away from the Sun, the longer it took to orbit it. The movement could only make sense if the Sun, not the Earth, was motionless. He saw that the patterns in the sky had shifted. And he dared to write the words “What appears to us as the motions of the Sun arise not from its motion but from the motion of the Earth.” Copernicus still believed in the idea of the perfect circular orbit, though, so his model was still not completely accurate. He compiled his theories in his book entitled De revolutionibus orbium colestium, or The Revolution of the Heavenly Orbs.
The Copernican model of the universe was different from everything that others believed. His thinking was so alien, no one could even comprehend such a thought. The Roman Catholic Church almost took it to be sacrilege, and many, such as the great Martin Luther, complained. In 1616, the Pope put Copernicus’ book on the Church’s list of prohibited books, where it stayed for over 200 years. Not long after, Galileo comes forth and states the same ideas as Copernicus. With his new telescope, Galileo discovered moons around Jupiter and to him, this proved Copernicus right. He defended Copernicus and worked on his own book, Dialogue Concerning the Two Chief World Systems. Then in 1632, his book was also banned, and he was brought to trial and ended up denying any belief in the Copernican system.
Tycho Brahe was a Danish astronomer that Kepler worked with in Tycho’s observatory near Prague. Tycho feared Kepler would overshadow him, so he with held much of his data. When he suddenly died, his full data was available. Not long after, Kepler discovered the 3 laws of planetary motion. In the Tychonic Universe, all the planets revolved around the Sun, and the Sun with other planets revolved around the Earth. In a way, this universe seemed to be a compromise between Aristotle and Copernicus.
All previous astronomers believed that things went in a circular orbit. One night, Kepler realized that Mars’ orbit was elliptical, not circular. This eventually led him to Kepler’s Laws of Planetary Motion. These laws are as follows:
1. The Sun is not at the center of a planets orbit, but is offset slightly in an ellipse
2. A planet moves faster when closer to the Sun and slower when farther away
3. The inner planets move faster around the Sun than the outer ones
These laws are what make up the Keplerian model of the universe and what he based his theories on.
Newton studied Kepler’s Laws of Planetary Motion and he decided that space was held together by a force, which he named gravity. He reasoned that the larger an object was, the stronger its pull of gravity would be. Newton explained how gravity was responsible for keeping the planets in the orbits described by Copernicus and Galileo. He also discovered certain laws of motion which are as follows:
1. Any object not moving will stand still unless affected by an outside force. If the object is moving, it will continue to move at a constant speed in a straight line.
2. When a force causes change in the speed of an object, the rate of change is in proportion to the strength of that force
He used his laws of motion and gravity to explain why the Moon orbits Earth. The same laws were applied to other planets and their satellites and to the solar system as a whole.
