A transit circle is a telescope mounted so that it can only move along the meridian from the zenith to the horizon, that is, about a fixed horizontal axis in a north-south vertical plane. Transit circles, also known as meridian circles, are used to measure accurately the altitudes of stars and to time their passage across the observer's meridian. Shown here is the Airy Transit Circle, which was set up at the Royal Greenwich Observatory in 1850 when George Airy was the Astronomer Royal.