ASTRONOMY

science that deals with the origin, evolution, composition, distance, and motion of all bodies and scattered matter in the universe. It includes astrophysics, which discusses the physical properties and structure of all cosmic matter. Astronomy is the most ancient of the sciences, having existed since the dawn of recorded civilization. Much of the earliest knowledge of the celestial bodies is often credited to the Babylonians. They are thought to have recognized a number of prominent constellations as early as 3000 BC and to have developed a calendar based on the regularity of certain astronomical events some centuries thereafter. The ancient Greeks introduced various influential cosmological ideas. During the 6th century BC Pythagoras proposed the notion of a spherical Earth and a universe populated by objects whose motions were governed by the harmonious relations of natural laws. Later Greek philosophers taught that the sky was a hollow globe surrounding the Earth and having the stars inlaid like jewels on its inner surface. The sky was supported on an axis thrust through the Earth; on this axis the sky rotated westward daily, causing the celestial bodies to rise and set. During the 2nd century AD Ptolemy (Claudius Ptolemaus), one of the most celebrated of the ancient Greek astronomers, put forth a conception of an Earth-centred (geocentric) universe that influenced astronomical thought for more than 1,300 years. In the Ptolemaic system, each planet moved in a small circle, the epicycle, in the period of its actual revolution around the heavens relative to the Sun's position. Meanwhile, the centre of this circle moved eastward around the Earth on a larger circle in the observed period of the planet's revolution relative to the stars. During the 16th century the Polish astronomer Nicolaus Copernicus disposed of much of the complexity of the Ptolemaic system by assigning the central position to the Sun. In this revolutionary system the Earth, attended by the Moon, became one of the planets revolving around the Sun. Copernicus also proposed the daily axial rotation of the Earth from west to east, so that the daily circling of the heavenly bodies around the Earth results simply from its apparent motion. Published in De revolutionibus orbium coelestium libri VI (Six Books Concerning the Revolutions of the Heavenly Orbs) in 1543, the Copernican heliocentric theory ushered in the age of modern astronomy. The 17th century witnessed several momentous developments that led to major advances in astronomy. These were the discovery of the principles of planetary motion by Johannes Kepler, the application of the telescope to astronomical observation by Galileo Galilei, and the formulation of the laws of motion and gravitation by Isaac Newton. Other significant contributions followed in rapid succession. In 1750 Thomas Wright, for example, postulated that the universe was made up of numerous galaxies. Later in the century another English astronomer, William Herschel, undertook the first thorough telescopic survey of the heavens and established the foundations of modern stellar astronomy. Spectroscopy and photography were adopted for astronomical research in the 19th century. They enabled investigators to measure the quantity and quality of light emitted by stars and nebulae (clouds of interstellar gas and dust), making it possible for them to conduct studies of the brightness, temperature, and chemical composition of such cosmic objects. It was soon recognized that the properties of all celestial bodies, including the planets of the solar system, could only be understood in terms of the physics of their atmosphere and interior. The trend toward the application of physical laws to the interpretation of observational data gained impetus during the early 1920s, and many astronomers began referring to themselves as astrophysicists. This tendency continues to prevail. The major areas of current interestX-ray astronomy, gamma-ray astronomy, infrared astronomy, and radio astronomyare all basically concerned with physics and engineering, the knowledge of the latter having utmost importance in the construction of observational instruments and auxiliary equipment. Technological advances such as electronic radar and radio units, high-speed computers, electronic radiation detectors, and Earth-orbiting observatories and long-range planetary probes have greatly broadened the scope of both theoretical and observational research on astronomical phenomena. science that deals with the origin, evolution, composition, distance, and motion of all bodies and scattered matter in the universe. It includes astrophysics, which discusses the physical properties and structure of all cosmic matter. Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, initially for calendrical and astrological purposes and later for navigational applications and scientific interest. The catalog of objects now studied is much broader and includes, in order of increasing distances, the solar system, the stars that make up the Milky Way Galaxy, and other more distant stellar objects and galaxies. With the advent of scientific space probes, the Earth also has come to be studied as one of the planets, though its more detailed investigation remains the domain of the geologic sciences. Additional reading Antonie Pannekoek, A History of Astronomy (1961, reissued 1969; originally published in Dutch, 1951); and Stephen Toulmin and June Goodfield, The Fabric of the Heavens (1961), are comprehensive surveys. Thomas S. Kuhn, The Copernican Revolution: Planetary Astronomy in the Development of Western Thought (1957), discusses the ancient geocentric theory of the universe and the heliocentric system that replaced it. Charles A. Whitney, The Discovery of Our Galaxy (1971, reprinted 1988), explains theories and observations in stellar astronomy from the 18th through the 20th centuries. Otto Struve and Velta Zebergs, Astronomy of the 20th Century (1962), studies the developments and achievements of the first 50 years of the century. The same period is covered in a later study from the ongoing series The General History of Astronomy: Owen Gingerich (ed.), Astrophysics and Twentieth-Century Astronomy to 1950 (1984).Literature on current knowledge in astronomy is voluminous. C.W. Allen, Astrophysical Quantities, 3rd ed. (1973, reprinted 1983), provides a compact compilation of data; and Kenneth R. Lang, Astrophysical Formulae: A Compendium for the Physicist and Astrophysicist, 2nd rev. ed. (1980), is a comprehensive reference source, with extensive formulas and background data. Robert Burnham, Jr., Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, rev. ed., 3 vol. (1978), is a guide to stars, nebulas, and galaxies. Arthur P. Norton, Norton's Star Atlas and Reference Handbook (Epoch 1950.0), 17th ed., edited by Gilbert E. Satterthwaite (1978, reprinted 1986), is a popular atlas. Introductory texts include George O. Abell, David Morrison, and Sidney C. Wolff, Exploration of the Universe, 5th ed. (1987), a comprehensive classic; and A.E. Roy and D. Clarke, Astronomy: Principles and Practice, 3rd ed. (1988), a survey of experimental basics.The scope and methodology of astronomy are explored in specialized works: J. Kelly Beatty, Brian O'Leary, and Andrew Chaikin, The New Solar System, 2nd ed. (1982); Peter H. Cadogan, The Moon: Our Sister Planet (1981); John C. Brandt and Robert D. Chapman, Introduction to Comets (1981); Gordon Walker, Astronomical Observations: An Optical Perspective (1987); R.J. Tayler, The Stars: Their Structure and Evolution (1970, reissued 1981); Carl E. Fichtel and Jacob I. Trombka, Gamma Ray Astrophysics: New Insights into the Universe (1981); Claus E. Rolfs and William S. Rodney, Cauldrons in the Cosmos: Nuclear Astrophysics (1988); Dimitri Mihalas and James Binney, Galactic Astronomy: Structure and Kinematics, 2nd ed. (1981); John D. Kraus et al., Radio Astronomy, 2nd ed. (1986); M.S. Longair, High Energy Astrophysics: An Informal Introduction for Students of Physics and Astronomy (1981); Michael Rowan-Robinson, The Cosmological Distance Ladder: Distance and Time in the Universe (1985); and Edward R. Harrison, Cosmology, the Science of the Universe (1981). For up-to-date reviews of specialized subjects and for current listings of observational information, consult the Annual Review of Astronomy and Astrophysics, Annual Review of Earth and Planetary Sciences, The Astronomical Almanac (annual), and The Observer's Handbook (annual).