The discovery of the telescope changed the face of astronomy forever. A dutch lens maker, Hans Lippershey invented the telescope in 1608. In the same year, Johannes Kepler, published his discoveries on the elliptical motion of planets.
With the help of the telescope, Galileo Galilei succeeded in making several discoveries. He found that Venus exhibited phases like the Moon, that the Moon itself had mountains and 'seas', that Jupiter was orbited by four small satellites of its own, etc.
Sir Isaac Newton invented a new type of telescope, using a mirror, instead of a lens for improved performance.