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Encyclopedia Britannica



HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up)

This article appears in Volume V13, Page 231 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: HEG-HIG
HELIOSTAT (from Gr. rtXtor, the sun, vrares, 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.
'
Dollond
  provides for changing the power by sliding the lens d nearer to or farther from a.
Many forms of heliostats have been devised, the earliest having I been described by Wilhelm Jacob s'Gravesande in the 3rd edition of his Physices elementa (1742). One of the simplest consists of a I
plane mirror rigidly connected with a revolving axis so that the angle between the normal to the mirror and the axis of the
instrument
  equals half the sun's polar distance, the mirror being adjusted so that the normal has the same right ascension as the sun. It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will re-main equal throughout the day, and therefore this device reflects the rays in the direction of the earth's axis; a second fixed mirror reflects them in any other fixed direction. Foucault's
heliostat
  reflects the rays horizontally Fin. t. in any required direction. The principle
of the apparatus may be explained
by reference to fig. 1. The axis of -rotation AB bears a rigidly
attached rod DBC inclined to it at an angle equal to the sun's polar
distance. By adjusting the right ascension of the plane ABC and
rotating the axis with the angular velocity of the sun, it follows that
BC will be the direction of the solar rays
throughout the day. X is the mirror
rotating about the point E, and placed so
that (if EB is the
horizontal
  direction in
which the rays are to be reflected) (t) the
normal CE to the mirror is jointed to BC
at C and is equal in length to BE, (2) the
rod DBC passes through a slot in a rod ED
fixed to, and in the plane of, the mirror.
Since CE equals BE these directions are
equally inclined to, and coplanar with, the
normal to the mirror. Hence light incident
along the direction BC will be reflected
along CE. Silbermann's
heliostat
  reflects
the rays in any direction. The principle
may be explained by means of fig. 2. AB
a. is the axis of rotation, BC an adjustable
rod as in Foucault's construction, and BD is another rod which can be set to the direction in which the rays are to be reflected. The rods BC and DB carry two small rods EF, GF jointed at F; at this
joint
  there is a pin which slides in a slot on the rod BH, which is normal to the mirror X. Theto the framework pate, the members of which are parallel to the incident and reflected rays SO, OR, and the
diagonal
  pf is perpendicular to the mirror. The framework is attached to two independent circular arcs Cs and rr' having their centres at 0 and provided with clamps D and A on the axis F of the instrument. The arc Cs is graduated, and is set so that the angle COD equals the complement of the sun's declination. This can be effected (after setting the axis) by rotating Cs until a needle indicates true time on the hour dial B. The arc rr is set so as to reflect the rays in the required direction. The axis F of the instrument is set at an angle equal to the
latitude
  of the place of observation and in the meridian by means of the screw K, and rotated by clockwork contained in the barrel H. The setting in the meridian is effected by turning the instrument after setting for
latitude
  until a pin-hole aperture s and a small screen P, placed so that Ps is parallel to CO, are in a line with the sun.
Many other forms of heliostats have been designed, the
chief
  difference consisting in the mechanical devices for maintaining the constant direction of the reflecting ray. One of the most important applications of the heliostat is as an adjunct to the newer forms of
horizontal
  telescopes (q.v.) and in conjunction with spectroscopic telescopes in observations of eclipses.


End of Article: HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up)


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