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The science of space beyond Earth's surface and lower atmosphere. In addition to observational astronomy, it includes such branches as astrometry, the measurement of the positions and movements of celestial bodies; astrophysics, the physics of stars, galaxies, and other objects in the universe; celestial mechanics, which specializes in orbit calculation, including the influence of gravity, mass, acceleration, and inertia; and cosmology, which investigates the origins and evolution of the universe.

History of astronomy

Born at the crossroads of agriculture and religion, astronomy, the earliest of sciences, was of great practical importance in ancient civilization. Before 2000 BC, Babylonians, Chinese, and Egyptians all sowed their crops according to calendars computed from the regular motions of the Sun and Moon.

Although early Greek philosophers were more concerned with the physical nature of the universe than with precise observation, later Greek scientists (for example, Aristarchus and Hipparchus) returned to the problems of positional astronomy. The vast achievement of Greek astronomy was epitomized in the writings of Claudius Ptolemy. His Almagest, passing through Arabic translations, was eventually transmitted to medieval Europe and remained the chief authority among astronomers for more than 1400 years.

Throughout this period the main purpose of positional astronomy had been to assist in the casting of accurate horoscopes, the twin studies of astronomy and astrology having not yet parted company. The structure of the universe meanwhile remained the reserve of Aristotelian physics. The work of Copernicus represented an early attempt to harmonize and improved positional astronomy with a true physical theory of planetary motion. Against the judgment of antiquity that the Sun, Moon, and planets circled the Earth as lanterns set in a series of concentric transparent shells; in his de Revolutionibus (1543) Copernicus argued that the Sun lay motionless at the center of the planetary system.

Although the Copernican (or heliocentric) hypothesis proved to be a sound basis for the computation of navigators' tables (the needs for which were stimulating renewed interest in astronomy), it did not become unassailably established in astronomical theory until Newton published his mathematical derivation of Kepler's laws in 1687. In the meanwhile Kepler, working on the superb observational data of Tycho Brahe, had shown the orbit of Mars to be elliptical and not circular and Galileo had used the newly invented telescope to discover sunspots, the phase of Venus and four moons of Jupiter.

Since the 17th century the development of astronomy has followed on successive improvements in the design of telescopes. In 1781 William Herschel discovered Uranus, the first discovery of a new planet to be made in historical times. Measurement of the parallax of a few stars in 1838 first allowed the estimation of interstellar distances. Analysis of the Fraunhofer lines in the solar spectrum gave scientists the first indication of the chemical composition of the stars.

In the 20th century and up to the present day, the scope of observational astronomy has been extended to electromagnetic radiation emitted by celestial objects from the shortest wavelengths (gamma rays) to the longest radio waves), taking in X-rays, ultraviolet, optical, and infrared. As the Earth's atmosphere absorbs most of these radiations from space, telescopes have been launched into orbit to gain an unobstructed view of the heavens.

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