He developed a model known as the Tychonic system which placed the Earth at the center of the universe but had the Sun, Moon, and other planets orbiting the Sun. This model was an attempt to reconcile the observations that supported the heliocentric model (like phases of Venus) with the belief that the Earth was stationary.
While his model ultimately proved incorrect, it was a significant step in the transition from geocentrism to heliocentrism.
