SITTIUS, a German astronomer and chronologer, born at Groschleben in Thuringia in 1556. He studied at Helmstadt, 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 at Wittenberg, both of which he declined. He agreed, however, to conduct the school of music, established at Pforzheim, an office which he afterwards exchanged for a similar situation at Leipzig. 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, though 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 Melodie condende ratio. He died at Leipzig in 1617.