Kepler’s Battle With Mars: "Brahe’s data, obtained with the naked eye, were indeed accurate: to 1/30th the angular size of the Moon. Tycho Brahe’s data were so precise, it quickly became clear that neither the Ptolemaic nor the Copernican systems fit the data well." 
"However, although the Copernican system was the less precise model, Kepler, accepted the heliocentric hypothesis. He first used the observations of Mars to obtain the orbit of Earth. Then, he mapped the Martian data from the geocentric frame to a heliocentric frame with ..."
"...the Sun displaced from the center of the orbit of Mars, which he took to be circular. The model worked much better than both the Ptolemaic and Copernican systems, but discrepancies remained. However, Kepler concluded that a different curve would work much better."
From the slides of "Physics at the precision frontier: then, now, and tomorrow," by Harrison B. Prosper at RADCOR-LoopFest 2021 [PDF] indico.cern.ch/event/958085/c… Figure from "Tycho and Kepler: Solid Myth versus Subtle Truth," by Gingerich and Voelkel jstor.org/stable/40972003
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