Simon Marius, German astronomer and academic (died 1624)

Simon Marius was a German astronomer. He was born in Gunzenhausen, near Nuremberg, but spent most of his life in the city of Ansbach. He is best known for being among the first observers of the four largest moons of Jupiter, and his publication of his discovery led to charges of plagiarism.

January 20 is the 20th day of the year in the Gregorian calendar; 345 days remain until the end of the year.