Antique ships Pelorus, Royal Navy / British Admiralty pattern.

Antique Pelorus or bearing plate. British Admiralty pattern Contained in the original hardwood box. Patt 42.

Box dimensions L 20cm x D 20cm x H 13.5cm.

A Pelorus is a navigational instrument used by the ships navigator to aid the safe navigation of a ship. It is used to determine the angular distance between objects (point of land, lighthouse, buoys, another ship, stars and planets) and the ship's bow.  The Pelorus can be used to measure relative bearing (relative to the ship's bow= 0 degrees) or be used to measure true/magnetic/ compass headings if you know the ship's heading (ship's heading plus Pelorus angular bearing).