With a telescope, Mayr observed the four satellites of Jupiter, accurately determining their periods of revolution. While Galileo called them the “Medicean stars,” Mayr named them Europa, Io, Ganymede and Callisto, names which are still used today.
In this work Mayr also considered Tycho’s objection to Copernicus based upon star sizes. The telescope did not solve the problem, for Mayr saw fixed stars through the telescope as circular disks of measurable size. For Mayr, Tycho’s objection seemed strengthened rather than diminished by the telescopic evidence.
Galileo similarly affirmed that stars appear as measurable disks, rather than pinpoints of light. Galileo bisected star disks with a micrometer affixed to the telescope. Galileo did not address Tycho’s argument in print.