Astronomy is a natural science that is the study of celestial objects such as moons, planets, stars, nebulae, and galaxies, the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma-ray bursts, and cosmic microwave background radiation.
A related but distinct subject, cosmology, is concerned with studying the universe as a whole.
Astronomy is one of the oldest sciences.
Prehistoric cultures have left astronomical artifacts such as the Egyptian monuments and Nubian monuments, and early civilisations such as the Babylonians, Greeks, Chinese, Indians, Iranians and Maya performed methodical observations of the night sky.
However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is nowadays often considered to be synonymous with astrophysics.
During the 20th century, the field of professional astronomy split into observational and theoretical branches.
is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics. This includes basic observations of the movement of the sun (for example observations of the solstice and equinox), moon, stars and planets.
is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena.
The two fields complement each other, with theoretical astronomy seeking to explain the observational results and observations being used to confirm theoretical results.
Astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Amateur astronomers have contributed to many important astronomical discoveries.
Astronomy is not to be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin they are now entirely distinct.
The word "astronomy" comes from the Greek "aster" meaning star and the suffix "_nomy" meaning "laws of" the Greek word nomos means law or rule.