Kepler's discovery of elliptical orbits marks the beginning of modern, scientific astronomy. He based his explanations upon observations, rather than making the observations fit an assumed model of the universe. Prior to Kepler, planets were believed to orbit Earth in circular paths. The many variations of their motions were accommodated by adding more and more complex sets of circles upon circles in order to rationalize the observations. Although this worked moderately well, the future position of planets could only be roughly predicted.

Kepler changed all of that. Altogether, he discovered three relationships, now called "Kepler's laws" that describe the orbital motion of the planets. 

1. Law of Ellipses (1609). The orbit of each planet is an ellipse, with the Sun located at one focus. Earth is closest to the Sun in January and farthest from the Sun in July as it travels along its elliptical orbit.
2. Law of Equal Areas (1609). A line drawn from the planet to the Sun sweeps out equal areas in equal times. This geometric description captures the fact that a planet's orbital velocity varies in a regular way -- the farther the planet is from the Sun, the more slowly it moves along its orbit. Earth moves fastest in January, and slowest in July.
3. Harmonic Law (1618). The square of the sidereal period of a planet is directly proportional to the cube of the semimajor axis of its orbit. In non-technical terms, planets move more slowly, and in a predictable way, the greater their distance from the Sun. Mercury speeds once around the Sun in just 88 days, Earth takes one year, Mars almost two years and distant Pluto almost 250 years.