AstronomyAstronomy, which etymologically means "laws of the stars", is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. Astronomy is often associated with astrophysics. Astronomy is one of the few sciences where amateurs still play an active role, especially in the discovery and monitoring of transient phenomena. This 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. The divisions are not unique, however, and the intersections, as well as astronomers who work in several areas, are the rule more than the exception. A first distinction is between theoretical and observational astronomy and astrophysics. 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. Fields of study are also categorized in another two main ways: by subject, usually according to the region of space (e.g. Galactic astronomy) or problems addressed (such as star formation or cosmology); and according to the means of obtaining the data (e.g. optical astronomy or radioastronomy) By way of obtaining information In astronomy, the main way of obtaining information is through the detection and analysis of electromagnetic radiation, photons, but we also receive information from outside the earth 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: * Optical astronomy refers to the techniques used to detect and analyze light in and slightly around the wavelengths than 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 using infrared radiation (wavelengths longer than the red light). Again, the most common tool is the telescope, but with instruments sensitive to longer wavelengths; the telescope itself can be 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 Optical and radio astronomy can be done using 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 many 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, the nature of the stars and planets, and so forth, most of which are contradicted by modern astronomy; 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 some Arabic astronomers. The renaissance came to astronomy with the work of Copernicus, who proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by the likes of Galileo Galilei and Johannes Kepler. The latter of these was the first to provide 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, the final explanation of the motions of the planets. Astrophysics was a later development, which only became possible once it was understood that the elements that made up the "celestial objects" were the same that made up the Earth, and that the same laws of physics applied. Stars were found to be far away objects, and 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, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot 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.