In 1974 I saw a photograph of a device called a nocturnal as part of the inventory of an 18th Century American Clockmaker. I didn't quite understand its function, so I manufactured a copy to play with.
What it does is tell the time at night. One of the two indices (one for ursa major; one for ursa minor) is set to the month and day on the outer ring. Polaris is sighted through the center hole, and the arm is rotated to align with the guard stars of the constellation chosen. The time is read on the inner ring. It was a common instrument from the 14th through the 19th Centuries--not only for getting the time at night, but for setting clocks. It has the advantage over the sun dial in that neither compass variation nor the equation of time need be known. Nifty, huh?