The Collaborative International Dictionary of English v.0.48:

Siderostat \Sid"er*o*stat\, n. [L. sidus, sideris, a star + Gr.
   ??? standing, fixed, fr. ???? to place.] (Astron.)
   An apparatus consisting essentially of a mirror moved by
   clockwork so as to throw the rays of the sun or a star in a
   fixed direction; -- a more general term for heliostat.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Heliostat \He"li*o*stat\, n. [Helio- + Gr. ? placed, standing,
   fr. ? to place, stand: cf. F. h['e]liostate.]
   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]