As we have seen, Copernicanism remained a theory because there were strong scientific objections to it. What was needed was a new thinker with new tools. And that man duly arrived in the form of the Italian Galileo Galilei (1564 - 1642).
Galileo was a polymath whom many observers believe to be the founder of modern science. In 1609 he turned the newly developed telescope to the sky and made many discoveries: he observed the phases of Venus and the moons of Jupiter; both discoveries which strengthened the case for Copernicanism. He saw that the moon was a world not too dissimilar to Earth (the maria at the time being regarded as actual seas of water, not basalt).
However, Galileo made powerful enemies in the Church by his inability to suffer fools gladly and in 1632 he was called before the Inquisition and forced to disavow his belief in Copernicanism. He remained under house arrest for the rest of his life.