Sidereus Nuncius

Galileo began his telescopic observations in the later part of 1609, and by March 1610 was able to publish a small book, The Starry Messenger (Sidereus Nuncius), describing some of his discoveries: mountains on the Moon, lesser moons in orbit around Jupiter, and the resolution of what had been thought to be very cloudy masses in the sky (nebulae) into collections of stars too faint to see individually without a telescope. Other observations followed, including the phases of Venus and the existence of sunspots.