17th-century English nocturnal

This is a typical example of a 17th-century English nocturnal, an instrument designed for telling the time at night. It is made of boxwood with a heart shape cut out of the handle, which also has the words 'BOTH BEARS' stamped into it. On the back is a circular table for the rule of the Pole Star (used when measuring the observer's latitude from the altitude of the Pole Star), which surrounds a 16-point compass rose. Date and hour scales appear on the front of the main body. On top of this is a rotatable disc with an hour scale, a lunar age scale and two shaped pointers, one marked 'GB', the other marked 'LB' - indicating Great Bear and Little Bear. A rotatable pointer sits on top.

Nocturnals are known to have been used by at least the 10th century, but it was in the 16th and 17th centuries that they became common. They were used to tell the time from the position of the Great Bear, and often also the Little Bear, relative to the Pole Star. To use the nocturnal, the oberver set the pointer for the constellation being observed (Great Bear or Little Bear) aginst the appropriate date on the date scale. They then looked at the Pole Star through the hole in the centre and lined up the long pointer with the two stars in the Great or Little Bear that align with the Pole Star. The time is shown by the position of the pointer against the hour scale on the rotatable disc.