Description
Astronomy is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe. Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, it required the invention of the telescope before astronomy developed into a modern science. Since the 20th century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned with acquiring data, which involves building and maintaining instruments, as well as processing the results. Theoretical astrophysics is concerned with ascertaining the observational implications of computer or analytic models. The two fields complement each other, with theoretical astronomy seeking to explain the observational results. Astronomical observations can be used to test fundamental theories in physics, such as general relativity. Historically, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Modern astronomy is not to be confused with astrology, the belief system that claims human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, most thinkers in both fields believe they are now distinct. In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as the length of the year. As civilizations developed, most notably Babylonia, Egypt, ancient Greece, India, and China, astronomical observatories were assembled and ideas on the nature of the universe began to be explored. Early ideas on the motions of the planets were developed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. These included speculations on the spherical nature of the Earth and Moon, and the rotation and movement of the Earth through the heavens. A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1,000 B.C. by the Chinese. The Chaldeans discovered that eclipses recurred in a repeating cycle known as a saros. In the second century B.C., the size and distance of the Moon were estimated by Hipparchus. During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe until the 13th century.