Another astronomer who found fault with the state of astronomy in the sixteenth century was Danishman Tycho Brahe (he is usually referred to as “Tycho,” after common Scandinavian usage). Tycho, a nobleman, took an early interest in astronomy. As a teenager, he recognized the need for long-term, systematic observation of the heavens to correct the errors found in existing planetary tables and star charts. To produce more accurate data, he would also need better instruments.

Tycho persuaded the king of Denmark to grant him a fiefdom on the island of Hven and the finances needed to build an observatory there. The observatory, which he called Uraniborg (“Heavenly Castle”), was completed in 1580. Tycho also designed and constructed his own instruments. Tycho and his assistants made multiple observations using these instruments throughout the 1580’s and early 1590’s, compiling the most accurate data on stellar and planetary positions to date. Tycho left Denmark for Prague in 1597, finding a new patron in the Holy Roman Emperor Rudolf II.

While Tycho appreciated the mathematical advantages of the Copernican theory over the Ptolemaic theory, he maintained the traditional belief in an unmoving Earth. If the Earth was indeed orbiting the Sun, the nearest stars should appear to move relative to stars farther away from an observer on Earth, but Tycho never detected such a movement with his instruments. Either the Earth had to be at rest, or the stars had to be at such a distance – at least 700 times further out than Saturn, according to Tycho’s calculations – that their apparent movement was too small to detect. Unable to accept such a vast universe, Tycho envisioned a modified version of the Copernican system, in which the Earth sits at the center of the cosmos, and the moon and Sun revolve around it. The other five planets orbit the Sun, and the stars lie just outside the region of the planets.

Johannes Kepler, Tycho’s most famous assistant, was a strong believer in the Platonic ideas of a simple, harmonic universe. Kepler was assigned to observe the movements of the planet Mars, which greatly deviated from the circular model created by Ptolemy. After Tycho’s death in 1601, Kepler was able to use Tycho’s observational records, along with the heliocentric theory of Copernicus and English philosopher William Gilbert’s theory of magnets, to devise a new model of planetary orbits and the Sun’s influence on them. Kepler worked out that the planetary orbits were elliptical rather than circular, and formulated the mathematical rules which govern the period and size of a planet’s orbit.