What is astronomy?
Astronomy is the branch of Earth Science that studies the motions and natures of celestial bodies, such as PLANETS, STARS, and GALAXIES; more generally, the study of matter and energy in the universe at large. Astronomy is perhaps the oldest of the pure sciences. In many primitive civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict events. Astronomical observations provided a basis for the CALENDAR by determining the units of MONTH and YEAR. Later, astronomy served in navigation and timekeeping. The earliest astronomers were priests, and no attempt was made to separate astronomy from the pseudoscience of ASTROLOGY. Astronomy reached its highest development in the ancient world with the Greeks of the Alexandrian school in the Hellenistic period. ARISTARCHUS OF SAMOS determined the sizes and distances of the MOON and SUN and advocated a heliocentric (sun-centered) cosmology. ERATOSTHENES made the first accurate measurement of the actual size of the earth. The greatest astronomer of antiquity, HIPPARCHUS, devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon. Using such a system, PTOLEMY predicted the motions of the planets with considerable accuracy (see PTOLEMAIC SYSTEM). One of the landmarks of the scientific revolution of the 16th and 17th cent. was Nicholas COPERNICUS' revival (1543) of the heliocentric theory (see COPERNICAN SYSTEM). The next great astronomer, Tycho BRAHE, compiled (1576-97) the most accurate and complete astronomical observations yet produced. Johannes KEPLER's study of Brahe's observations led him to the three laws of planetary motion that bear his name (see KEPLER'S LAWS). GALILEO Galilei, the first to make astronomical use of the TELESCOPE, provided persuasive evidence (e.g., his discovery of the four largest moons of JUPITER and the phases of VENUS) for the Copernican cosmology. Sir Isaac NEWTON, possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and PHYSICS. His laws of motion and theory of universal GRAVITATION, published in 1687, provided a physical, dynamic basis for the merely descriptive laws of Kepler. By the early 19th cent. the science of CELESTIAL MECHANICS had reached a highly developed state through the work of Alexis Clairaut, Jean d'ALEMBERT, Leonhard EULER, Joseph LAGRANGE, Pierre LAPLACE, and others. In 1838, Friedrich BESSEL made the first measurement of the distance to a star (see PARALLAX). Astronomy was revolutionized in the second half of the 19th cent. by techniques based on photography and the SPECTROSCOPE. Interest shifted from determining the positions and distances of stars to studying their physical composition (see STELLAR EVOLUTION). With the construction of ever more powerful telescopes (see OBSERVATORY), the boundaries of the known universe constantly increased. Harlow SHAPLEY determined the size and shape of our galaxy, the MILKY WAY. Edwin HUBBLE's study of the distant galaxies led him to conclude that the universe is expanding (see HUBBLE'S LAW). Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, were formulated (see COSMOLOGY). Since the mid-20th cent. the frontiers of astronomy have been expanded by SPACE EXPLORATION and observations in new parts of the spectrum, e.g., gamma-ray astronomy, RADIO ASTRONOMY, ultraviolet astronomy, and X-ray astronomy. The new observational techniques have led to the discovery of strange new astronomical objects, e.g., PULSARS, QUASARS, and black holes.
