Nicolaus Copernicus (1473-1543) , born the son of a Polish merchant, he had the best education possible for that time period. When he completed his education in Italy, he returned to Poland and assumed a position at his uncle's cathedral. (His father had died when he was very young and he was raised by his uncle.) His new position gave him ample time to pursue his interests in several different areas, including astronomy. He observed a rare conjunction when he was 31 years old. This led him to consider the following idea: The prediction of planetary positions would be much simpler if we said that the Earth rotates on its own axis and that the Earth as well as the other 5 known planets orbit the Sun (in circular orbits).

Tycho Brache (1546-1601), was also highly educated . He was very eccentric. He was a very skilled craftsman and a very skilled observer of the sky. For twenty years (1576-1596) he made meticulous measurements of the positions of the known planets. He did not try to make any sense out of the data that he accumulated.

Johannes Kepler (1571-1630), born the son of a mercenary soldier, received his college education at a Lutheran seminary and then at University of Tübingen ( More biographical information). While at the University he studied astronomy and mathematics. He studied astronomy under Michael Maestlin (1550 - 1631), one of the leading astronomers of this time period. At the age of 30, he was hired by Tycho Brache as his assistant. Tycho died about a year later. Kepler, after a legal battle, "inherited" Tycho's 20 years worth of data. He spent the remaining 29 years of his life trying to find patterns in the data. As a result of his efforts, he came up with 3 empirical laws that described the motions of the 5 then know planets. We refer to these results as Kepler's Laws of Planetary motion. Keep in mind that these are not physical laws, they are empirical laws.

1. Planets move in an elliptical orbit, with the Sun at one focus.

2. An imaginary line drawn from the Sun to a planet sweeps out equal areas inside of the ellipse in equal periods of time.

3. The square of a planet's orbital period is directly proportional to the cube of it's semi-major axis.

Kepler made no attempt to explain this "Law's" in terms of physical theories. That task was completed by Isaac Newton. Click here for a log-log plot of the period vs. radius for the planets.

Galileo Galilei (1564-1642), born the son of a musician, also received an excellent university education. ( More biographical information) In addition to his working in the areas of physics and astronomy, he was also know as a mathematician and a philosopher. His major contributions to astronomy were a direct result of his using a telescope to study the sky. ( Thomas Harriot was another early user of telescopes in astronomy.) He found out that:

1. Jupiter had four satellites that revolved around it, like the planets revolved around the Sun.
2. That Venus exhibited phases, like the Moon.
3. That there were mountains on the Moon.
4. That there were spots on the Sun.
5. That the glow of the Milky Way is due to large numbers of stars that cannot be resolved with the naked eye. This confirmed the idea of Democritus (c. 400 BC)

Check out The Galileo Project, a hypertext source of information on the life and work of Galileo.

Isaac Newton (1642-1727) , was the son of a farmer, received his education at Cambridge University. (More biographical information) Using his so called Laws of Motion and his law of Universal Gravitation, Newton was able to establish the physical principles behind Kepler's "Laws" of Planetary Motion.

Newton published his work in a book called The Principia, in 1687. With that, the revolution started by Copernicus in 1543 (when he published De Revolutionibus) was over. It took about 144 years, but, there would be no turning back.

Back to T. J. Keefe's Home Page