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The 16th century brought to Prague some of the glamour of the Italian Renaissance. Under the auspices of the Emperor Rudolph II many branches of art and sciences began to flourish. It was on the initiative of the Czech physician and astronomer Tadeas Hajek of Hájek, author of several important astronomical treatises, that Rudolph II invited the prominent Danish astronomer Tycho Brahe to the Bohemian Court. Tycho came to Prague at the beginning of the summer 1599 and began to form a significant centre of astronomical studies modelled on his home observatory at Uraniborg on the island of Hven which he had had to abandon because of disputes with the sovereign.
Tycho Brahe immediately invited the young and excellent Johannes Kepler, who came to Prague in 1600 and met with Tycho for the first time on the February 3. Brahe expected that Kepler would help him with the mathematical elaboration of his geocentric planetary system (Moon and Sun move around the earth, but the orbits of all planets are centred on the Sun). Neverheless, their famous collaboration was not so splendid and did not last for long.
Tycho performed his astronomical observations in the castle of Benatky on the Jizera River, about 40 km north of Prague. Then, at the beginning of autumn 1600, he moved to Prague and started to made his measurements from the terrace of "Belvedere", the renaissance style Royal Summer Palace of Queen Anne built 1538-1563 by the architect Paolo della Stella.
Tycho Brahe took advantage of the presence of two constructors of precise observational instruments in Prague, Jost Burgi (one of the inventors of logarithms) and Erasmus Habermel. Habermel provided for observers a sextant which permitted angles to be read with a precision of 2 minutes of arc. This instrument has been preserved and can be seen in the Prague National Technical Museum.
Unfortunately, Tycho Brahe suddenly died on October 24, 1601, only two years after coming to Bohemia. His tombstone is in the Týn Church on the Prague Old Town Square.
The Emperor promoted Kepler to the rank of imperial mathematician. His task was to elaborate the Rudolphine Tables - the new planetary tables based on Tycho's exceptionally accurate observations. This became a unique source of data which was used by Kepler for the determination of the Mars orbit - the crucial result for the discovery of the fundamental laws of planetary motion which significantly improved the Copernicus system. In 1605 he definitely decided that the orbit of Mars is elliptical and that the Sun is placed in one of the two focal points. In 70 chapters of his large work "Astronomia nova" (published in Prague in 1609, the manuscript itself having been fully completed by 1605) Kepler deduced and formulated the first two of his famous laws of planetary motion (the third law was discovered in 1618). Modern astronomy was born.
In the Preface to his treatise "Astronomia nova" Kepler also wrote "Axioms of the true doctrine of gravitation", an important link in the evolution of celestial mechanics anticipating the law of universal gravitation. It is there that one can find formulations such as "If two stones were placed in any given part of the universe, near to each other and outside the sphere of force of a third body, then the two stones, like two magnetic bodies, would unite in some intermediate point, each approaching the other through a distance proportional to the mass of the other".
Among other works of Kepler in Prague are his observations in 1604 of a new star in the constellation of Serpentarius (Kepler's supernova) and a sunspot (by "camera obscura"). He also completed several writings, in particular "Dioptrice" from 1611 containing the theory of the astronomical telescope.
