This replica of a nocturnal was made by a craftsman in Massachusetts. The nocturnal is used to tell the local time at night. When dials are set for the date, one index arm is lined up with the pointer stars in either Ursa Major or Ursa Minor and the other directly towards the zenith. The time can then be read off the dials. These instruments were used in the 17th century and 18th centuries, in an era where portable watches were too expensive for mariners.