Copernicus (15th century A.D.)

Since the time of Aristarchus, Copernicus was the first to propose that the Earth was not the center of the solar system but in fact that the sun was the center of the solar system. This new system is referred to as the heliocentric system. (Helios was the Greek god of the sun.) Copernicus however, did not eliminate the need for perfect circles in astronomical movements. Therefore, he retained the ideas of epicycles established by Ptolemy. Holding on to epicycles would make the Copernican system not much more accurate than the Ptolemaic system in predicting planetary positions.

The advantage of the Copernican system was that it better explained retrograde motion. The idea of retrograde motion under this system was that the Earth was passing the planet. Therefore parallax was causing the position of the planet to apparently move in the opposite direction of its actual motion. The diagram below shows the orbit of the Earth and the orbit of Mars along with the changing position of Mars against the background stars.

Copernicus would be persecuted for his beliefs and this persecution caused him to hesitate the publication of his book until near his death.

Tycho Brahe (16th century A.D.)

Brahe was most famous for his accurate observations. He was able to construct an observatory to measure the positions of the stars and planets. The accuracy of these measurements was greater than had ever been made before. Once all his data was collected, he hired a student named Kepler to assist him with processing the data. Brahe would, unfortunately, die before establishing the rules for planetary motion.

Kepler (16th century A.D.)

Kepler used the data collected by Brahe and himself and established three laws concerning the motion of the planets around the sun. The importance of these three laws is twofold. First, it requires that the sun be the center of the solar system and not the Earth. This heliocentric system was not well accepted even at the time of Kepler. Secondly, it accurately predicted the positions of the planets. These predictions were much more accurate than the Ptolemaic system.

Kepler's three laws were established by a guess and check method. This is referred to as an empirical method. That is to say, these laws were not derived from mathematical equations and given laws of motion. This will be very different than the laws of motion and the methods of Newton.

Kepler's first law of motion states "planets orbit the sun in elliptical paths with the sun at one focus". Your book shows a simple method for drawing an ellipse. Each tack represents a focus. Try drawing your own ellipses with the tacks at various distances from each other. These drawings will show ellipses with different eccentricities. If both tacks are at the same place, the ellipse is a circle, and the eccentricity is equal to zero.

Kepler's second law of motion states "an imaginary line from the focus (the sun) to the planet sweeps out equal areas in equal time intervals". In English this means that when the planet is near the sun it moves with a fast speed and when the planet is far from the sun it moves slowly.

Kepler's third law of motion states "the ratio of the cube of a planet's average distance from the sun to the square of its orbital period is the same for each planet". In English this means that a planet close to the sun orbits in a smaller time period than a planet far from the sun. Not only does a farther planet have a larger distance to travel, but it travels more slowly.

Even with these laws established, the heliocentric system was not yet accepted.

This page was last updated on 06/06/01.