astronomy

astronomy Branch of science studied since ancient times and concerned with the universe and its components in terms of the relative motions of celestial bodies, their positions on the celestial sphere, physical and chemical structure, evolution and the phenomena occurring on them. It includes celestial mechanics, astrophysics, cosmology, and astrometry. Waves in all regions of the electromagnetic spectrum can now be studied either with ground-based instruments or by observations and measurements made from satellites, space probes and rockets.

History

Astronomy was first used practically to develop a calendar, the units of which were determined by observing the heavens. The Chinese had a calendar in the 14th century bc. The Greeks developed the science between 600 bc and ad 200. Thales introduced geometrical ideas and Pythagoras saw the universe as a series of concentric spheres. Aristotle believed the Earth to be stationary but he explained lunar eclipses correctly. Aristarchus put forward a heliocentric theory. Hipparchus used trigonometry to determine astronomical distances. The system devised by Ptolemy was a geometrical representation of the Solar System that predicted the motions of the planets with great accuracy. From then on astronomy remained dormant until the scientific revolution of the 16th and 17th centuries, when Copernicus stated his theory that the Earth rotates on its axis and, with all the other planets, revolves round the Sun. This had a profound effect upon contemporary religion and philosophy. Kepler and his laws of planetary motion refined the theory of heliocentric motion, and his contemporary, Galileo, made use of the telescope and discovered the moons of Jupiter. Isaac Newton combined the sciences of astronomy and physics. His laws of motion and universal theory of gravitation provided a physical basis for Kepler's laws and enabled the later prediction of Halley's comet and the discovery of the planets Uranus, Neptune, and Pluto. by the early 19th century the science of celestial mechanics (the study of the motions of bodies in space as they move under the influence of their mutual gravitation) had become highly advanced and new mathematical techniques permitted the solution of the remaining problems of classical gravitation theory as applied to the Solar System. In the second half of the 19th century astronomy was revolutionized by the introduction of techniques based on photography and spectroscopy. These encouraged investigation into the physical composition of stars. Ejnar Hertzsprung and H. N. Russell studied the relationship between the colour of a star and its luminosity. by this time larger telescopes were being constructed, which extended the limits of the universe known to man. Harlow Shapley determined the shape and size of our galaxy and E. P. Hubble's study of distant galaxies led to his theory of an expanding universe. The Big Bang and steady-state theory of the origins of the universe were formulated. In recent years, space exploration and observation in different parts of the electromagnetic spectrum have contributed to the discovery and postulation of such phenomena as the quasar, pulsar, and black hole. There are various branches of modern astronomy. Optical astronomy studies sources of light in space. Light rays can penetrate the atmosphere but, because of disturbances, many observations are now made from above the atmosphere. Gamma-ray, infrared, ultraviolet, and X-ray astronomy study the emission of radiation (at all wavelengths) from astronomical objects. Higher wavelengths can be studied from the ground, while lower wavelengths require the use of satellites and balloons. Other branches within astronomy include radar astronomy and radio astronomy.