#OTD in 1609.
Galileo Galilei demonstrates his first telescope, with a magnification of about 8 or 9, to Venetian lawmakers.
Based only on uncertain descriptions of the first practical telescope which Hans Lippershey tried to patent in the Netherlands in 1608, Galileo made a telescope with about 3x magnification. He published his initial telescopic astronomical observations in March 1610 in a brief treatise entitled Sidereus Nuncius.