n. [ Helio- + -meter: cf. F. héliomètre. ] (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.

n. [ Photo- + heliometer. ] (Astron.) A double-lens instrument for measuring slight variations of the sun's diameter by photography, utilizing the common chord of two overlapping images. [ Webster 1913 Suppl. ]

n. [ Gr. &unr_; fire + &unr_; sun + -meter. ] (Physics) An instrument for measuring the direct heating effect of the sun's rays. [ 1913 Webster ]