Not long ago I posted about "alidade" a device for taking navigational bearings.
measuring the horizontal azimuth. Today I encountered "pelorus" another
name for such a device.Dictionary says it is an eponym.
pelorus
n.
5? after L Pelorus, pilot of Hannibal‘s ship6 Naut. a device for taking bearings, consisting of a flat metal ring, equipped with sighting vanes or a small telescope, that fits over a compass card or gyrocompass