Parallactic Angle. Angle between hour circle of an observed heavenly body and the arc of a circle of altitude intercepted by the body and observer's zenith. Is angle X in PZX triangle.
Also called 'Angle of Position'.
Parallactic Inequality. Variation in tidal phenomena that is due to variation in Moon's distance from Earth.
Parallax. Apparent change in position of an observed object when due to change in position of observer. 2. Parallax of sextant is equal to angle, at an observed object, between a line from centre of index glass and another line from horizon glass. Error is quite negligible unless observed object be very near.