What is the meridian?
Meridian circle, also called transit circle, is a device used in astronomy to measure the position of the star when it passes or crosses, the local meridian. From the observer's point of view, the local meridian is a large circle passing through the zenith, a point directly above the head and the northern and southern heavenly poles, which are projections from the Earth's poles. The device is mounted in such a way as to make observation only along the meridian between the horizon and the zenith. It first developed at the end of the 17th century, and the Meridian Circle played a crucial role in astrometry, measurement of positions and movements of astronomical bodies.
The devices are usually refractive telescopes with lens diameters below 8 inches (about 20 cm). Looking through the eyepiece, the field of view can be divided by fine lines that parallel with merid. Traditionally, they were used to measure the transit of the star and derive the moment in which it was on the local Meridian. In the modern circle of the meridian, these measurements are performed electronically and calculations are carried out by computer.
observation with a meridian circle also measures declination and a true rise, two coordinates that define a point in the sky using a equatorial coordinate system. Declination is the distance of the building north or south of the heavenly equator, projection of the Earth equator, expressed in degrees. This position can be described as an astronomical equivalent of latitude. Early star maps were designed using a Meridian circle to measure transit and declination time.
Right rise, also known as an hour's angle, is the degree of position to the east along the heavenly equator from folk equinox, the point where the sun passes in the spring. The position changes with time, so the correct rise must be recorded with reference to the -eter observation has been made. It can be measured in stages or hours where it is 360 degrees for 24 hours. When mapping the sky, this position measurement is an analogue of the terrestrial length.
In addition to astrometry has a meridian circlePractical applications on the ground. By observing Meridian transit times of known stars, local length and time can be calculated. Such observations were the most accurate method of determining the time before the development of the atomic clock.
Technical advances have significantly extended the accuracy of the poles and most of their function is now automated. With a space platform for measuring the type of transit circle it was realized at the satellite of Hipparcos European Space Agency. Delcope Carlsburg Meridian in the Canary Islands is a modern automated tool that is used to specify earlier observations and record positions of asteroids.