Tycho Brahe 1546-1601, planetary motion

Tycho Brahe built instruments to observe celestial bodies. Using his instruments he could measure angles to an accuracy that had not been possible in the past. He built quadrants and sextant; using these instruments and a compass he could, among other things, measure the heights of celestial bodies above the horizon. Tycho Brahe's observations were later used by Johannes Kepler who used the data to discover the laws of planetary motion.

A quadrant

A quadrant is a quarter of a circle Using a quadrant one can measure the height above the horizon of a celestial body. Tycho Brahe built a mural quadrant at the wall of his observatory Uranienborg.

A sextant

A sextant is one sixth of a circle. Guess what an octant is!

Tycho Brahe and geocentrism

Tycho Brahe would only believe in what he could observe. Since he could not observe any stellar parallax, he concluded that the earth did not move. Another explanation for him not seeing any stellar parallax could be that the stars were too far away, making an angle even smaller than the angles Tycho Brahe could measure. It would take another couple of centuries before a stellar parallax could be observed.