Astronomy

File:Moon.crater.arp.750pix.jpg

Lunar astronomy: the large crater is Daedalus, 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 58 miles (93 km).

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, 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 and for thousands of years was the same with Astrology, a scientific methodology existing at the time of Ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). Historically, amateurs 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 originated from astrology, the study of celestial influences in the natural world and human affairs. Ancient astrologers/astronomers who first began observations of movements of the planets and stars relative to the earth correlated planetary and stellar motions with happenings in the world. These classical astrologers, of many cultures, were natural astronomers; inventing the mathematical principles, tools and copious obvservations that gave birth to modern astronomy - with branches that apply precise observational and data-collecting instruments to weigh, measure, and map the universe.

The study of the cosmos with the greatest possible mathematical accuracy to collect phyical data, is now called Astronomy. The study of the motions of the Sun, Moon, planets and stars and their mathematical aspects relative to Earth, and correlated to human events in the natural world environment is called Astrology.

Divisions

Before the extensive contributions of ancient Greece earlier civilizations of the Mayan, Chaldean, Sumarian, Eygptian and Mesopotamian cultures possessed an astounding knowledge of astronomy. By the emergence of ancient Greek culture, this consisted largely of astrometry, measuring positions of stars and planets in the sky and correlating their effects on earth.

Hipparchus (circa. 100 B.C.)is considered one of the greatest Greek astrologer/astronomers. His mapping of the Sun, Moon, planets and stars are so accurate at times that latter astronomers used his work on the Lunar Nodes to predict eclipses of the Sun and Moon. Hipparchus' also introduced the idea of comparative stellar magnitudes - now commonly used by modern astronomers.

Claudius Ptolemy, the Second Century A.D. Eygptian-based astrologer, authored major works on his stellar observations in books, Almagest and Tetrabiblos (four books); contain his extensive findings on mapping the cosmos using the prior works of Hipparchus. Ptolemy, as an astrologer, also held a geocentric view of the solar system to make forecasts of events on earth including his use of astronomy in weather prediction.

After the decline of the Roman Empire, and the subsequent European Dark Ages, (700 1200 A.D.) an Islamic empire that stretched from Southern Europe to Central Asia gave way to a new crop of astrologer/astronomers who would come to be reknown for their refinements of the astrolabe , the telescope and invention of mathematics and techniques of algebra, geometry and trigonometry to map the skies and track the movement of planets and stars. Excellent observers and stellar map-maker; many stars given Arabic names remain in wide use in modern astronomy. It was the Arab astrologers who provided the essential bridge needed for classical Greek astronomy and Islamic improvement to the scholars and Christian monks of the European Middle Ages.

Later, the emergence of the Age of Reason in Europe by the mid-17th Century, a rich database of thousands of years of recorded celestial observations by ancient and classical astrologer/astronomers led to a Renaissance of scientfic thought also called the Age of Enlightenment. Western civilization re-emerged with vigor and the works of men like Tycho Brahe, Copernicus Kepler and most notably, Isaac Newton, ushered in a new age of exploration of the universe. Newton, whose revolutionary work led to the development of celestial mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular - is among one of the most brilliant names in modern astronomy. Much of the effort in these two areas, once done largely by hand, is highly automated in modern science. Motions and positions of objects are now more easily determined, and modern astronomy is more concerned with observing and understanding the physical nature of celestial objects.

Since the twentieth century, the field of professional astronomy has 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 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.

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.

• Astrology: the study of the positions of the celestial objects relative to the Earth and how these positions correlate to events in the lives of individuals, cultures, nations and the natural Earth environment.
• 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.
• 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
• Astrophilosophy

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

Ways of obtaining information

Radio telescopes are among many different tools used by astronomers

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 receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
• 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 atmosphere is transparent at the wavelengths being detected. 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 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.

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 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.

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 center 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 center 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 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.

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 Tycho Brahe's work 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.

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 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. 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 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. 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.

Timelines in astronomy

File:Ant.nebula.arp.600pix.jpg

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

• Artificial satellites and space probes
• Astronomical maps, catalogs, 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

See also

• List of astronomical topics
• Astronomers and Astrophysicists
• Space exploration
• Space science
• Astronomical symbols
• Astronomical cycles
• Astronomical naming conventions
• Astronomical object
• Astronomical observatories
• Astronomy organizations

• Celestial navigation

Astronomy tools

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

External Links

• Space.com
• Astronomy.com
• AbsoluteAstronomy.com
• Bad Astronomy
• Nasa
• Run4Space Forum
• Astronomy Picture of the Day
• New Scientist Space
• Sky & Telescope
• American Astronomical Society
• Planet Surveyor
• The Virtual Sky