The piece is a replica of a Nocturnal and Tide Computer from 1570, signed by Humphrey Cole, that can be found in the British Museum's collection.
One of its faces is a nocturnal clock and the other is a tide computer. The nocturnal or star clock is an artifact that is used to obtain the time at night using the fixed stars in the sky. The abacus or tide computer is used to figure the time of the tides, that greatly affect navigation both in harbors and for fishing.
This reproduction was made as faithful as possible to the original but, at the same time, updated and furnished with certain essential elements that make it useful in present times.