HELIOSTAT (from Gr. [Greek: helios], the sun, [Greek: statos], fixed,
set up), an instrument which will reflect the rays of the sun in a fixed
direction notwithstanding the motion of the sun. The optical apparatus
generally consists of a mirror mounted on an axis parallel to the axis
of the earth, and rotated with the same angular velocity as the sun.
This construction assumes that the sun describes daily a small circle
about the pole of the celestial sphere, and ignores any diurnal
variation in the declination. This variation is, however, so small that
it can be neglected for most purposes.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: From Jamin and Bouty, _Cours de physique_,
Gauthier-Villars.