2, in o which the curve QOP is the evolute and QPR FIG.
(3) The angle QOP, which the radius vector makes with the fundamental plane.
The angle QOP is the latitude of the place and the angle NOQ its longitude.
After the image of the body is brought into coincidence with the cross threads, the instrument is turned through 180° on the axis, which results in the line of sight of the telescope pointing in a certain direction OQ, determined by the condition QOZ = ZOP. The telescope is then a second time pointed at the object by being moved through the angle QOP. Either of the angles QOZ and ZOP is then one half that through which the telescope has been turned, which may be measured by a graduated circle, and which is the zenith distance of the object measured from the direction of the axis OZ.