target, as seen from the mirror, are very different. Because of the disadvantages, this design has never been commonly used, but some people do experiment with it. Typically, the heliostat mirror moves at a rate that is 1/2 the angular motion of the sun. There is another arrangement that satisfies the definition of a heliostat yet has a mirror motion that is 2/3rd of the motion of the sun. Many other types of heliostat have also occasionally been used. In the very earliest heliostats, for example, which were used for daylighting in ancient Egypt, servants or slaves kept the mirrors aligned manually, without using any kind of mechanism. (There are places in Egypt where this is done today, for the benefit of tourists. In the movie The Fifth Element an Egyptian boy holds a mirror to illuminate a wall inside a cave for a fictional archaeologist.) Elaborate clockwork heliostats were made during the 19th Century which could reflect sunlight to a target in any direction using only a single mirror, minimizing light losses, and which automatically compensated for the sun's seasonal movements. Some of these devices are still to be seen in museums, but they are not used for practical purposes today. Amateurs sometimes come up with ad hoc designs which work approximately, in some particular location, without any theoretical justification. An essentially limitless number of such designs are possible. See also Heliograph, a similar non-tracking device, used for communication Renewable energy Solar cell Solar cooker Solar energy Solar thermal energy Solar tracker References External links Field of 63 heliostats at the Odeillo Solar Furnace Solar Furnaces at Odeillo Overview of heliostat reflector materials Sunalign free heliostat software and related material Solar energy