He made these discoveries through an exhaustive process of trial and error. The work was made more difficult because he had to decipher the motion of a planet around the sun from the moving earth. In other words, he had to distinguish between the part of the planet's motion due to its path around the sun and the part due to our motion around the sun. He did all these tedious and difficult calculations without even the aid of a slide rule. In the process he formulated what are now called Kepler's Three Laws of Planetary Motion.
Planetary orbits are ellipses with the sun at one focus.
Figure 1 shows the shape and features of an ellipse. Planetary orbits are not as eccentric as the one drawn here, however. The joining a planet to the sun sweeps out equal amounts of area in equal amounts of time. Figure 2 shows this area. If the time interval is equal on both sides, the green area will equal the blue area.