The son of a mercenary soldier and tavern keeper later accused in magic was carried away by the astronomy in his early years: his mother showed a comet and lunar eclipse to the boy. Studying at the university, he heard about Kopernik’s heliocentric system and became its passionate supporter at once. The two first laws of planetary motion were published by Kepler in 1609 in his book New Astronomy. Out of precaution, he related them to Mars alone. One could not help being too careful living in this period. At the time when Kepler was formulating the third law, his mother was under arrest accused in witchery. In his book Harmonices Mundi (Harmony of the World), the astronomer mentioned that the idea of third law suddenly came to his mind on Mach 8, 1618. By the middle of May, he finished the calculations and made sure that the law is correct: the squares of the orbital periods of the planets were directly proportional to the cubes of the semi-major axes of their orbits.