heliostat

n. [ Helio- + Gr. &unr_; placed, standing, fr. &unr_; to place, stand: cf. F. héliostate. ] An instrument consisting of a mirror moved by clockwork, by which a sunbeam is made apparently stationary, by being steadily directed to one spot during the whole of its diurnal period; also, a geodetic heliotrope. [ 1913 Webster ]