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Astronomy (from

cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole.[1]

Astronomy is one of the oldest natural sciences. The early civilizations in

Greeks, Indians, Egyptians, 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. Nowadays, professional astronomy is often said to be the same as astrophysics.[2]

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs play an

transient events. Amateur astronomers
have helped with many important discoveries, such as finding new comets.


19th-century Quito Astronomical Observatory is located 12 minutes south of the Equator in Quito, Ecuador.[3]

Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.[4] Although the two fields share a common origin, they are now entirely distinct.[5]

Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are synonyms.[6][7][8] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties,"[9] while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[10] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[11] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[6] Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,[7] and many professional astronomers have physics rather than astronomy degrees.[8] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy & Astrophysics.


A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit

Ancient times

In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly 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 and in understanding the length of the year.[12]

Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in

Ptolemaic system, named after Ptolemy.[13]

Jains at The Schoyen Collection, London. Above: its manuscript from c. 1500 AD.[14]

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among

Alexandria on the Oxus
, present-day Afghanistan 3rd–2nd century BC.

Following the Babylonians, significant advances in astronomy were made in

planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[21]

Middle Ages

Medieval Europe housed a number of important astronomers. Richard of Wallingford (1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes and could predict eclipses. Nicole Oresme (1320–1382) and Jean Buridan (1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia) which was able to show planets were capable of motion without the intervention of angels.[22] Georg von Peuerbach (1423–1461) and Regiomontanus (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.

Abū Ishāq Ibrāhīm al-Zarqālī, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[27][28]

It is also believed that the ruins at

Post-classical West Africa, Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in August 1583.[31][32]
Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during the pre-colonial Middle Ages, but modern discoveries show otherwise.[33][34][35][36]

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.[37]

Scientific revolution

Galileo's sketches and observations of the Moon
revealed that the surface was mountainous.

During the

celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope.[39]

Improvements in the size and quality of the telescope led to further discoveries. The English astronomer

Nicolas Louis de Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[41]

During the 18–19th centuries, the study of the

Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[42]

Significant advances in astronomy came about with the introduction of new technology, including the

spectroscope and photography. Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.[27]

The existence of the Earth's galaxy, the

gravitational waves in the previous September.[44][45]

Observational astronomy

The main source of information about

visible light, or more generally electromagnetic radiation.[46] Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum
on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.

Radio astronomy

Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range.[47] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[47]

Although some

interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.[11][47]

A wide variety of other objects are observable at radio wavelengths, including

active galactic nuclei.[11][47]

Infrared astronomy

ALMA Observatory is one of the highest observatory sites on Earth. Atacama, Chile.[48]

Infrared astronomy is founded on the detection and analysis of

star clusters.[49][50]
With the exception of infrared
wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.[51] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.[52]

Optical astronomy

Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility
(right) is an example of a telescope that operates only at near-infrared wavelengths.