CALVISIUS, SETHUS (1556-1617), a German astronomer and chronologer, was born at Grosehleben, in Thuringia, in 1556. He studied at Hehnsthdt, where he made great progress in classical literature, as well as in the sciences in which he afterwards became so distinguished. He was offered a professorship of mathematics at Frankfort, and afterwards one at Wittenberg, both of which he declined. He agreed, however, to conduct the school of music, established at Pforte, - an office which he afterwards exchanged for a similar situation at Leipsic. At Frankfort he published, in 1585, his Opus Chronologicum, a work compiled on astronomical principles. He likewise organized a system of chronology, embodying the history of the world, upon an ingenious and original plan, highly commended by Casaubon and Scaliger. This work, which was strongly condemned in the Index Expurgatorius, has been frequently reprinted. In 1612 Calvisius published his Elenchus Calendarii Gregoriani, et duplex Calendarii melioris forma, in which he attempts to prove the inadequacy of the Gregorian calendar, and proposes to introduce a new system based upon astronomical principles. The only proof now extant of his musical knowledge is his treatise entitled Melodite condendce ratio. He died at Leipsic in 1617.