Instrument for finding the time at night by observing the positions of the celestial bodies or the lunar phases.

The sidereal dial usually consists of superimposed disks of different diameters, calibrated for a particular latitude. The reference point is the Pole Star, which shows the celestial north pole, and is sighted through the central hole. Thanks to a movable index arm, the observer can track the rotation of a star around the polar axis, thus obtaining the desired time.

The moon dial, based on the phases of the Moon, is used to measure their durations. It consists of superimposed disks, one of which has a hole through which the observer can see a chart of the phases of the moon below, arranged in precise time intervals.