Museo Galileo
Virtual Museum
Tycho Brahe's system
Video   Text


Tycho Brahe built the large astronomical observatory of Uraniborg in Hveen, where he observed the sky with impressive instruments for more than 20 years.

Tycho was convinced of the Earth's absolute immobility, evidenced by the fact that a stone dropped from the top of a tower falls to its base. If the Earth rotated on its axis, the stone would have fallen west of the tower.

However, Tycho rejected the Ptolemaic system, which held phenomena such as comets to be internal to the Earth's atmosphere.

In 1572, a nova appeared in the constellation of Cassiopea, and Tycho tried to measure its distance by means of triangulation. The narrowness of the angle he found proved that the nova lay far beyond the Moon. Using the same method, Tycho demonstrated that even the comet of 1577 moved beyond the Moon's sky, on a circular orbit around the Sun, at a non-uniform pace.

Tycho examined the Sun's annual travel and regarded it as uniform along a circumference eccentric to the Earth. For the monthly travel of the Moon, he developed instead a model consisting of five circles revolving in uniform motion; the model enabled him to determine the Moon's position relative to the Sun and the stars with unprecedented precision.

In 1582, thanks to new triangulations, Tycho asserted that Mars when in opposition to the Sun was closer to the Earth than the Sun itself. From this, he concluded that Mars, as well, revolved around the Sun. But since the planet's orbit intersected the Sun's orbit, Tycho concluded that Mars could not lie on a solid crystal sphere.

He therefore eliminated the celestial spheres of the Aristotelian-Ptolemaic tradition and asserted the fluidity of the heavens. Tycho extended the circumsolar movement of Mars to the other planets. Under the sphere of the fixed stars, Mercury, Venus, Jupiter, and Saturn revolved around the Sun as well. At the same time, the Sun carried them with it around the immobile Earth.