Which Renaissance astronomer formulated the Tychonic system?

The Renaissance astronomer who formulated the Tychonic system was Tycho Brahe.

Announced in 1588, the Tychonic system was a geoheliocentric model designed to combine features of the older Ptolemaic system with the mathematical advantages of Copernican astronomy. In Brahe’s arrangement, the Earth remains stationary at the center; the Sun and Moon orbit Earth, while the other known planets orbit the Sun.

Brahe admired Copernicus but rejected the idea that Earth moved, partly because he considered it inconsistent with contemporary physics and partly because astronomical observations had not yet detected annual stellar parallax. His model could also account for observations such as the phases of Venus while retaining a stationary Earth.

The system was later displaced as telescopic evidence and improved theory supported heliocentrism. Brahe’s exceptionally precise pre-telescope observations nevertheless became crucial to Johannes Kepler, who used them—especially observations of Mars—to formulate his laws of planetary motion. Thus Brahe’s model was ultimately superseded, but his measurements helped transform astronomy.