Astronomy (from the Greek words astro = star and nomos = law) is the
scientific study of celestial objects (such as stars, planets, comets,
and galaxies) and phenomena that originate outside the Earth's
atmosphere (such as the cosmic background radiation). It is concerned
with the evolution, physics, chemistry, meteorology, 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, 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, the making of calendars, and even astrology,
but professional astronomy is nowadays often considered to be
synonymous with astrophysics. Since the 20th century, the field of
professional astronomy split into observational and theoretical
branches. Observational astronomy is focused on acquiring and
analyzing data, mainly using basic principles of physics. Theoretical
astronomy is oriented towards 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.
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.
Old or even ancient astronomy is not to be confused with astrology,
the belief system that claims that human affairs are correlated with
the positions of celestial objects. Although the two fields share a
common origin and a part of their methods (namely, the use of
ephemerides), they are distinct.