The nocturnal was a primitive navigation tool used to tell time by the celestial clock (stars).

One ring was set to the date, Polaris was sighted through the hole in the center, and the arm was swung around to align with pointer stars - those in the Big Dipper, Little Dipper, or Cassiopeia.

Where the arm crossed the marked ring, the time was read.

The device was also used to measure Polaris's distance in minutes of arc from true north so a correction could be applied.