'Heliometer' definitions:

Definition of 'heliometer'

noun

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope

From: GCIDE

Heliometer \He`li*om"e*ter\, n. [Helio- + -meter: cf. F. h['e]liom[`e]tre.] (Astron.) An instrument devised originally for measuring the diameter of the sun; now employed for delicate measurements of the distance and relative direction of two stars too far apart to be easily measured in the field of view of an ordinary telescope. Heliometric