heliometer

noun

Etymology: From helio- + -meter, from Ancient Greek Ἥλιος (Hḗlios), from ἥλιος (hḗlios, “sun”), and French mètre, from Ancient Greek μέτρον (métron, “measure”).

1. An astronomical instrument, based on a telescope, for measuring the diameter of the sun; now used to measure the angular distance between stars and other celestial bodies.