German astronomer who named the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. All four are named after mythological figures with whom Jupiter fell in love. He and Italian astronomer Galileo Galilei (Galileo) both claimed to have discovered them, about 1610, and it is likely both did so independently. A dispute over priority resulted in unwarranted obloquy for Marius. The two were antagonists for the rest of their lives, and on several occasions Galileo attacked Marius in print and accused him of plagiarizing his work.

Marius studied briefly with Danish astronomer Tycho Brahe (Brahe, Tycho) and later became one of the first astronomers to use a telescope. He was the first to publish, in 1611, the telescopic observation of the Andromeda Galaxy, describing the sight as “like a candle seen at night through a horn” (referring to horn lanterns, then common). He was also among the first to observe sunspots (sunspot).