An Etymological Dictionary of Astronomy and Astrophysics

A flat mirror with a clock-drive mounted in such a way that it moves from east to west to compensate for the apparent
rotation of the Earth in order that the image of a particular area of sky remains fixed in the focal plane. See also → siderostat and → heliostat.

Etymology (EN): Coelostat, from L. coelo-, for caeli-, combination form of coelum “sky” + -stat prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Âsmândâštâr, from âsmân, → sky, + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support.”