From the Super Mario Wiki
The Telescope is an item in the PC release of Mario's Time Machine. It is a telescope that Galileo Galilei created and perfected himself after reading reports from other scientists. He was able to use it to examine many astronomical phenomena for the first time, notably the four largest moons of Jupiter that are now called the Galilean moons in honor of him. In 1610, as Galileo is about to look at Jupiter's moons for the first time, Bowser time travels to Padua and steals the Telescope, then exhibits it in his museum alongside the other items that he stole (according to Mario's Time Machine). The Telescope is then taken by Mario, who travels back in time to Padua and shows it to random people in the hopes of returning it to its owner. An unnamed poet and one of Galileo's students both recognize the Telescope, and eventually Mario is absolutely certain and brings it to Galileo. Galileo immediately uses it to look at Jupiter and confirms that there are moons circling Jupiter.