Short answers, pulled from the story.
Simon Marius was a German astronomer born on the 10th of January 1573 in Gunzenhausen, near Nuremberg, who died on the 5th of January 1625. He is best known for being among the first observers of the four largest moons of Jupiter and for giving those moons the names Io, Europa, Ganymede, and Callisto, which remain in use today.
Both made independent observations at nearly the same time. Marius recorded his first observation on the 29th of December 1609 (Julian calendar), which converts to the 8th of January 1610 in the Gregorian calendar, one day after Galileo's letter describing the moons. A scientific committee in the Netherlands ruled in 1903 that Marius's discovery was independent.
Galileo accused Marius of plagiarism in The Assayer in 1623, after Marius published Mundus Iovialis in 1614 claiming to have found Jupiter's moons about a month before Galileo. Marius was also linked in Galileo's mind to his student Baldassarre Capra, who had separately plagiarized Galileo in 1607.
A scientific committee in the Netherlands examined the evidence in 1903 and ruled in favor of Marius's independent discoveries. Johannes Bosscha published the findings in 1907, nearly 300 years after Galileo's accusations had tainted Marius's reputation.
Marius published Mundus Iovialis in 1614, describing the planet Jupiter and its four major moons. He had previously announced the discovery in a local almanac in 1611.
Marius observed the Andromeda nebula in 1612, measuring its diameter and describing its light as brightening toward the center like a candle shining through horn. He also observed the location of Tycho Brahe's supernova of 1572 and derived more precise orbital periods for the Galilean moons than Galileo did.