Which German astronomer discovered the four large moons of Jupiter in 1610, independently of Galileo (January 7th) and just one day later (January 8th)?

The German astronomer was Simon Marius.

The dispute between Simon Marius and Galileo Galilei over the discovery of Jupiter's moons became one of the most famous scientific controversies of the 17th century. In January 1610, both astronomers independently observed the four large moons of Jupiter. Marius named them Io, Europa, Ganymede and Callisto, the names still used today for Jupiter's Galilean moons.

Galileo made his observations first and published his findings in the "Sidereus Nuncius" later that year, giving him priority for the discovery. Marius, who was working in Nuremberg, made similar observations a few days later, around 8 January 1610, and published his own results, but Galileo had already taken credit for the discovery.

https://en.wikipedia.org/wiki/Simon_Marius

https://en.wikipedia.org/wiki/Galilean_moons