Our present concern is with his contribution to the Copernican theory.
Galileo himself records in a letter to Kepler that he became a convert to this theory at an early day.

He was not enabled, however, to make any marked contribution to the subject, beyond the influence of his general teachings, until about the year 1610.

The brilliant contributions which he made were due largely to a single discovery--namely, that of the telescope.

Hitherto the astronomical observations had been made with the unaided eye.

Glass lenses had been known since the thirteenth century, but, until now, no one had thought of their possible use as aids to distant vision.