Nocturnal, Antique Navigational Tool

Imagine navigating the open seas in the 17th century. As you sail through the vast open water, how would you determine the time without access to today's technology?

This navigation tool, known as a nocturnal, was used to calculate time at night. Similar to an astrolabe or a sundial clock, this nocturnal was one of the few ever found on a shipwreck. It was discovered in 1996 among the wreckage of the La Belle, one of three ships that French explorer René Robert Cavelier, Sieur de La Salle sailed into Texas's Matagorda Bay in 1685. This nocturnal, made of English boxwood, has three independently rotating disks held together at the center by a brass fixture. A pointer—much like a hand on a clock—determines the time at night based on the position of the North Star in relation to another constellation, such as Ursa Minor (the Little Dipper). Engravings on both the larger disk (calendar, compass degrees, and constellations), and the smaller disk (an additional calendar)  are  written in English, indicating the nocturnal was likely made in England.