Who published the first telescopic observations of Jupiter's four largest moons?

Galileo Galilei published the first telescopic observations of Jupiter’s four largest moons. He observed them in January 1610 and initially thought they were fixed stars near Jupiter. Over several nights, their changing positions showed that they were orbiting the planet.

Galileo published his observations in Sidereus Nuncius (The Starry Messenger) in March 1610. He called the moons the “Medicean stars” to honor Cosimo II de’ Medici and his family. They are now known as Io, Europa, Ganymede, and Callisto.

Simon Marius independently observed the moons around the same time and published his account in 1614. However, Galileo’s earlier publication established him as the first to publish the discovery. The observations also provided important evidence that not every celestial body revolved around Earth, supporting the Copernican model.