The possibility of the telescope

Up to about the end of the 15th century mankind was in a condition of helpless ignorance with regard to the nature, the distances, and the dimensions or (except two) the form of the various bodies which constitute the visible universe.
No human eye had ever beheld either stars or planets as other than dimensionless points. Their motions and positions had been carefully observed, systematised and theorised upon ; but they were unapproachable. What a wonderful revelation then must that have been which Galileo's telescope opened up ! practically diminishing the distance some 20 or 30 times. True, his was a very simple affair, and feeble in its infancy; but it revealed some most important facts. It served to show that the planets at least were globes, some of them of vast bulk : it opened up to human vision for the first time the wonderful mountainous scenery of the Moon.
But perhaps its most important service was to establish beyond question the Copernican theory of astronomy by revealing the phases of the inner planets—also the moons of Jupiter—a Copernican system in miniature.