Brahe and Kepler

Tycho Brahe was born in Denmark in 1546. He was a wealthy man that was pressured to study law but chose to pursue astronomy instead. Tycho’s detailed observations of supernovas and comets helped prove that the “heavens” or space do in fact change. In his observatory, he would track objects in their entire visible orbit across the sky with the naked eye. He too proposed his own view of the universe in the sense that the earth was the center and the sun and moon orbited around it. All while the other five known planets orbited around the sun.

Towards the end of the 16th century, a German astronomer by the name of Johannes Kepler came to work with Tycho as his assistant. Tycho did not like “sharing” his observations, notes, and measurements of the planets and their orbits. Therefore, he had Kepler work on understanding the orbit of Mars. Other astronomers had noted that Mars appeared to move backwards in its orbit and they did not know why. During Kepler’s observations he compared with with Tycho’s observations and came to the conclusion that the planets revolved around the Sun. Tycho suggested they moved around the Sun in circles. But Kepler’s observations showed that they moved around the Sun in stretched out circles called ellipses. This observation took Kepler almost a decade to realize and did not publish his findings until after Tycho’s death.

There are now three laws that Kepler came up with about planetary motion. These laws describe orbital shapes, changing speeds, and the lengths of planetary years.

Three Laws:

  1. The orbit of a planet about the Sun is an ellipse with the sun at one focus.
  2. A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.
  3. The square of a planet’s sidereal period around the Sun is directly proportional to the cube of the length of its orbit’s semi-major axis.


From Georgia Virtual Learning Astronomy