This horary disk is used to determine the hour by day and also serves as a lunar-solar calendar. The recto comprises three superposed disks, of which two have an index. The verso carries the hour lines, the shadow square, and an alidade with sights. It also bears the name of the maker, Simon Keill—on whom we have no information—and the date 1647. The instrument is engraved with the zodiac signs. Provenance: Vincenzo Viviani bequest.