When Kepler started his experiments, he knew he needed the most accurate measurements of the planets’ movements available.
Tycho Brahe was a Danish astronomer who had his own observatory and the most advanced instruments of the time.
He had giant sextants and quadrants and a tool called an equatorial armillary that measured the angles between the stars and the planets.
Brahe’s instruments and the meticulous care he took in his work gave him the most accurate observations of the skies that had ever been made.
As accurate as his measurements were, however, Brahe still believed Earth to be the center of the universe. He did believe the other planets revolved around the Sun, but thought the whole solar system revolved around Earth at a very great distance.
Kepler was more interested in Brahe’s records than his theories, and he was hired to work as Brahe’s assistant.
Within a year, Brahe died and left Kepler all his notes and calculations. It was the accuracy of these records that allowed Kepler to make his discovery.
As a tribute to his fellow astronomer, Kepler completed Brahe’s Rudolphine Tables, the first nautical almanac that showed the planetary positions for years to come.
The main motive for Kepler’s discoveries was to adjust the recorded observations to take account of Copernicus’s discovery that the Earth as the observation point was not stationary but orbited round the Sun.
Two of Kepler’s discoveries are contained in the Introduction to his Astronomia Nova (1609). They are that the velocity of the planets is inversely related to their distances from the Sun and also that the planetary orbits are elliptical. Greater mathematical precision is given to these discoveries in Kepler’s later works and also the concept of the foci. These discoveries are not as well known as they ought to be because of religious prohibition.
There is a little known paper by Kepler Concerning Conic Sections included in his book on Optics published in 1604. This paper investigates the works of Apollonius of Perga and Eutocius of Ascalon who were trying to bring together the various conic sections, the straight line, the circle, the ellipse, the parabola and the hyperbola. Kepler wrongly thought that his own discovery of the focus applied with pins and thread would help in this. It was not until 1618 that Kepler recognised that the focus was the location of the Sun in the orbiting planets.