A pelorus is a navigational tool used by the navigator and others responsible for the safe navigation of a ship to determine the angular distance between any object (point of land, light house, 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 measure).