Astrometry is a branch of astronomy that analyzes the positions and motions of celestial objects. It has four classes: small-field, semiglobal, terrestrial, and space astrometry. The oldest branch of astronomy, astrometry has evolved from using instruments like the quadrant and gnomon to more sophisticated ones like the telescope and micrometer. Two methods, absolute and differential, can determine the position of stars. Fundamental catalogs must be periodically reviewed due to the constant motion of celestial objects. Astronomers use instruments like radios and interferometers to establish the positions of distant celestial objects.
There are several branches of astronomy, one of which mainly deals with the analysis of the motions and positions of celestial objects. This branch, called astrometry, also involves measuring other quantities, such as the diameter of planets, and evaluating the orbits of the components of double stars. The four classes of astrometry are small-field astrometry, semiglobal astrometry, terrestrial astrometry, and space astrometry. Astrometry is also referred to as positional astronomy due to its primary role in examining the positions of stars. It is considered one of the oldest branches of astronomy.
In ancient times, one of the quantities determined in astronomy was the altitude of celestial objects using instruments such as the quadrant, Jacob’s staff and the gnomon. These instruments, however, were not considered good enough to obtain accurate measurements. With the invention of more sophisticated instruments such as the telescope, pendulum clock and micrometer in the 17th century, measurements became more and more accurate. In the 17th century, astronomers discovered that stars have their own motion, which has been called proper motion. Since then, determining the positions of stars and measuring stellar parallaxes, or differences in the apparent position of a star as seen from two different points, caused by the motion of the Earth around the Sun, became two important goals in astronomy.
Two coordinates can give the position of a celestial body, usually designated as declination and right ascension. Also, the position of the stars can be determined using two methods: absolute method and differential method. The absolute method can be performed by reading the altitude of a star on the transit circle and timing its transit so that the star’s coordinates are measured independently of those of other stars. Meanwhile, the differential method can be performed by comparing the position of a star with those of other stars, which are called fundamental stars. For differential observations, the most widely used method is the photographic method, in which a star whose position is being measured is photographed with fundamental stars, with measurements made on the photographic plate itself.
No matter how great these methods may be, however, periodic review of fundamental catalogs is important simply because all celestial objects are constantly in motion. Furthermore, the establishment of parallax can only be calculated at a distance of about 3,000 light years. Beyond this distance, astronomers can only estimate the motions and distances of celestial bodies according to various astrophysical hypotheses. To establish the positions of very distant celestial objects, astronomers use instruments such as radios and interferometers. The use of astrometric satellites, such as the one launched in 1989 called Hipparchos, is also important in the field of astrometry.
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