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Astronomy, which etymologically means "law of the stars",(From Greek): άστρο, + 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 all the objects that can be observed in the sky (and are outside the earth), as well as all the processes involving them. What subfields it includes is a question whose answer has been changing with the ages. During part of the 20th century astronomy was considered to be separated in astrometry, celestial mechanics and astrophysics. Of the three, astrophysics has gained certain preeminence, as that can be seen reflected in the naming of the University departments and institutions involved in astronomical research: the oldest ones are invariably Astronomy departments and institutes, the newest ones tend to include astrophysics in their names, sometimes excluding the word astronomy, to emphasize the nature of its research. Furthermore, astrophysical research, specially in theoretical astrophysics can be charried out by people whose backgroud is in physics or mathematics rather than astronomy.

File:Moon.crater.arp.300pix.jpg
Lunar astronomy: the far side of Earth's Moon. The large impact basin pictured is Crater 308. It spans about 30 kilometers (19 miles) and was photographed by the crew of Apollo 11 as they circled the Moon in 1969.
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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 which 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; astronomy embraces the scientific method, while astrology, with no basis in science, does not.

Divisions of astronomy

Given its huge scope, astronomy is divided into different branches. A first main distinction is between theoretical and observational astronomy. Observers use a variety of means to obtain data about different phenomena, data that is then used by theorists to create and constrain theories and models, to explain observations and to predict new ones; observer and theorist are not necessarily different persons, and rather than two perfectly delimited fields, there is a continuum of scientists that put more or less emphasis in observation or theory.

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 (esentially, what region of the spectrum is used). While the first division is applicable to both orbservers and theorist, the second one applies only to observers (and imperfectly), as theorists try to use all the information available, in all wavelenght, and observer often observe in more than one region of the spectrum.

By subject or problem adressed

File:Dust.devil.mars.arp.300pix.jpg
Planetary astronomy, or Planetary Sciences: a Martian dust devil. Photographed by the NASA Global Surveyor in Mars orbit, the long dark streak is formed by the movement of a swirling column of Martian atmosphere (with similarities to a terrestrial tornado). The dust devil (the black spot) is climbing the crater wall. Dust devils occur when the atmosphere is heated by a warm surface and begins to spin as it rises. The streaks in the right hand half of the picture are sand dunes on the crater floor.
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  • Astrometry: Position of object in the sky and their changes.
  • Observational cosmology: Study of the universe as a whole and its evolution.
  • Galactic astronomy: Used to be the study of the structure and components of our galaxy. Now includes study of other galaxies that can be observed in detail.
  • Extragalactic astronomy: Study of objects (mainly galaxies) outside ours.
  • Galaxy formation and evolution: Study of the formation of the galaxies, and its evolution to the observed present state.
  • Star formation: 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.
  • Stellar evolution: Study of the evolution of stars from its formation to its end as a stellar remnant.
  • Stellar astronomy: Study of stars, in general.

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

By ways of obtaining information

In astronomy, the main way of obtaining information is through the detection and analysis of electromagnetic radiation, photons, but information is also carried by cosmic rays, neutrinos, 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:

File:Grav.lens1.arp.300pix.jpg
Extragalactic astronomy: gravitational lensing. This Hubble Space Telescope image shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens effect of the cluster of yellow, elliptical and spiral galaxies near the photograph's center. The gravitational lens is produced by the cluster's tremendous gravitational field that bends light to magnify, brighten and distort the image of a more distant object.
Click on the picture for more information and a larger version.

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 , and 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.

The study of astronomy almost stopped during the middle ages, except for the work of Arabic astronomers. In the late 9th century the Islamic astronomer al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani) wrote extensively on the motion of celestial bodies. In the 12th century, his works were translated into Latin, and it is said that Dante got his astronomical knowledge from al-Farghani's books.

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, also known as the tilt of the Earth's axis relative to the Sun. As we know today, the Earth's tilt is approximately 23o34', and al-Khujandi measured it as being 23o32'19". Using this information, he also compiled a list of latitudes and longitudes of major cities.

Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami) was a great Persian scientist, philosopher, and poet who lived from 1048-1131. He compiled many astronomical tables and performed a reformation of the calendar which was more accurate than the Julian and came close to the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156 days long, which is accurate to the 6th decimal place.

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 which described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not understand the reasons 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 far away 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.

File:Ant.nebula.arp.300pix.jpg
Stellar astronomy,Stellar evolution: The Ant planetary nebula. The ejection of gas, from the dying star at the center, has intriguing symmetrical patterns unlike the chaotic patterns expected from an ordinary explosion. Scientists using Hubble would like to understand how a spherical star can produce such prominent symmetries in the gas that it ejects.
Click on the picture for more information and a larger version.

See also

Astronomy Tools

http://www.asimpleclick.com/nasa_related.htm for additional info

Organizations

References

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