Astronomy

Astronomy, science dealing with all the celestial bodies in the universe, including the planets and their satellites, comets and meteors, the stars and interstellar matter, the star systems known as galaxies, clusters of galaxies, and quasars. Modern astronomy is divided into several branches: astrometry, the observational study of the positions and motions of these bodies; celestial mechanics, the mathematical study of their motions as explained by the theory of gravitation; astrophysics, the study of their chemical composition and physical condition from spectrum analysis and the laws of physics; and cosmology, the study of the universe as a whole.

Ancient Origins
The curiosity of ancient peoples concerning day and night and the Sun, Moon, and stars led eventually to the observation that the heavenly bodies appear to move in a regular manner that is useful in defining time and direction on the Earth. Astronomy grew out of problems originating with the first civilizations, the need to establish the proper times for planting and harvesting crops and for religious celebrations and to find bearings and positions on long trading journeys or voyages. See Archaeoastronomy.

To ancient peoples the sky showed many regularities of behaviour. The bright Sun, which divided day from night, rose every morning from one direction, the east, moved steadily across the sky during the day, and set in a nearly opposite direction, the west. At night the stars could be seen to follow a similar course, seeming to rotate in permanent groupings, called constellations, around a fixed point in the sky, which is known as the celestial pole.

People noticed that daytime and night-time were unequal in length. On long days, seen from the North Temperate Zone, the Sun rose north of east and climbed high in the sky at noon; on days with long nights the Sun rose south of east and did not climb so high. Observation of the stars that appear in the west after sunset or in the east before...