Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernovaexplosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.

Etymology

Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "rule") means study of celestial objects.^[1] Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. The two fields share a common origin but became distinct, astronomy being supported by physics while astrology is not.^[2]

Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are broadly synonymous in modern usage.^[3][4][5] In dictionary definitions, "astronomy" is "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties",^[6] while "astrophysics" is the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".^[7]Sometimes, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" means the qualitative study of the subject, whereas "astrophysics" is the physics-oriented version of the subject.^[8] Some fields, such as astrometry, are in this sense purely astronomy rather than also astrophysics. Research departments may use "astronomy" and "astrophysics" according to whether the department is historically affiliated with a physics department,^[4] and many professional astronomers have physics rather than astronomy degrees.^[5] Thus, in modern use, the two terms are often used interchangeably.^[3]

Pre-historic

Megaliths from Nabta Playa, constructed by Neolithic populations,^[9]located in Aswan, Upper Egypt.^[10]

The Nebra sky disc found on Mittenberg hill in Germany and dated to c. 1800–1600 BCE.^[11]

The initial development of astronomy was driven by practical needs like agricultural calendars. Before recorded history archeological sites such as Stonehenge provide evidence of ancient interest in astronomical observations.^[12]: 15  Evidence also comes from artefacts such as the Nebra sky discinlaid with symbols interpreted as a sun, moon, and stars including a cluster of seven stars.^[13][14][15]Megalithic structures located in Nabta Playa, Upper Egypt featured astronomical calendar arrangements in alignment with the heliacal rising of Sirius and supported calibration the yearly calendar for the annual Nile flood.^[16]

Classical

A Babylonian planisphere (7th century BCE) was an early astronomical instrument. Its use of sexagesimals (e.g. 12, 24, 60, 360) is still being used today through having been broadly adopted for timekeepingand astrometry.^[17]

Civilizations such as Egypt, Mesopotamia, Greece, India, China independently but with cross-cultural influences created astronomical observatories and developed ideas on the nature of the Universe, along with calendars and astronomical instruments.^[18] A key early development was the beginning of mathematical and scientific astronomy among the Babylonians, laying the foundations for astronomical traditions in other civilizations.^[19] The Babylonians discovered that lunar eclipses recurred in the saros cycle of 223 synodic months.^[20]

Following the Babylonians, significant advances were made in ancient Greece and the Hellenistic world. Greek astronomy sought a rational, physical explanation for celestial phenomena.^[21] In the 4th century BC, Heracleides Ponticus was the first to proposed that the Earth rotates on its own axis.^[22]In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model.^[23] 

In the 2nd century BC, Hipparchus calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe.^[24] He also observed the small drift in the positions of the equinoxes and solstices with respect to the fixed stars that we now know is caused by precession.^[12] Hipparchus also created a catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.^[25] The Antikythera mechanism (c. 150–80 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.^[26]

Post-classical

After the classical Greek era, astronomy was dominated by the geocentric model of the Universe, or the Ptolemaic system, named after Claudius Ptolemy. His 13-volume astronomy work, named the Almagest in its Arabic translation, became the primary reference for over a thousand years.^[27]: 196  In this system, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it.^[28] 

While the system would eventually be discredited, it gave the most accurate predictions for the positions of astronomical bodies available at that time.^[27]

With the arrival of Hellenistic astronomy in India through trade and cultural contacts, Indian astronomy entered a new phase during the early centuries CE.^[29] Earlier indigenous traditions, such as those recorded in the Vedāṅga Jyotiṣa, provided calendrical foundations, while Greek astronomical models were later integrated by scholars including Āryabhaṭa, Varāhamihira, and Brahmagupta.^[30] Āryabhaṭa notably improved methods for calculating planetary motions and eclipses.^[31] In the later medieval period, the Kerala school contributed to astronomy through refined observational practices and more accurate planetary and eclipse calculations.^[32]

Portrait of Alfraganus in the Compilatio astronomica, 1493. Islamic astronomerscollected and translated Indian, Persian and Greektexts, adding their own work.^[33]

Astronomy flourished in the medieval Islamic world. Astronomical observatories were established there by the early 9th century.^[34][35][36] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars.^[37] The SN 1006 supernova, the brightest apparent magnitude stellar event in the last 1000 years, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006.^[38] Iranian scholar Al-Biruni observed that, contrary to Ptolemy, the Sun's apogee (highest point in the heavens) was mobile, not fixed.^[39][40] Arabic astronomers introduced many Arabic names now used for individual stars.^[41]

The ruins at Great Zimbabwe and Timbuktu^[42] may have housed astronomical observatories.^[43] In Post-classical West Africa, astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens and diagrams of orbits of the other planets based on complex mathematical calculations.^[44] Songhai historian Mahmud Kati documented a meteor shower in 1583.^[45]

In medieval Europe, Richard of Wallingford (1292–1336) invented the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies,^[46] as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes.^[47] Nicole Oresme (1320–1382) discussed evidence for the rotation of the Earth.^[48] 

Jean Buridan (1300–1361) developed the theory of impetus, describing motions including of the celestial bodies.^[49][50] For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.^[51]