Themis (solar power plant) in the context of Odeillo

A heliostat (from Ancient Greek ἥλιος (hḗlĭos) 'sun' and στατός (stătós) 'standing')is a device that reflects sunlight toward a target, turning to compensate for the Sun's apparent motion.The reflector is usually a plane mirror.

The target may be a physical object, distant from the heliostat, or a direction in space. To do this, the reflective surface of the mirror is kept perpendicular to the bisector of the angle between the directions of the Sun and the target as seen from the mirror. In almost every case, the target is stationary relative to the heliostat, so the light is reflected in a fixed direction. According to contemporary sources the heliostata, as it was called at first, was invented by Willem 's Gravesande (1688–1742). Other contenders are Giovanni Alfonso Borelli (1608–1679) and Daniel Gabriel Fahrenheit (1686–1736). A heliostat designed by George Johnstone Storey is in the Science Museum Group collection.