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The Nocturnal
The Nocturnal is a Star Clock. Using the North Star, and the guide stars of either Ursa Major or Ursa Minor, the Noctunal was used to tell the time.
This instrument was used in the 13th through 18th centuries.

The Nocturnal is a Star Clock. The larger outer dial has the days and months of the year marked upon it. The smaller diameter dial has hours and minutes marked on it. Notice the two tabs on the smaller dial, labeled LB and GB. LB stands for Little Bear and GB stands for Great Bear. The user selects which constellation to use for telling time. The long edge of the tab is set on the day of the year by rotating the smaller dial. The Nocturnal is held upright, with the small point at the top pointing upwards, and the user sights at the North Star through the hole in the center. The long arm on the dial is rotated until it rests upon the guide stars: Kochab and Pherkad for the Little bear, or Dubhe and Mirek for the Great Bear. Once aligned, the time is read off the time dial at the point where the long arm crosses the time dial.