The pelorus is a simple compass card equipped with a rotating sighting vane. There is no magnetic needle in a pelorus. When a target is observed, the sighting vane is rotated until the object can be observed through it. The bearing can then be read off the card.

On a vessel at sea the compass card would be set with 000° aligned along the length of the vessel. The Relative bearing, that is the bearing of the object relative to the vessel's heading, can then be read off. The relative bearing of a target directly in front of the vessel will always be 000° regardless of the vessel's course.

From the static position of Worms Head Lookout, 000° is aligned with true North. Readings are therefore true compass bearings.

'A pelorus is a navigational device, named after Hannibal’s pilot of two centuries BC.'
The Oxford Companion to Ships and the Sea.