The parallactic angle is zero when the object crosses the meridian.

With an altazimuth mount, those directions are rotated by the amount of the parallactic angle.

It is usually denoted q. In the triangle zenith-object-celestial pole, the parallactic angle will be the angle at the celestial object.

The orientation of the disk of the Moon, as related to the horizon, changes throughout its diurnal motion and the parallactic angle changes equivalently.

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.

If this angle is measured from the North point on the limb, it can be converted to an angle measured from the zenith point (the vertex) as seen by an observer by subtracting the parallactic angle.