A meridian circle, also called a transit circle, is a device used in astronomy to measure the position of a star as it transits, or crosses, the local meridian. From the perspective of the observer, the local meridian is a great circle passing through the zenith, a point directly overhead, and the north and south celestial poles, which are projections from the Earth's poles. The device is mounted in such a way as to allow observations to be made only along the meridian between the horizon and zenith. First developed in the late 17th century, the meridian circle has played a vital role in astrometry, the measurement of positions and motions of astronomical bodies.
The instruments are typically refractor telescopes with lens diameters under 8 inches (about 20 cm). Looking through the eyepiece, the field of view may be divided by fine lines that parallel the meridian. Traditionally, these were used to measure a star's transit and derive the instant at which it was on the local meridian. In a modern meridian circle, these measurements are made electronically and calculations performed by a computer.
Observations with the meridian circle also measure the declination and right ascension, the two coordinates that define a point in the sky using the equatorial coordinate system. The declination is an object's distance north or south of the celestial equator, a projection of the Earth's equator, expressed in degrees. This position can be described as the astronomical equivalent of latitude. Early star maps were constructed using the meridian circle to measure the time of transit and declination.
Right ascension, also known as hour angle, is a measure of position eastward along the celestial equator from the vernal equinox, the point where the sun crosses it in the spring. The position changes with time, so the right ascension must be recorded with reference to the year the observation was made. It can be measured in either degrees or hours, where 24 hours equals 360 degrees. When mapping the sky, this positional measurement is the analog of earthly longitude.
In addition to astrometry, the meridian circle has practical applications on Earth. By observing the meridian transit times of known stars, local longitude and time can be calculated. Such observations were the most accurate method of determining time before the development of the atomic clock.
Technical advances have greatly extended the accuracy of meridian circles and much of their function is now automated. A space platform for transit circle type measurements was realized with the European Space Agency's Hipparcos satellite. The Carlsburg Meridian Telescope in the Canary Islands is a modern, automated instrument that is used to refine earlier observations and record the positions of asteroids.