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( Astrometry)
Astrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies. Although once thought of as an esoteric field with little useful application for the future,[citation needed] the information obtained by astrometric measurements is now very important in contemporary research into the kinematics and physical origin of our Solar System and our Galaxy, the Milky Way. Astrometry is that branch of astronomy, which deals with precise measurement and explanation of position & movement of stars and other celestial bodies leading to kinematics. The history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis and Aristillus to discover the earth’s precession. In doing so, he also developed the brightness scale still in use today.[1] Astrometry was studied extensively in Islamic astronomy, which produced many star catalogues during the Islamic Golden Age. In 850, Alfraganus wrote Kitab fi Jawani (A compendium of the science of stars), which gave revised values for the obliquity of the ecliptic, the precessional movement of the apogees of the sun and the moon, and the circumference of the earth.[2] Albatenius (853-929) gave times for the new moon and lengths for the solar year and sidereal year, and worked on the phenomenon of parallax.[3] In the 10th century, Azophi carried out observations on the stars and described their positions, magnitudes, brightness, and colour, and gave drawings for each constellation, in his Book of Fixed Stars. Ibn Yunus observed more than 10,000 entries for the sun's position for many years using a large astrolabe with a diameter of nearly 1.4 metres. His observations on eclipses were still used centuries later in Simon Newcomb's investigations on the motion of the moon, while his other observations inspired Laplace's Obliquity of the Ecliptic and Inequalities of Jupiter and Saturn's.[4] Abu-Mahmud al-Khujandi relatively accurately computed the axial tilt to be 23°32'19" (23.53°),[5] which was a significant improvement over the Greek and Indian estimates of 23°51'20" (23.86°) and 24°,[6] and still very close to the modern measurement of 23°26' (23.44°).
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