Parallactic angle

In spherical astronomy, the parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hour circle of the object. It is usually denoted q. In the triangle zenith—object—celestial pole, the parallactic angle will be the position angle of the zenith at the celestial object. Despite its name, this angle is unrelated with parallax. The parallactic angle is 0° or 180° when the object crosses the meridian.

Uses

For ground-based observatories, the Earth atmosphere acts like a prism which disperses light

of different wavelengths such that a star generates a rainbow along the direction that points

to the zenith. So given an astronomical picture with a coordinate system with a known direction

to the Celestial pole, the parallactic angle represents the direction of that prismatic effect relative

to that reference direction.

[In that sense, the stretched image of the star caused by atmospheric dispersion is vaguely similar to the (apparent) motion of the star that could have been produced by its finite distance to the observer, i.e., its parallax.]

Knowledge of that angle is needed to align Atmospheric Dispersion Correctors with the beam axis of the telescope .

Depending on the type of mount of the telescope, this angle may also affect the orientation of the celestial object's disk as seen in a telescope. With an equatorial mount, the cardinal points of the celestial object's disk are aligned with the vertical and horizontal direction of the view in the telescope.