Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects and phenomena that originate outside the Earth's atmosphere, such as stars, planets, comets, auroras, galaxies, and the cosmic background radiation. It is concerned with the formation and development of the universe, the evolution and physical and chemical properties of celestial objects and the calculation of their motions. Astronomical observations are not only relevant for astronomy as such, but provide essential information for the verification of fundamental theories in physics, such as general relativity theory. Complementary to observational astronomy, theoretical astrophysics seeks to explain astronomical phenomena.

Astronomy is one of the oldest sciences with a scientific methodology existing at the time of ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). 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 as practised is not to be confused with astrology, the belief system that states that people's destiny and human affairs in general are correlated to the positions of celestial objects in the skies. Although the two fields share a common origin, they are quite different; astronomers employ the scientific method, while astrologers do not.

Contents [hide] 1 Divisions 1.1 By subject or problem addressed 1.2 Ways of obtaining information 2 History of astronomy 3 Timelines in astronomy 4 See also 4.1 Astronomy tools 5 External links

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Divisions

A giant Hubble mosaic of the Crab nebula

In Babylon, and ancient Greece, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. Later, with the work of astronomers Kepler and Newton, whose work led to the development of celestial mechanics, the mathematical prediction of the motions of celestial bodies interacting gravitationally became the focus of astronomy. This was applied to solar system objects in particular. Motions and positions of objects are now more easily determined, and modern astronomy is more concerned with observing and understanding the actual physical nature of celestial objects.

Since the twentieth century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned mostly with acquiring data, which involves building and maintaining instruments and processing the results; this branch is at times referred to as "astrometry" or simply as "astronomy". Theoretical astrophysics is concerned mainly with ascertaining the observational implications of different models, and involves working with computer or analytic models.

The fields of study can also be categorized in other ways. Categorization by the region of space under study (for example, Galactic astronomy, Planetary Sciences); by subject, such as star formation or cosmology; or by the method used for obtaining information.

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By subject or problem addressed

Planetary astronomy, or Planetary Sciences: a dust devil on Mars. Photographed by Mars Global Surveyor, the long dark streak is formed by a moving swirling column of Martian atmosphere (with similarities to a terrestrial tornado). The dust devil itself (the black spot) is climbing the crater wall. The streaks on the right are sand dunes on the crater floor.

• Astrometry: the study of the position of the objects in the sky and their changes of the position in the sky. Defines the system of coordinates used and the kinematics of objects in our galaxy.
• Astrophysics: the study of physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.
• Cosmology: the study of the origin of the universe and its evolution. The study of cosmology is theoretical astrophysics at its largest scale.
• Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.
• Galactic astronomy: the study of the structure and components of our galaxy and of other galaxies.
• Extragalactic astronomy: the study of objects (mainly galaxies) outside our galaxy.
• Stellar astronomy: the study of the stars.
• Stellar evolution: the study of the evolution of stars from their formation to their end as a stellar remnant.
• Star formation: the study of the condition and processes that led to the formation of stars in the interior of gas clouds, and the process of formation itself.
• Planetary Sciences: the study of the planets of the Solar System.
• Astrobiology: the study of the advent and evolution of biological systems in the Universe.

Other disciplines that may be considered part of astronomy:

• Archaeoastronomy
• Astrochemistry
• Astrosociobiology

See the list of astronomical topics for a more exhaustive list of astronomy related pages.

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Ways of obtaining information

Main article: Observational astronomy.

In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future.

A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:

• Optical astronomy is the part of astronomy that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.
• Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Space telescopes are also used to avoid atmospheric thermal emission, atmospheric opacity, and the effects of astronomical seeing at infrared and other wavelengths.
• Radio astronomy detects radiation of millimetre to dekametre wavelength. The radio telescope receivers are similar to those used in radio broadcast transmission but much more sensitive.
• High-energy astronomy includes X-ray astronomy, gamma ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays.

Optical and radio astronomy can be performed with ground-based observatories, because the Earth's atmosphere is transparent at the wavelengths being detected. Infrared radiation is heavily absorbed by atmospheric water vapour, so infrared observatories have to be located in high, dry places or in space.

The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

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History of astronomy

Extragalactic astronomy: gravitational lensing. This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the photograph's centre. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.

Main article: History of astronomy.

In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. Aristotle said that the Earth was the centre of the Universe and everything rotated around it in orbits that were perfect circles. Aristotle had to be right because people thought that Earth had to be in the centre with everything rotating around it because the wind would not scatter leaves, and birds would only fly in one direction. For a long time, people thought that Aristotle was right, but it is probable that Aristotle accidentally did more to hinder our knowledge than help it.

The first ancestor of an astronomy computer, was the Antikythera mechanism, an ancient Greek device for calculating the movements of planets, dating from about 80 BC. It was discovered in an ancient shipwreck off the Greek island of Antikythera, between Kythera and Crete. The device became famous for its use of a differential gear, previously believed to have been invented in the 16th century and the miniaturization and complexity of its parts, comparable to a clock made in the 18th century. The original mechanism is displayed in the Bronze collection of the National Archaeological Museum of Athens, accompanied by a replica. Another replica is on display at the American Computer Museum in Bozeman, Montana.

The Hindu vedic text, Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal; see Biblical cosmology. In 500 AD, Aryabhata presented a mathematical system that described the earth as spinning on its axis and considered the motions of the planets with respect to the sun.

East Asia's first astronomical observatory was developed in Silla, one of Three Kingdoms of Korea, under the reign of Queen Seondeok of Silla. It was termed Cheomsongdae, and is one of the oldest surviving scientific installations on Earth. It means star-gazing tower in Korean.

Although classical astronomy was one of the seven key subjects taught at medieval universities in Europe, observational astronomy was mostly stagnant in medieval Europe until XIII century, when lived astronomers and astrologers as Johannes de Sacrobosco, in England, and Guido Bonatti from Forlì, in Italy.

Tycho Brahe's work is very famous, in the 16th Century. However, observational astronomy flourished in the Iranian world and other parts of Islamic realm. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the Persian astronomer al-Khujandi, who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyám performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. Abraham Zacuto was responsible in the 15th century for the adaptations of astronomical theory for the practical needs of Portuguese caravel expeditions.

Table of astronomy, from the 1728 Cyclopaedia.

In Europe during the Renaissance, Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the centre. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.

Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes. The existence of Earth's galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law, and cosmological abundances of elements.

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Timelines in astronomy

Stellar astronomy, Stellar Evolution: The Ant planetary nebula. Ejecting gas from the dying centre star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.

• Artificial satellites and space probes
• Astronomical maps, catalogues, and surveys
• Big Bang
• Black hole physics
• Cosmic microwave background astronomy
• Cosmology
• Galaxies, clusters of galaxies, and large scale structure
• Interstellar medium and intergalactic medium
• Natural satellites
• Other background radiation fields
• Solar astronomy
• Solar system astronomy
• Stellar astronomy
• Telescopes, observatories, and observing technology
• White dwarfs, neutron stars, and supernovae

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See also

Find more information on Astronomy by searching Wikipedia's sister projects:

Dictionary definitions from Wiktionary
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Quotations from Wikiquote
Source texts from Wikisource
Images and media from Commons
News stories from Wikinews

• List of astronomical topics
• Astronomers and Astrophysicists
• Astronomical symbols
• Astronomical cycles
• Astronomical naming conventions
• Astronomical object
• Astronomical observatories
• Astronomy organisations
• Celestial navigation
• Space exploration
• Space science
• Astronomical objects named after people

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Astronomy tools

• Binoculars
• Telescope
• Computers
• Calculator
• Observatory
• Space observatory
• Maksutov telescope

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External links

• AbsoluteAstronomy.com
• NASA
• Southern Hemisphere Astronomy
• Astronomy Picture of the Day
• New Scientist Space
• Sky & Telescope
• American Astronomical Society
• Planet Surveyor
• Utah Skies Astronomy Information by and for Amateur Astronomers
• ebook History of Astronomy, George Forbes (1849-1936
• The Amazing Sky
• Space and Astronautics News
• Lunataker Astronomie Page
• Night Sky Info
• IceInSpace The Australian Amateur Astronomy Community

General subfields within astronomy	Edit
Astrophysics | Cosmology | Galactic astronomy | Extragalactic astronomy | Galaxy formation and evolution | Planetary science | Stellar astronomy | Stellar evolution | Star formation

General subfields within the Natural sciences
Astronomy | Biology | Chemistry | Earth science | Ecology | Physical science | Physics