Heliostats are scientific instruments that use a mirror to reflect the light of the sun in a specific direction. Since the position of the sun in relation to the Earth changes throughout the day, most heliostats have some sort of mechanism built in to track this movement. Simple ones use an ordinary clockwork mechanism to follow the sun, while more complex heliostats include trackers that sense the sun's position in the sky or incorporate microprocessing software that compensates for the angle of the sun throughout the solar year.
The word heliostat is a compound of the Greek word for "sun," helios, and the word for "still" or "stationary," stat, and it appears to have been coined in the mid 18th century. As with many scientific inventions from that period, a number of inventors have been credited for the device, making it unclear who is actually responsible for it. The most frequent use of heliostats in the 18th century was for astronomy observations and optical experiments. In the 20th century, arrays of mirrors were used in a range of mostly scientific applications, including electricity generation, astronomy, architecture, and solar lighting.
In some ways, the heliostat works rather like the sunflower, which tracks the movement of the sun across the sky to maximize exposure. Mechanical heliostats work in a similar way, and usually with the same end goal of harnessing the sun's energy. The most basic heliostat involves a pivoting clockwork mechanism with a mirror positioned on it to follow the sun's progress across the sky and reflect its light to a set point. In a laboratory setting, where the device can be moved to compensate for changes in the sun's angle, this type of device is adequate. More complex automated heliostats use software and sensors to orient themselves in relationship to the sun. Companies that sell heliostats to provide architectural lighting accents or integral lighting solutions usually offer them with embedded microprocessors capable of calculating solar positioning algorithms.
Massive arrays of heliostats that reflect light to specific locations and distribution systems can be found in high rise buildings, on solar energy farms, as architectural effects on structures all over the world, and in many astronomy laboratories. Use of heliostats in architecture is growing, as sunlight is effective, free, and pleasing ambient lighting. In labs, heliostats are used for solar observations, including measurements of solar radiation, as well as for energy production, heat generation, and other applications. Many labs also have siderostats, designed for tracking stars other than the sun.