Astrophysics (Index)About

meridian circle

(transit circle)
(instrument for timing the passage of stars over a meridian)

A meridian circle (or transit circle) is an instrument designed specifically to time a star's passage over the meridian where the instrument is located (its local meridian), information useful for determining the star's right ascension. The instrument is mounted so as to be aimed only along its local meridian, varying only north/south, i.e., aiming has just one degree of freedom. The precise time that a star is showing in the instrument can be recorded. To assist in the precision, the view typically included cross-hairs, and precision machinery was used to set and measure the aiming angle, often including a circular structure, on the order of one-to-ten feet in diameter, perhaps the reason the instrument is referred to as a circle.

Meridian circles were commonly used before modern astrometric methods. One doesn't necessarily incorporate a telescope, but during the history of their use, that became common for the most advanced, in which case they can be thought of as a specific type of transit telescope, i.e., a meridian telescope.

The name transit circle does not implicitly suggest alignment along a meridian, but astronomical instruments referred to as transit circles were often (perhaps usually) meridian circles. The passage of a star through the view of a telescope is known as a transit. A transit circle is essentially the same as a ship navigator's transit, but enlarged.

(telescope type,instrument type,astrometry)
Further reading:

Referenced by pages:
Carlsberg Meridian Catalogue (CMC)