These will be introduced below. Kepler's most detail study was on Mars for which Tycho had an extensive set of data. Kepler discovered that planets' orbits around the Sun were elliptical. The more elongated the ellipse is, the closer the foci are to the ends of the ellipse. The widest diameter of the ellipse is called the major axis and the smallest diameter is called the minor axis.
Diameters pass through the center of the ellipse.
The center of the ellipse is the point at the middle of the line that connects the two foci. The roundness of the ellipse is measured by the eccentricity of the ellipse which is defined as the ratio of the distance between the foci to the length of the major axis. If eccentricity is zero then we have a circle. The most elongated ellipse is the one having the maximum eccentricity of one. The place where a planet is closest to the Sun is called perihelion and the place where the planet is farthest away is called aphelion.