On 25th of August 1609 Galileo Galilei demonstrated his first telescope to Venetian lawmakers, including the Doge (ruler) of Venice, Leonardo Donato.

While Galileo did not invent the telescope, he did design and build telescopes with increasingly higher magnifying power for his own use and to present to his patrons. He was a skilled instrument maker, and his telescopes were known for their high quality.

Galileo’s first telescope was basically a tube containing two lenses. His first attempt was a three-power instrument; this was followed by one that magnified objects approximately nine times. He showed the latter device to the Venetian senate, hoping to impress them with its commercial and military potential. American Institute of Physics

His book, Sidereus nuncius or The Starry Messenger was first published in 1610 and made him famous. In it he reported on his observations of the Moon, Jupiter and the Milky Way. These and subsequent observations and his interpretations of them eventually led to the demise of the geocentric Ptolemaic model of the universe and the adoption of a heliocentric model as proposed in 1543 by Copernicus. -Australia Telescope National Facility