How accurate were Tycho Brahe's planetary observations compared with earlier observations?

Tycho Brahe’s planetary observations were significantly more accurate than those available in earlier astronomical records. Before his work, existing almanacs and ephemerides—such as those based on Copernican tables—were often found to be grossly inaccurate, sometimes missing celestial events by several days.1

Tycho’s measurements, which were the most precise possible prior to the invention of the telescope, achieved an unprecedented accuracy of approximately 2 arc minutes (where 1 arc minute is 1/60 of a degree). This level of precision was so reliable that when Johannes Kepler later attempted to model planetary motion, he was unable to reconcile his initial circular theories with Tycho’s data to better than 8 minutes of arc. Because he trusted the 2-arc-minute accuracy of Tycho’s observations, Kepler was compelled to abandon circular orbits in favor of ellipses, ultimately leading to his laws of planetary motion.213