Astronomy is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe. Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, it required the invention of the telescope before astronomy developed into a modern science.

Since the 20th century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned with acquiring data, which involves building and maintaining instruments, as well as processing the results. Theoretical astrophysics is concerned with ascertaining the observational implications of computer or analytic models. The two fields complement each other, with theoretical astronomy seeking to explain the observational results.

Astronomical observations can be used to test fundamental theories in physics, such as general relativity. Historically, amateur astronomers 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 is not to be confused with astrology, the belief system that claims human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, most thinkers in both fields believe they are now distinct.

In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as the length of the year.

As civilizations developed, most notably Babylonia, Egypt, ancient Greece, India, and China, astronomical observatories were assembled and ideas on the nature of the universe began to be explored. Early ideas on the motions of the planets were developed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. These included speculations on the spherical nature of the Earth and Moon, and the rotation and movement of the Earth through the heavens.

A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1,000 B.C. by the Chinese. The Chaldeans discovered that eclipses recurred in a repeating cycle known as a saros. In the second century B.C., the size and distance of the Moon were estimated by Hipparchus. During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe until the 13th century. However, observational astronomy flourished in the Persian Empire and other parts of the Islamic world. Islamic astronomers introduced many names that are now used for individual stars.

During the Renaissance, Nicolaus 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.

Further discoveries paralleled the improvements in size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made an extensive catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.

During the nineteenth century, attention to the three body problem by Euler, Clairaut and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrance and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes.

The existence of Earth's galaxy, the Milky Way, as a separate group of stars was only proved 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.

The Telescope:

A telescope is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types.

An optical telescope is an optical tool that gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied. Optical telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses.

Single-dish Radio telescopes are focusing radio antennae often having a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. As of 2005, the current record array size is many times the width of the Earth, utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes.

X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola. High energy particle telescopes detect a flux of particles, usually originating at an astronomical sourcee.

The Hubble Space Telescope:

The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble for his discovery of galaxies outside the Milky Way and his creation of Hubble's Law, which calculates the rate at which the universe is expanding. Its position outside the Earth's atmosphere allows it to take sharp optical images of very faint objects, and since its launch in 1990, it has become one of the most important instruments in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics. Hubble's Ultra Deep Field is the deepest (most sensitive) astronomical optical image ever taken.

From its original conception in 1946 until its launch, the project to build a space telescope was beset by delays and budget problems. Immediately after its launch, it was found that the main mirror suffered from spherical aberration, severely compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its planned quality and became a vital research tool as well as a public relations boon for astronomy. The HST is part of NASA's Great Observatories series, with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

The future of Hubble is currently uncertain. Its stabilizing gyroscopes need replacing, and without intervention to boost its orbit it will re-enter the Earth's atmosphere sometime after 2010. Following the Columbia Space Shuttle disaster, NASA decided that a repair mission by astronauts would be unreasonably dangerous. The organization later reconsidered this position, but a final servicing mission still depends on the success of the Space Shuttle program in overcoming the design flaws which led to the Columbia disaster.

Hubble's successor telescope, the James Webb Space Telescope (JWST), is due to be launched in 2013 and will be far superior to Hubble for most astronomical research programs. However, the JWST will only observe in infrared, so it will not replace Hubble's ability to observe in the visible part of the spectrum.

The history of the Hubble Space Telescope can be traced back as far as 1946, when astronomer Lyman Spitzer wrote a paper entitled Astronomical advantages of an extra-terrestrial observatory. In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes: First, the angular resolution (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere which causes stars to twinkle and is known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.1 arcsec for a telescope with a mirror 2.5 m in diameter. The second major advantage would be that a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere.

Astronomy Resource Books And Information For Space Enthusiasts

 Astronomy Today (5th Edition)

An Introduction to Modern Astrophysics

The Essential Cosmic Perspective (3rd Edition)

Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity

The Cosmic Perspective w/CD

Astronomy: A Beginner's Guide to the Universe (5th Edition)

 The History Of Interstellar Space:

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by Francis Bacon in 1626 where he wrote: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." Later, natural philosopher Robert Boyle surmised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty."

Before modern electromagnetic theory early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as R. H. Patterson wrote in 1862, "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces" .

The advent of deep photographic imaging allowed Barnard to produce the first images of dark nebulae silhouetted against the background star field of the Galaxy. In 1904 Hartmann detected spectroscopic absorption lines towards a pair of binary stars that could not have come from the stars themselves. The growing evidence for interstellar material led William Henry Pickering to comment in 1912 that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars..."

The same year Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain down on the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year the Norwegian explorer and physicist Kristian Birkeland wrote: 'It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space.'

In 1930, Samuel L. Thorndike notes that ".. it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles from the Sun emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy".

Solar Systems - Planets:

The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, currently defines "planet" as a celestial body that, within the Solar System:

(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and
(c) has cleared the neighborhood around its orbit;

or within another system:

 (i) is in orbit around a star or stellar remnants;
(ii) has a mass below the limiting mass for thermonuclear fusion of deuterium; and
(iii) is above the minimum mass/size requirement for planetary status in the Solar System.

As a result of this definition, the Solar System is now considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Those objects which fulfil criteria (a) & (b), but not (c) - Ceres, Pluto, and Eris - are categorized as dwarf planets. Prior to the adoption of the 2006 resolution, there was no formal scientific definition of "planet". Without one, the Solar System had been considered to have differing numbers of planets over the years, including Pluto, Ceres and several asteroids at various stages.

Beyond the Solar System, there have been more than two hundred objects discovered orbiting other stars. However, while a formal definition for planets within the Solar System now exists, the IAU's position on those in other systems remains only a working definition in place since 2003. The IAU has not yet taken a position on whether free-floating objects of planetary mass outside star systems count as planets, except to exclude those in young star clusters.

Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic, which is roughly parallel to the Sun's equator. The planets lie very close to the ecliptic, while comets and kuiper belt objects often lie at significant angles to it. All of the planets, and most other objects, also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the smallest orbital circumference, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly.

A planet's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion, while its farthest point from the Sun is called its aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits, with large differences between perihelion and aphelion.

Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the average distance between the Earth and the Sun, or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun, while Jupiter lies at roughly 5.2 AU.

Informally, the Solar System is sometimes divided into separate "zones"; the first zone, known as the inner Solar System, comprises the inner planets and the main asteroid belt. The outer solar system is sometimes defined as everything beyond the asteroids; however, it is also the name often given to the region beyond Neptune, with the gas giants as a separate "middle zone."

One common misconception with regards to the Solar System is that the orbits of the major objects (planets, Pluto, and asteroids) are equidistant. Due to the vast distances involved, many representations of the Solar System tend to simplify these orbits, with equal spacing between each object. However, with certain exceptions, it can generally be stated that the farther a planet or belt is from the Sun, the greater the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, whereas Jupiter lies 1.9 AU from the farthest extent of the asteroid belt, and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the respective orbital distances.respective orbital distances.