an instrument consisting of a mirror moved by clockwork, for reflecting the sun's rays in a fixed direction.
Origin of heliostat
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
an astronomical instrument used to reflect the light of the sun in a constant direction
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012