The nocturnal was a common instrument used by seamen of the sixteenth and seventeenth centuries to determine the time by observing the apparent rotation of a or b Ursae majoris or of b Ursae minoris around the pole star, Polaris. These instruments generally have four main parts: 1) the main plate with handle having graduated circles for months and hours, 2) the inner plate with pointers for a or b Ursae majoris and b Ursae minoris, having graduated circles for hours and days of the month (this disk is sometimes saw-toothed to aid in determining its relative position in the dark), 3) an index arm to line up with the indicator stars, and 4) a pierced bolt which also serves as a site to line up the Pole star.