Tycho proposed an experiment that would prove the earth goes around the sun.

Figure 1.3 Schematic Representation of stellar Parallax. Distant stars act as a fixed reference coordinate system. Nearby stars, when observed 6 months apart, will show a small movement with respect to the background of fixed stars. At position 1, the nearby star would be viewed against a background that contained star B while 6 months later, at position 2, the nearby star would be viewed against a background that contained star A.

Tycho Brahe 1546 - 1601

He built a very elaborate observatory at Ven that made use of siting tubes but no telescopes since they hadn't been invented yet!

He essentially used a series of long "sextans" that could measure fairly accurately the angle of a star above the horizon. The azimuth was measured in degrees from some starting point in this circular observatory out portals that were equally spaced around the floor.

Tycho's experiment was to measure Stellar Parallax:

The Tycho Parallax Simulator

Upon failing to detect stellar parallax he proposed a strange hybrid model where the planets orbit the sun but that whole system orbits around the earth.

This hybrid model doesn't work at all with respect to either retrograde motions or predicting planetary positions, but it does explain why Venus and Mercury would always be seen in the sky, relatively near the Sun. The model, however, also allows Mars (as well as Jupiter and Saturn) to come much closer to the Earth than the standard heliocentric model. This means there would be times when Mars should be really bright in the sky.

Despite this highly erroneous model, there are many overall contributions of Tycho:

1. He made the most precise observations that had yet been made by devising the best instruments available before the invention of the telescope.

2. His observations of planetary motion, particularly that of Mars, provided the crucial data for later astronomers like Kepler to construct our present model of the solar system.

3. He made observations of a supernova (literally: nova= "new star") in 1572 (we now know that a supernova is an exploding star, not a new star).

   This was a "star" that appeared suddenly where none had been seen before, and was visible for about 18 months before fading from view. Since this clearly represented a change in the sky, prevailing opinion held that the supernova was not really a star but some local phenomenon in the atmosphere (remember: the heavens were supposed to be unchanging in the Aristotelian view). Brahe's meticulous observations showed that the supernova did not change positions with respect to the other stars (no parallax). Therefore, it was a real star, not a local object. This was early evidence against the immutable nature of the heavens.

   

4. Brahe made careful observations of a comet in 1577. By measuring the parallax for the comet, he was able to show that the comet was further away than the Moon. This contradicted the teachings of Aristotle, who had held that comets were atmospheric phenomena ("gases burning in the atmosphere" was a common explanation among Aristotelians). As for the case of the supernova, comets represented an obvious change in a celestial sphere that was supposed to be unchanging; furthermore, it was very difficult to ascribe uniform circular motion to a comet.

5. He made the best measurements that had yet been made in the search for stellar parallax. Upon finding no parallax for the stars, he (correctly) concluded that either
   • the earth was motionless at the center of the Universe, or

   • the stars were so far away that their parallax was too small to measure.

   Brahe did not believe that the stars could possibly be so far away and so concluded that the Earth was the center of the Universe and that Copernicus was wrong.

6. Brahe proposed a model of the Solar System that was intermediate between the Ptolemaic and Copernican models (it had the Earth at the center). It proved to be incorrect, but was the most widely accepted model of the Solar System for a brief period of time.

Thus, Brahe's ideas about his data were not always correct, but the quality of the observations themselves was central to the development of modern astronomy.