Kepler himself was, at various times, to feel the restraint of ecclesiastical opposition, though he was never subjected to direct persecution, as was his friend and contemporary, Galileo.

At the very outset of Kepler's career there was, indeed, question as to the publication of a work he had written, because that work took for granted the truth of the Copernican doctrine.

This work appeared, however, in the year 1596.

It bore the title Mysterium Cosmographium, and it attempted to explain the positions of the various planetary bodies.

Copernicus had devoted much time to observation of the planets with reference to measuring their distance, and his efforts had been attended with considerable success.