By comparing the parallel columns, the reader can determine which events were contemporaneous, or how many years separated two different events. To place all the events on the same time scale, Eusebius used an Anno Mundi (A.M.) era, meaning that events were dated from the supposed beginning of the world as computed from the Book of Genesis in the Hebrew Pentateuch. According to the computation Eusebius used, this occurred in 5199 B.C. The Chronicon of Eusebius was widely used in the medieval world to establish the dates and times of historical events. Subsequent chronographers, such as George Syncellus (died circa 811), analyzed and elaborated on the Chronicon by comparing with other chronologies. The last great chronographer was Joseph Justus Scaliger (1540-1609) who reconstructed the lost Chronicon and synchronized all of ancient history in his two major works, "De emendatione temporum" (1583) and "Thesaurus temporum" (1606). Much of modern historical datings and chronology of the ancient world ultimately derives from these two works. Scaliger invented the concept of the Julian Day which is still used as the standard unified scale of time for both historians and astronomers.