What’s a Meridian Circle?

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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 observer’s point of view, 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 terrestrial poles. The device is mounted in such a way as to allow observations only along the meridian between the horizon and the zenith. First developed in the late 17th century, the meridian circle has played a vital role in astrometry, the measurement of the positions and motions of astronomical bodies.

The instruments are typically refracting telescopes with lenses less than 8 inches (about 20 cm) in diameter. When looking through the eyepiece, the field of view can be divided by fine lines parallel to the meridian. Traditionally, these were used to measure the transit of a star and determine when it was on the local meridian. In a modern meridian circle, these measurements are taken electronically and calculations are done by a computer.

Meridian circle observations also measure declination and right ascension, the two coordinates that define a point in the sky using the equatorial coordinate system. Declination is the distance of an object north or south of the celestial equator, a projection of the earth’s equator, expressed in degrees. This location can be described as the astronomical equivalent of latitude. The first star charts were constructed using the meridian circle to measure transit and declination times.

Right ascension, also known as the hour angle, is a measure of your eastward position along the celestial equator from the vernal equinox, the point where the sun crosses it in spring. Location changes with time, so right ascension should be recorded with reference to the year the observation was made. It can be measured in degrees or hours, where 24 hours equals 360 degrees. When mapping the sky, this measure of position is analogous to Earth’s longitude.

In addition to astrometry, the meridian circle has practical applications on Earth. By looking at 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 expanded the accuracy of meridian circles and much of their function is now automated. With the Hipparcos satellite of the European Space Agency, a space platform for measurements of the type of transit circle has been created. The Carlsburg Meridian Telescope in the Canary Islands is a modern, automated instrument that is being used to refine previous observations and record the positions of asteroids.

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