Final answer:
Tycho Brahe's contribution to astronomy was his accurate observation of planetary positions, which enabled Johannes Kepler to derive the laws of planetary motion.
Step-by-step explanation:
The great contribution of Tycho Brahe was to observe planetary positions with sufficient accuracy so that Kepler could later use the data to discover the laws of planetary motion. Brahe's meticulous work at his observatory enabled Kepler to derive the three fundamental laws of planetary motion. While Brahe himself did not have the ability to fully analyze the data or create a new model to replace Ptolemy's, he was instrumental in collecting extensive and precise observations over almost 20 years. These observations would prove to be invaluable to Johannes Kepler, who, after Brahe's death, used them to establish the mathematical foundation of the heliocentric model of the solar system, which was first proposed by Copernicus and later solidified by the work of Galileo and Newton.