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What is Astronomy?

Astronomy is the science that deals with the origin, evolution, composition, distance, and motion of all bodies and scattered matter in the universe. It includes astrophysics, which discusses the physical properties and structure of all cosmic matter.

Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, initially for calendrical and astrological purposes and later for navigational applications and scientific interest. The catalog of objects now studied is much broader and includes, in order of increasing distances, the solar system, the stars that make up the Milky Way Galaxy, and other more distant stellar objects and galaxies. With the advent of scientific space probes, the Earth also has come to be studied as one of the planets, though its more detailed investigation remains the domain of the geologic sciences.

Impact of Astronomy

No area of science is totally self-contained. Discoveries in one field find applications in others, often unpredictably. There are various notable examples of this involving astronomical studies. Newton's laws of motion and gravity emerged from the analysis of planetary and lunar orbits. The behaviour of nuclear matter and of some elementary particles is now better understood as a result of measurements of neutron stars and the cosmological helium abundance, respectively. Study of the theory of synchrotron radiation was greatly stimulated by its detection in radiation emitted by the Crab Nebula, and machines are now being used to produce synchrotron radiation to probe the structure of materials.

Astronomical knowledge also has had impact outside of science. The earliest calendars were based on astronomical observations of the cycles of repeated solar and lunar positions. Also, for centuries, familiarity with the positions and apparent motions of the stars enabled sea voyagers to navigate with reasonable accuracy. Perhaps the single greatest effect that astronomical studies have had on modern society has been in molding the attitude of its members. The development of what is known as the scientific method was strongly influenced by astronomical observations. The power of science to provide the basis for accurate predictions of such phenomena as eclipses and the positions of the planets and later, so dramatically, of comets has generated an attitude toward science that remains an important social force today.

Telescopic Observations

Before Galileo's use of telescopes for astronomical investigations in 1609, all observations were made by naked eye, with corresponding limits on the faintness and degree of detail that could be seen. Since that time, telescopes have become central to astronomy. With apertures much larger than the pupil of the human eye, telescopes permit the study of faint and distant objects. In addition, sufficient radiant energy can be collected in short time intervals to permit rapid fluctuations in intensity to be detected. Galileo

Optical telescopes are either refractors or reflectors that use lenses or mirrors, respectively, for their main light-collecting elements (objectives). Refractors are effectively limited to apertures with a diameter of 102 centimetres (40 inches) or less because of problems inherent in the use of large lenses. These distort under their own weight and can be supported only around the perimeter; an appreciable amount of light is lost due to absorption in the glass. Large-aperture refractors are extremely long and require large and expensive domes. The largest modern telescopes are all reflectors. They are not subject to the chromatic problems of refractors, can be better supported mechanically, and can be housed in smaller domes because of their more compact dimensions.

The atmosphere does not transmit radiation of all wavelengths equally well. This restricts ground-based astronomy to the visible and radio regions of the spectrum, with some relatively narrow “windows” in the nearer infrared. Longer infrared wavelengths are heavily absorbed by atmospheric water vapour and carbon dioxide. Atmospheric effects can be reduced by careful site selection and by carrying out observations at high altitudes. Most major optical observatories are now located on high mountains, well away from cities and their reflected lights. Infrared telescopes have been constructed atop Mauna Kea in Hawaii and in the Canary Islands where atmospheric humidity is very low. Airborne telescopes designed mainly for infrared observations, such as the one aboard the Kuiper Airborne Observatory, operate at
an altitude of about 12,000 metres (40,000 feet), with flight durations limited to a few hours. Telescopes for infrared-, X-, and gamma-ray observations have been carried
to altitudes of more than 30,000 metres by balloons. Higher altitudes can be attained during short-duration rocket flights for ultraviolet observations. Telescopes for all wavelengths from infrared to gamma rays have been carried by remote-controlled Earth-orbiting satellites as well as by some manned space vehicles. The Hubble Space Telescope, launched in 1990 and fully operational since late 1993, has provided remarkably detailed images of not only familar objects like nebulae and galaxies,
but also of objects invisible to terrestrial observatories.

Angular resolution better than one milliarcsecond has been achieved at radio wavelengths by using several telescopes in an array. In such an arrangement, the effective aperture (D) becomes the greatest distance between component telescopes. For example, in the Very Large Array (VLA), operated near Socorro, N.M., by the National Radio Astronomy Observatory, 27 movable radio dishes are set out along tracks that extend for nearly 21 kilometres. In a technique called very long baseline interferometry (VLBI), simultaneous observations are made with radio telescopes thousands of kilometres apart.

The Earth is a moving platform for astronomical observations. It is important that the specification of precise celestial coordinates be made in ways that correct for telescope location, the position of the Earth in its orbit around the Sun, and the epoch of observation, since the Earth's axis of rotation moves slowly over the years. Time measurements are now based on atomic clocks rather than on the Earth's rotation, and telescopes can be driven continuously to compensate for the planet's rotation so as to permit tracking of a given astronomical object.


Use of Radiation Detectors

The atmosphere does not transmit radiation of all wavelengths equally well. This restricts ground-based astronomy to the visible and radio regions of the spectrum, with some relatively narrow “windows” in the nearer infrared. Longer infrared wavelengths are heavily absorbed by atmospheric water vapour and carbon dioxide. Atmospheric effects can be reduced by careful site selection and by carrying out observations at high altitudes. Most major optical observatories are now located on high mountains, well away from cities and their reflected lights. Infrared telescopes have been constructed atop Mauna Kea in Hawaii and in the Canary Islands where atmospheric humidity is very low. Airborne telescopes designed mainly for infrared observations, such as the one aboard the Kuiper Airborne Observatory, operate at an altitude of about 12,000 metres (40,000 feet), with flight durations limited to a few hours. Telescopes for infrared-, X-, and gamma-ray observations have been carried to altitudes of more than 30,000 metres by balloons. Higher altitudes can be attained during short-duration rocket flights for ultraviolet observations. Telescopes for all wavelengths from infrared to gamma rays have been carried by remote-controlled Earth-orbiting satellites as well as by some manned space vehicles. The Hubble Space Telescope, launched in 1990 and fully operational since late 1993, has provided remarkably detailed images of not only familar objects like nebulae and galaxies, but also of objects invisible to terrestrial observatories.

Angular resolution better than one milliarcsecond has been achieved at radio wavelengths by using several telescopes in an array. In such an arrangement, the effective aperture (D) becomes the greatest distance between component telescopes. For example, in the Very Large Array (VLA), operated near Socorro, N.M., by the National Radio Astronomy Observatory, 27 movable radio dishes are set out along tracks that extend for nearly 21 kilometres. In a technique called very long baseline interferometry (VLBI), simultaneous observations are made with radio telescopes thousands of kilometres apart.

The Earth is a moving platform for astronomical observations. It is important that the specification of precise celestial coordinates be made in ways that correct for telescope location, the position of the Earth in its orbit around the Sun, and the epoch of observation, since the Earth's axis of rotation moves slowly over the years. Time measurements are now based on atomic clocks rather than on the Earth's rotation, and telescopes can be driven continuously to compensate for the planet's rotation so as to permit tracking of a given astronomical object.

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