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

Oh my. And here I wanted Alidade to be a big city in Australia, and pelorus the distal end of the stomach...

(See, Hogwarts is wherever you want it to be! )

Dear wofahulicodoc: you got those back-azimuth.