Although Galileo rushed to print his telescopic observations, he did not invent the telescope. Jacob Metius was one of several Dutchmen with a claim to the invention of the telescope. This book by Jacob’s brother mentions Jacob’s telescopic observations of the satellites of Jupiter.
The Sidereus nuncius (Venice, 1610) of Galileo is the first published report of telescopic observations. In that work, Galileo described the four moons of Jupiter, mountains on the Moon and countless new stars in the Milky Way. Although Galileo improved the telescope, he did not invent it. Rather, two Dutchmen have strong claims to its invention: Hans Lippershey and Jacob Metius. The latter left no written description of observations. However, this work by his brother, Adriaan, refers to Jacob’s observations of the satellites of Jupiter (”planetae circa Jovem vagantes,” p. 2). Adriaan Metius, a professor of mathematics and astronomy at Franeker from 1600 until his death, supported Tycho’s cosmology. Descartes attended his lectures in 1629.