|See also heliostat Compare coelostat an astronomical instrument consisting essentially of a plane mirror driven about two axes so that light from a celestial body, esp the sun, is reflected along a constant direction for a long period of time|
|[C19: from sidero-, from Latin sidus a star + |
any of a class of astronomical instruments consisting of a flat mirror that is turned slowly by a motor to reflect a given region of the sky continuously into a fixed telescope. In the traditional siderostat, the mirror is rotated by a lever arm connected to a motor that turns at a rate of one revolution every 24 hours. This so-called Foucault siderostat provides a fixed but rotating image. In recent years the Foucault siderostat has been largely supplanted by the heliostat (q.v.), which is a polar siderostat, and by the coelostat (q.v.).
Learn more about siderostat with a free trial on Britannica.com.