In 1610 Galileo Galilei peered through the elaborately designed, but entirely primitive by today's standards, telescope pictured above, or one similar. With it he discovered the four largest of Jupiter's moons, Io, Europa, Ganymede, and Callisto.
He also observed the phases of Venus which was key to his conclusion that we live in a heliocentric universe.
The scope above is a refracting scope as opposed to Isaac Newton's later reflecting scope.
A traditional refracting scope is a simple technology wherein the light is focused by the primary lens and observed through the secondary lens.
A reflecting scope, on the other hand, takes the light in through a lens which is bounced off the primary mirror at the bottom of the scope and then bounced again off of a secondary mirror which sends the light through the eyepiece, normally toward the top of the scope.