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The term astronomy is derived from two Greek words: astron, meaning star, and nemein, meaning to arrange or distribute. It is one of the oldest sciences. The field has historically attracted people who have a natural fascination with our universe. Astronomers have traditionally been driven by a desire to learn; the pursuit of practical applications of astronomy has, for many, been secondary.

One of the earliest practical applications, the establishment of a calendar based on celestial movement, was pursued by many ancient civilizations, including the Babylonians, Chinese, Mayans, Europeans, and Egyptians. A chief aim of early astronomers was to study the motion of the bodies in the sky in order to create a calendar that could be used to predict certain celestial events and provide a more orderly structure to social life. The ancient Babylonians were among the first to construct a calendar based on the movement of the sun and the phases of the moon; their calendar has been found to have been accurate within minutes. In Europe, stone mounds constructed by ancient inhabitants also attest to astronomical work. Stonehenge in England is one of the largest and most famous of these mounds.

Ancient Greek astronomers introduced a new concept of astronomy by attempting to identify the physical structure of the universe, a branch of astronomy that has become known as cosmology. Astronomers such as Aristotle, Apollonius, Hipparchus, and Ptolemy succeeded in describing the universe in terms of circular movements. Their discoveries and theories were adopted by astronomers throughout much of the world. Modern astronomy was born with the theory of the sun-centered universe, first proposed by Nicolaus Copernicus in the 16th century. Copernicus's discovery revolutionized the field of astronomy and later would have a dramatic impact on many aspects of science.

After thousands of years, astronomers had succeeded in developing accurate predictions of celestial events. Next, they turned to newly evolving areas of astronomy, those of identifying the structure of the universe and of understanding the physical nature of the bodies they observed. Astronomers were aided by the invention of telescopes, and, as these increased in power, astronomers began to make new discoveries in the skies. For much of history, for example, it was believed that there were only five planets in the solar system. By the end of the 17th century, that number had increased to six, and, over the next two centuries, three more planets, Neptune, Uranus, and Pluto (now reclassified as a dwarf planet), were discovered.

Astronomers have always relied heavily on tools to bring faraway worlds close enough for study. As technology has evolved, so then has the field of astronomy. Spectroscopy, invented in the 19th century, allowed astronomers to identify the elements that make up the composition of the planets and other celestial bodies and gave rise to a new branch of astronomy, called astrophysics, which describes the components of the universe by measuring such information as temperature, chemical composition, pressure, and density. Later, photography, too, became an important research aid. In the early years of the 20th century, new discoveries further revolutionized the field of astronomy, particularly with the discovery of other galaxies beyond our own. The understanding grew that the universe was constructed of many millions of galaxies, each an island in an infiniteness of space.

By the middle of the 20th century, scientists had learned how to send rockets, and later manned spacecraft, into space to gain a closer view of the universe surrounding us. Using satellites and unmanned space probes, astronomers have been able to "travel" far into the solar system and beyond. A major event in astronomy occurred with the launching in 1990 of the powerful Hubble Space Telescope, which orbits the Earth and continues to send back information and photographs of events and phenomena across the universe.

Space scientists are currently working on a replacement for Hubble: the James Webb Space Telescope. This new telescope (called "Webb" or "JWST" for short), will have 10 times the power of the Hubble Space Telescope. The telescope is expected to launch in 2021. (Visit for more information on this new telescope.)