• (n.) The apart or practice of measuring the diameters of heavenly bodies, their relative distances, etc. See Heliometer.
He`li·om'e·try noun The apart or practice of measuring the diameters of heavenly bodies, their relative distances, etc. See Heliometer . 
heliometry The use of a telescope with a movable split-lens objective, primarily to measure the angular distance between two stars; originally used to measure the diameter of the sun.