The
meridian circle is an instrument for timing of the passage of
stars across the local
meridian, an event known as a
transit, while at the same time measuring their angular distance from the
nadir. These are special purpose
telescopes mounted so as to allow pointing only in the
meridian, the
great circle through the north point of the horizon, the
zenith, the south point of the horizon, and the
nadir. Meridian telescopes rely on the rotation of the Earth to bring objects into their
field of view and are mounted on a fixed, horizontal, east-west axis.