Source : Webster's Revised Unabridged Dictionary (1913)

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}.