Galileo's Moon observations. Sketches of the Moon made by the Italian physicist, mathematician and astronomer Galileo Galilei (1564-1642). The text is in Latin. These pioneering observations were first made in 1610, using the telescopes that Galileo had built in 1608. Galileo saw that the terminator (the line between the Moon's night and day sides) was sometimes irregular (top) and sometimes smooth (bottom). He deduced that the irregularities were due to mountains on the Moon, the first time Earth-like objects had been discovered in the heavens. This challenged the existing worldview that said the heavens were perfect and unchanging. This page is from a 1653 edition of Sidereus Nuncius (March 1610).