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AstronomyAstronomy, which etymologically means "law of the stars", (from Greek: αστρονομία = άστρον + νόμος) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. It studies the origins, evolution, physical and chemical properties of objects that can be observed in the sky (and are outside the earth), as well as the processes involving them.
 , photographed by the crew of Apollo 11 as they circled the Moon in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 93 kilometers (58 miles).]]
Astronomy is one of the few sciences where amateurs still play an active role, especially in the discovery and monitoring of transient phenomena. Astronomy is not to be confused with astrology, a pseudoscience that attempts to predict a person's destiny by tracking the paths of astronomical objects. Although the two fields share a common origin, they are quite different; astronomers embrace the scientific method, while astrologers do not.
Divisions of astronomy
In its earliest days, going back to ancient Greece and other ancient civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. Later, the work of Kepler and Newton paved the way for celestial mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular. Much of the effort in these two areas, once done largely by hand, is highly automated nowadays, to the extent that they are rarely considered as independent disciplines anymore. Motions and positions of objects are now easily known, and modern astronomy concerns itself much more with trying to observe and understand the actual physical nature of celestial objects—what makes them "tick".
Ever since the twentieth century the field of professional astronomy has tended to split into observational astronomy and theoretical astrophysics. Although most astronomers incorporate elements of both into their research, because of the different skills involved, most professional astronomers tend to specialize in one or the other. Observational astronomy is concerned mostly with getting data, which involves building and maintaining instruments and processing the resulting data; this branch is at times referred to as "astrometry" or simply as "astronomy." Theoretical astrophysics is concerned mainly with figuring out the observational implications of different models, and involves working with computer or analytic models.
The fields of study are also categorized in another two ways: by "subject", usually according to the region of space (e.g. Galactic astronomy) or "problems addressed" (such as star formation or cosmology); or by the way used for obtaining information.
By subject or problem addressed
 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.]]
- Astrobiology: the study of the advent and evolution of biological systems in the universe.
- Astrometry: the study of the position of objects in the sky and their changes of position. Defines the system of coordinates used and the kinematics of objects in our galaxy.
- Cosmology: the study of the universe as a whole and its 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.
- Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.
- Planetary Sciences: the study of the planets of the solar system.
- 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.
Also, there are other disciplines that may be considered part of astronomy:
See list of astronomical topics for a more exhaustive list of astronomy-related pages.
Ways of obtaining information
In astronomy, information is mainly received from the detection and analysis of electromagnetic radiation,
photons, but information is also carried by cosmic rays, neutrinos, meteors, and, in the near future, gravitational waves (see LIGO and LISA).
A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
- Optical astronomy describes the techniques used to detect and analyze light in and slightly around the wavelengths that can be detected with the eyes (about 400 - 800 nm). The most common tool is the telescope, with electronic imagers and spectrographs.
- Infrared astronomy deals with the detection of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but with the instrument optimized for infrared. Space telescopes are also used to eliminate noise (electromagnetic interference) from the atmosphere.
- Radio astronomy uses completely different instruments to detect radiation of wavelengths of mm to cm. The receivers are similar to those used in radio broadcast transmission (which uses those wavelengths of radiation). See also Radio telescopes.
- High-energy astronomy
 . This image shows several blue, loop-shaped objects that are multiple images of the same galaxy. They have been duplicated by the gravitational lens effect of the cluster of yellow galaxies near the photograph's center. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.]]
Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at those wavelengths. Infrared light is heavily absorbed by
water vapor, so infrared observatories have to be located in high, dry places or in space.
The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy, UV astronomy and, except for a few wavelength "windows", Far infrared astronomy, so observations
can be carried out only from balloons or space observatories.
Short history
In the early part of its history, astronomy involved only the observation and predictions of the motions of the objects in the sky that could be seen with the naked eye. The 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 took the earth to spin on its axis and considered the motions of the planets with respect to the sun.
Astronomy was mostly stagnant in medieval Europe, but flourished meanwhile in the Arab world. The late 9th century Islamic astronomer al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir 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, Iran, by the astronomer al-Khujandi who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. In Persia, Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami) compiled many tables and performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian.
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. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. 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.
Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses and sizes. The existence of our 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. 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.
For a more detailed history of astronomy, see the history of astronomy.
 . The ejection of gas, from the dying star at the center, has symmetrical patterns unlike the chaotic patterns expected from an ordinary explosion.]]
Timelines in astronomy
See also
Astronomy tools
External links
Organizations
References: Formulas and Constants
External links
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Harmonia Macrocosmica by Andreas Cellarius
Atlas of the heavens as seen by the astronomers of the time of its 1661 printing: Copernicus, Ptolemy, Brahe, and Aratus. Entire book has been digitized and the images may be browsed or searched.
http://www.lib.utah.edu/digital/cellarius.html
History and Philosophy of Western Astronomy
The History chapter of an introductory astronomy course.
http://www.astronomynotes.com/history/s1.htm
Jewish Astronomy, From Ancient to Modern Times
Astronomy in Israel from Og's Circle to the Wise Observatory.
http://wise-obs.tau.ac.il/judaism/jewish_astro.html
The ÀryabhatÃya of Àryabhata
The oldest exact astronomic constant? The ratio of earth rotations to lunar orbits in Aryabhata's AD 498 writing.
http://www.jqjacobs.net/astro/aryabhata.html
Astronomy in Japan
Historical and modern Japanese astronomy, and its place in Japanese culture
http://www2.gol.com/users/stever/jastro.html
The Star of Bethlehem
An investigation of the science and history which bear on the mysterious star said to have accompanied the advent of Christ.
http://www.bethlehemstar.net/
Northern California History of Astronomy Luncheon and Discussion Association
Announcements of discussions (in Oakland, California), open to the public, on various astronomy history topics, and an archive of past discussions
http://nchalada.org/
Rundetaarn (The Round Tower)
Astronomy in Denmark: Brahe, Roemer, Hertzsprung.
http://www.rundetaarn.dk/engelsk/observatorium/history.htm
Hindu Cosmological Time Cycles
An accurate calendar and the progenitor of the sexagesimal (base 60) "degree, minutes, seconds" measurement system.
http://www.aaronsrod.com/time-cycles/index.html
History of Astronomy in Ancient India
Eclipse calculation, heliocentric theory, size of the world.
http://india.coolatlanta.com/GreatPages/sudheer/astro.html
History of Astronomy Pages
Frederik Kaiser (1808-1872) and the professionalisation of Dutch astronomy; The "Lost Letters" of J.C. Kapteyn (1851-1922); Many history of astronomy and history of science links with a Dutch flavour.
http://www.strw.leidenuniv.nl/~heijden/history.html
Astronomical References in the Ruba'iyât of Omar Khayyam
Excerpts and commentary with reproductions of some of Elihu Vedder's illustrations.
http://www.minaret.org/INSAP3.htm
HASTRO-L
Archive of the History of Astronomy Discussion Group, the mailing list for scholars in this field.
http://listserv.wvu.edu/archives/hastro-l.html
Tycho's Star Maps
Celestial atlases and globes on exhibit at the Royal Observatory, Greenwich.
http://www.nmm.ac.uk/server.php?request=setTemplate:singlecontent&contentTypeA=conWebDoc&contentId=388
X-ray Astronomy at Goddard
Describes observations using balloons, rockets and satellites.
http://lhea.gsfc.nasa.gov/docs/xray/history.html
Historical Astronomy Division
American Astronomical Society division devoted to history, with a link to pages on the history of the society itself.
http://www.aas.org/~had/had.html
A Brief SETI Chronology
A timeline of the search for extraterrestrial intelligence, from Morrison and Coconni to SETI@home.
http://www.setileague.org/general/history.htm
A Science Odyssey - Physics and Astronomy
PBS articles about 20th century astronomy and physics.
http://www.pbs.org/wgbh/aso/databank/physastro.html
Electronic Newsletter for the History of Astronomy
Complete archive and subscription information.
http://www.astro.uni-bonn.de/~pbrosche/aa/enha/
Biennial History of Astronomy Conferences
Workshops at Notre Dame, papers presented, abstracts, group pictures of attendees.
http://www.nd.edu/~histast4/index.html
From Stargazers to Starships
Tutorial/historical exposition of the motion of Earth in space, Newtonian mechanics and spaceflight, on a high school level.
http://www.phy6.org/stargaze/Sintro.htm
The Babylonian Theory of the Planets
A substantial look at the subject, and a review of N. M. Swerdlow's book.
http://www.maa.org/reviews/babplanets.html
The Manchester Astronomical Society History
The first hundred years, list of presidents, list of archived documents.
http://www.u-net.com/ph/mas/history.htm
Atlas Celeste
A rare celestial atlas discovered in the library of the Manchester Astronomical Society.
http://www.u-net.com/ph/mas/bevis/home.htm
Russell A. Hulse - Nobel Lecture
The discovery of the binary pulsar.
http://www.nobel.se/physics/laureates/1993/hulse-lecture.html
The Cosmic Microwave Background Radiation
Robert Woodrow Wilson Nobel Lecture.
http://www.nobel.se/physics/laureates/1978/wilson-lecture.html
Historiacoelestis
Yahoo group moderated by Stuart Williams, F.R.A.S.
http://groups.yahoo.com/group/historiacoelestis/
This Month in the History of Astronomy
Short descriptions of important discoveries, events, and birthdays sorted by month.
http://astro.martianbachelor.com/TMITHOA/
Digital Archive of Historical Astronomy Pictures
Images from the history of astronomy, old telescopes, pictures of astronomers, observatories.
http://www42.pair.com/infolund/bolaget/DAHAP/
Astronomy
Features history of discoveries and references.
http://jacq.istos.com.au/sundry/astro.html
The Society For The History Of Astronomy
Academic and popular topics, with a focus on Britain.
http://www.shastro.org.uk/
Heavenly Mathematics: Cultural Astronomy
An interdisciplinary course on cultural astronomy.
http://www.math.nus.edu.sg/aslaksen/teaching/heavenly.html
Islamic Astronomy
Article by Owen Gingerich explores refinement and criticism of Ptolemaic astronomy, including religious influences on direction, and describes precursors to and influences on Copernicus.
http://users.kfupm.edu.sa/phys/alshukri/PHYS215/Islamic%20astronomy.htm
Gene Smith's Brief History of Astronomy
Covers the development of this ancient science from days of Stonehenge (3100 BC) to the discovery of Pulsars (1968 AD). Includes related resource links.
http://cassfos02.ucsd.edu/public/tutorial/History.html
Understanding Tidal Friction
Article by Peter Brosche on gaining understanding of changes in the Moon's orbit.
http://www.tribunes.com/tribune/art98/bros.htm
Big Ear Radio Observatory
Radio astronomy and the search for extraterrestrial intelligence.
http://www.bigear.org/
Starry Messenger
Phase I of the Electronic History of Astronomy, developed in the Whipple Museum of the History of Science at Trinity College, Cambridge. Covers the history of instruments and techniques, themes such as astrology and calendar reform, and biographies of major historical astronomers.
http://www.hps.cam.ac.uk/starry/starrymessenger.html
History of SETI
An overview of the history of the search for extraterrestrial intelligence.
http://www.seti-inst.edu/seti/seti_background/Welcome.html
Roemer and the First Determination of the Velocity of Light
Burndy Library online publication of 1940 article detailing the background and details of Roemer's work.
http://dibinst.mit.edu/BURNDY/OnlinePubs/Roemer/index.html
Little Green Men, White Dwarfs or Pulsars?
A personal account by Jocelyn Bell Burnell on the discovery of pulsars.
http://www.bigear.org/vol1no1/burnell.htm
History of Astronomy
Offers a history of the field and science. Features links to related sites, awards and contact details. Provided by the Working Group for the History of Astronomy.
http://www.astro.uni-bonn.de/~pbrosche/astoria.html
Out of This World
Exhibition catalog for The Golden Age of the Celestial Atlas. Includes an historical essay and sample pictures.
http://www.lindahall.org/events_exhib/exhibit/ex_atlas.shtml
Astronomy in Sweden 1860-1940
From the Uppsala University Newsletter for History of Science.
http://www.astro.uu.se/history/swehistory_1860-1940.html
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