Sidereus Nuncius
The first telescopes appeared in the Netherlands in 1608 when Middelburg spectacle-maker Hans Lippershey tried to obtain a patent on one. By 1609 Galileo had heard about it and built his own improved version. He probably was not the first person to aim the new invention at the night sky but his was the first systematic (and published) study of celestial bodies using one. One of Galileo's first telescopes had 8x to 10x linear magnification and was made out of lenses that he had ground himself. This was increased to 20x linear magnification in the improved telescope he used to make the observations in "Sidereus Nuncius".
Linden Observatory Complex
Up until the 17th century, astronomy was positional and depended on observations of the naked eye. In the early 17th century a new instrument, the telescope was invented by Hans Lipperhey and quickly taken up and improved upon by Galileo Galilei. In 1610, armed with his 20 XX refractor telescope, Galileo discovered four moons of Jupiter and for the first time in the history of the world observed the movement of a satellite around a planet. Using his telescope Galileo also observed that the phases of Venus were similar to lunar phases and argued that these observations empirically supported the theory that the earth moves around the sun.