astronomy

Radio telescopes, like this one (the world's largest) at Arecibo, Puerto Rico, allow astronomers to analyse a broad range of low-frequency electromagnetic waves – visible light is only a small part of the electromagnetic spectrum. Pulsars and quasars were first discovered by radio telescopes.

The Hertzsprung-Russell diagram relates the brightness (or luminosity) of a star to its temperature. Most stars fall within a narrow diagonal band called the main sequence. A star moves off the main sequence when it grows old.

Believing that the theories of Ptolemy regarding the Earth as the centre of the universe were too complicated, Copernicus turned to earlier Greek astronomers such as Aristarchus and Hipparchus. His deduction that the Earth is a moving planet was developed by later astronomers such as Kepler and Galileo.

A NASA image of a spiral galaxy, one of the main classes of galaxy, of which Andromeda and our own Milky Way are examples. There may be billions of stars in such a system, held together by gravity. With the advanced technology now available to astronomers, especially the Hubble Space Telescope, the mysteries of galactic structure are being uncovered.

Illuminated treatise on astronomy, showing the cardinal points, from the 15th century. The reform of European astronomy began in the 15th century by astronomers and mathematicians seeking to clarify and correct astronomical texts (especially Ptolemy's); however it was not until the following century, with the discoveries of Copernicus, that the Sun was proved to be the centre of the planetary system.

Science of the celestial bodies: the Sun, the Moon, and the planets; the stars and galaxies; and all other objects in the universe. It is concerned with their positions, motions, distances, and physical conditions and with their origins and evolution. Astronomy thus divides into fields such as astrophysics, celestial mechanics, and cosmology. See also gamma-ray astronomy, infrared astronomy, radio astronomy, ultraviolet astronomy, and X-ray astronomy.

Greek astronomers

Astronomy is perhaps the oldest recorded science; there are observational records from ancient Babylonia, China, Egypt, and Mexico. The first true astronomers, however, were the Greeks, who deduced the Earth to be a sphere and attempted to measure its size. Ancient Greek astronomers included Thales and Pythagoras. Eratosthenes of Cyrene measured the size of the Earth with considerable accuracy. Star catalogues were drawn up, the most celebrated being that of Hipparchus. The Almagest, by Ptolemy of Alexandria, summarized Greek astronomy and survived in its Arabic translation. The Greeks still regarded the Earth as the centre of the universe, although this was doubted by some philosophers, notably Aristarchus of Samos, who maintained that the Earth moves around the Sun.

Ptolemy, the last famous astronomer of the Greek school, died in about AD 180, and little progress was made for some centuries.

Arab revival

The Arabs revived the science, developing the astrolabe and producing good star catalogues. Unfortunately, a general belief in the pseudoscience of astrology continued until the end of the Middle Ages (and has been revived from time to time).

The Sun at the centre

The dawn of a new era came in 1543, when a Polish canon, Copernicus, published a work entitled De revolutionibus orbium coelestium/On the Revolutions of the Heavenly Spheres, in which he demonstrated that the Sun, not the Earth, is the centre of our planetary system. (Copernicus was wrong in many respects – for instance, he still believed that all celestial orbits must be perfectly circular.) Tycho Brahe, a Dane, increased the accuracy of observations by means of improved instruments allied to his own personal skill, and his observations were used by German mathematician Johannes Kepler to prove the validity of the Copernican system. Considerable opposition existed, however, to removing the Earth from its central position in the universe; the Catholic Church was openly hostile to the idea, and, ironically, Brahe never accepted the idea that the Earth could move around the Sun. Yet before the end of the 17th century, the theoretical work of Isaac Newton had established celestial mechanics.

Galileo and the telescope

The first practical refracting telescope was invented about 1608, by Hans Lippershey in Holland, and was first applied to astronomy by Italian scientist Galileo in the winter of 1609–10. Immediately, Galileo made a series of spectacular discoveries. He found the four largest satellites of Jupiter, which gave strong support to the Copernican theory; he saw the craters of the Moon, the phases of Venus, and the myriad faint stars of our Galaxy, the Milky Way.

Galileo's most powerful telescope magnified only 30 times, but it was not long before larger telescopes were built and official observatories were established.

Galileo's telescope was a refractor; that is to say, it collected its light by means of a glass lens or object glass. Difficulties with his design led Newton, in 1671, to construct a reflector, in which the light is collected by means of a curved mirror.

Further discoveries

In the 17th and 18th centuries astronomers were mostly concerned with positional measurements. Uranus was discovered in 1781 by William Herschel, and this was soon followed by the discovery of the first four asteroids, Ceres in 1801, Pallas in 1802, Juno in 1804, and Vesta in 1807. In 1846 Neptune was located by Johann Galle, following calculations by British astronomer John Couch Adams and French astronomer Urbain Jean Joseph Leverrier. Also significant was the first measurement of the distance of a star, when in 1838 the German astronomer Friedrich Bessel measured the parallax of the star 61 Cygni, and calculated that it lies at a distance of about 6 light years (about half the modern value).

Astronomical spectroscopy was developed, first by Fraunhofer in Germany and then by people such as Pietro Angelo Secchi and William Huggins, while Gustav Kirchhoff successfully interpreted the spectra of the Sun and stars. By the 1860s good photographs of the Moon had been obtained, and by the end of the century photographic methods had started to play a leading role in research.

Galaxies

William Herschel investigated the shape of our Galaxy during the latter part of the 18th century and concluded that its stars are arranged roughly in the form of a double-convex lens. Basically Herschel was correct, although he placed our Sun near the centre of the system; in fact, it is well out towards the edge, and lies 25,000 light years from the galactic nucleus. Herschel also studied the luminous ‘clouds’ or nebulae, and made the tentative suggestion that those nebulae capable of resolution into stars might be separate galaxies, far outside our own Galaxy.

It was not until 1923 that US astronomer Edwin Hubble, using the 2.5 m/100 in reflector at the Mount Wilson Observatory, was able to verify this suggestion. It is now known that the ‘spiral nebulae’ are galaxies in their own right, and that they lie at immense distances. The most distant galaxy visible to the naked eye, the Great Spiral in Andromeda, is 2.2 million light years away; the most remote galaxy so far measured lies over 10 billion light years away. It was also found that galaxies tended to form groups, and that the groups were apparently receding from each other at speeds proportional to their distances.

A growing universe

This concept of an expanding and evolving universe at first rested largely on Hubble's law, relating the distance of objects to the amount their spectra shift towards red – the red shift. Subsequent evidence derived from objects studied in other parts of the electromagnetic spectrum, at radio and X-ray wavelengths, has provided confirmation. Radio astronomy established its place in probing the structure of the universe by demonstrating in 1954 that an optically visible distant galaxy was identical with a powerful radio source known as Cygnus A. Later analysis of the comparative number, strength, and distance of radio sources suggested that in the distant past these, including the quasars discovered in 1963, had been much more powerful and numerous than today. This fact suggested that the universe has been evolving from an origin, and is not of infinite age as expected under a steady-state theory.

The discovery in 1965 of microwave background radiation was evidence for the enormous temperature of the giant explosion, or Big Bang, that brought the universe into existence.

Further exploration

The siting of telescopes at new observatories in the previously neglected southern hemisphere has opened fresh areas of the sky to search. Australia has been in the forefront of these developments. The most remarkable recent extension of the powers of astronomy to explore the universe is in the use of rockets, satellites, space stations, and space probes. Even the range and accuracy of the conventional telescope may be greatly improved free from the Earth's atmosphere. The USA launched the Hubble Space Telescope, with a 2.4 m/94.5 in mirror, into permanent orbit in 1990. It detects celestial phenomena more distant (up to 14 billion light years) than any Earth-based telescope.

See also black hole and infrared radiation.

astronomy - events

c. 15 billion BC	World	The approximate date of the origin of the universe.
c. 13 billion BC	World	The hydrogen and helium atoms in the young universe begin to form into areas of greater and lesser density. The dense patches form into clusters of galaxies under the force of gravity.
c. 10 billion BC	World	Our Galaxy, the Milky Way, is probably formed at around this time.
c. 4600 million BC	World	Our Sun and our Solar System are formed.
c. 3500 BC	Europe	The Grand Menhir of Locmariaquer is erected in Brittany, France. Set among other standing stones, it is the largest in the world, at 20 m/66 ft high and 380 tonnes in weight.
c. 2800 BC	Europe	The Neolithic monument Stonehenge is built in England near Salisbury, Wiltshire, comprising a circular earthwork 97.5 m/320 ft in diameter with 56 small pits around the circumference (later known as the Aubrey holes). The position of the ‘heel stone’ outside the circle suggests a connection with Sun worship and observation. It is probably an astronomical observatory with religious functions; the motions of the Sun and Moon are followed with the aid of carefully aligned rocks.
c. 2700 BC	Mesopotamia	A lunar calendar is developed in Mesopotamia in which new months begin at each new moon. A year is 354 days long and the calendar is used primarily for administrative purposes.
1500 BC	China	Chinese astronomers make the earliest record of an appearance of a comet.
1361 BC	China	Chinese astronomers make the first recording of an eclipse of the Moon.
c. 1302 BC	China	The first recording of an eclipse of the Sun is made by Chinese astronomers.
1300 BC	China	The Shang dynasty in China establishes the solar year at 365 ¼ days. The calendar consists of 12 months of 30 days each, with intercalary months added to adjust the lunar year to the solar.
15 June 763 BC	Neo-Assyrian Empire	Assyrian archivists record an eclipse of the Sun. This is probably the event described in the Bible, in Amos 8:9.
28 May 585 BC	Greece	The first accurate prediction of an eclipse of the Sun is made in 585 BC, by the Greek philosopher Thales of Miletus.
c. 366 BC	Greece	Greek mathematician and astronomer Eudoxus of Cnidus builds an observatory and constructs a model of 27 nested spheres to give the first systematic explanation of the motion of the Sun, Moon, and planets around the Earth.
352 BC	China	Chinese astronomers make the earliest known record of a ‘visitor star’, probably a supernova.
c. 350 BC	Greece	Aristotle defends the doctrine that the Earth is a sphere, in De caelo/Concerning the Heavens, and estimates its circumference to be about 400,000 stadia (one stadium varied from 154 m/505 ft to 215 m/705 ft). It is the first scientific attempt to estimate the circumference of the Earth.
240 BC	China	Chinese astronomers make the first recorded observation that can definitely be associated with Halley's Comet.
c. 200 BC	Greece	The Greeks invent the astrolabe. It is used for observing the positions and altitudes of stars.
c. 165 BC	China, Former Han Empire	Chinese astronomers first observe and record sunspots. Continuous records of sunspots are kept by Imperial astronomers from 28 BC to AD 1638.
150	Greece, Egypt	The Egyptian astronomer Claudius Ptolemy publishes the work he calls ‘The Mathematical Collection’ but later known as the Almagest or ‘The Greatest’, an astronomical encyclopedia in 13 volumes that was to be highly influential for over a millennium.
516	India	The Indian astronomer and mathematician Aryabhata I produces his Aryabhatiya, a treatise on quadratic equations, the value of p, and other scientific problems, in which he adds tilted epicycles to the orbits of the planets to explain their movement.
772	Arabia	Muslim astronomer Al-Fazari translates the Indian astronomical compendium Mahasiddhanta/Treatise on Astronomy.
800	South America	An astronomical altar known as ‘the hitching post of the Sun’ is built in the Peruvian city of Machu Picchu, and is used to measure solar and lunar movements with great accuracy.
987	Central America	Toltec conquerors of the Central American Mayan city of Chichén Itzá construct monuments with ritual astronomical alignments to the rising and setting of the Sun and the sacred planet Venus.
1050	Europe	The astrolabe, a new device for making astronomical measurements and calculations, arrives in Europe from the East, where Muslim scientists developed it two centuries ago.
4 July 1054	world	A bright new star, visible in daylight, appears in the constellation Taurus. The supernova is observed in China and Korea, and is recorded in rock paintings in southwestern America.
1066	Normandy, France, England	The comet later known as Halley's Comet appears in the sky, and is taken as an omen by both the Norman and English sides before the Battle of Hastings. The victorious Normans record its appearance in the Bayeux Tapestry.
1091	Italy, England	The French-born Prior Walcher of Malvern Abbey, England, records his observations in Italy of an eclipse of the Moon. This is one of the earliest accurate western European observations of the phenomenon.
1519	Spain, Central America	The Spanish conquistador Hernán Cortés sends the Mayan ‘Dresden Codex’ to Charles V of Spain – it demonstrates the elaborate Mayan calendar based on the movements of the planet Venus.
1543	Poland	The Polish astronomer and priest Nicholas Copernicus has finally worked out to his satisfaction the details of the heliocentric theory, and they are published in his most important work De revolutionibus orbium coelestium/On the Revolutions of the Celestial Sphere as he lies dying of a cerebral haemorrhage.
11 November 1572	Denmark	The Danish astronomer Tycho Brahe observes a bright new star – a supernova – in the constellation Cassiopeia. It becomes known as ‘Tycho's Nova’.
1588	Europe	Giovanni Paolo Gallucci's Theatrum mundi/Theatre of the World features the first star chart marked with a celestial coordinate system.
1602	Denmark	The Danish astronomer Tycho Brahe's Astronomia instauratae progymnasmata/Introducing Exercises toward a Restored Astronomy is published posthumously, giving accurate positions for 777 fixed stars and a description of the 1572 supernova in Cassiopeia.
1603	Germany	The German astronomer Johann Bayer publishes his Uranometria star atlas, the most detailed yet, including the 12 new southern constellations, and introducing the practice of giving the stars Greek identifiers.
1609	Italy	Italian astronomer Galileo Galilei, having obtained a Dutch telescope, makes his own instruments, including one that magnifies objects 32 times. They are the first telescopes that can be used for astronomical observation.
1609	Germany	The German astronomer Johannes Kepler publishes his Astronomia nova/New Astronomy, which describes the orbit of Mars accurately and includes his first two laws of planetary motion which state that all planets move in elliptical orbits around the Sun, and that they sweep out equal areas in equal times.
1610	Italy	The Italian astronomer Galileo Galilei publishes Sidereus nuncius/The Starry Messenger, revealing his telescopic discoveries, including the moons of Jupiter, the phases of Venus, sunspots, and the curious shape of Saturn.
1611	Germany	The German astronomer Johannes Kepler publishes his Dioptrice/On Refraction, describing his investigations of light and optics.
1611	Germany	The German astronomer Simon Marius is the first to observe the Andromeda Nebula. He also discovers the four moons of Jupiter independently of Galileo, and names them Io, Europa, Ganymede, and Callisto.
1651	England	The English scientist William Gilbert's book A New Philosophy of Our Sublunar World is published posthumously, proposing theories that the fixed stars are not all at the same distance from Earth.
1755	Germany	In his Allgemeine Naturgeschichte und Theorie des Himmels/Universal Natural History and Theory of the Heavens, the German philosopher Immanuel Kant proposes a theory for the formation of the Solar System from a primordial nebula, predicts the existence of Uranus, and proposes that our Galaxy is just one of many in the universe.
1772	Germany	German astronomer Johann Elert Bode publicizes the Titius–Bode law, first proposed in 1766 by Johann Titius, which states that the distances to the planets are proportional to the terms of the series 0, 3, 6, 12, 24, …
13 March 1781	England	German-born English astronomer William Herschel discovers the planet Uranus.
1785	England	German-born English astronomer William Herschel argues in his work On the Construction of the Heavens that the Milky Way galaxy is composed of individual stars and is not some luminous fluid.
1786	England	German-born English astronomer William Herschel's Catalogue of Nebulae is published. It is a catalogue of nearly 2,500 nebulae.
1796	France	French mathematician and physicist Pierre-Simon Laplace publishes Exposition du système du monde/Account of the System of the World, in which he enunciates the ‘nebular hypothesis’ which forms the basis of modern theory, proposing that the Solar System formed from a cloud of gas.
1799	France	French mathematician and physicist Pierre-Simon Laplace discovers the invariability of planetary mean motions, and proves that the eccentricities and inclinations of planetary orbits to each other always remain small, constant, and self-correcting.
1800	England	German-born English astronomer William Herschel discovers the existence of infrared solar rays.
1802	England	English astronomer William Herschel discovers that some stars revolve around others, forming binary pairs. He catalogues 848 of them.
11 October 1838	Germany	Using the method of parallax, German astronomer Friedrich Bessel calculates the star 61 Cygni to be 10.3 light years away from Earth. It is the first determination of the distance of a star other than the Sun.
1842	Germany	The German astronomer Friedrich Bessel suggests that perturbations to the motion of Sirius are due to the existence of a companion star.
1842–1845	UK	Irish astronomer William Parsons (later Lord Rosse) builds the 180 cm/72 in reflecting telescope ‘Leviathan’.
23 September 1846	Germany	German astronomer Johann Gottfried Galle discovers the planet Neptune on the basis of French astronomer Urbain Le Verrier's calculations of its position.
1862	USA	The US astronomer Alvan Clark observes the companion star of Sirius – the first white dwarf to be discovered.
1864	England	By examining their spectra, English astronomer William Huggins demonstrates that the Orion Nebula (and hence all nebulae) consists of gases, while the Andromeda Nebula is composed of stars and is therefore a galaxy.
1872	USA	The US astronomer Henry Draper develops astronomical spectral photography and takes the first photograph of the spectrum of a star – that of Vega.
1881	USA	German-born US physicist Albert Michelson develops an interferometer to measure distances between stars.
1885–1890	South Africa	British astronomer David Gill photographs over 450,000 stars of 11th magnitude or brighter in the southern hemisphere, in South Africa.
1890	USA	The first version of the Henry Draper Star Catalogue is published. Produced by astronomers at Harvard College Observatory, it lists the position, magnitude, and type of over 10,000 stars, and begins the alphabetical system of naming stars according to temperature. Subsequent editions increase the listing to 400,000 stars.
1891	World	The ‘blink’ comparator is invented. It permits the discovery of objects in the Solar System by comparison of two photographs, taken a few hours apart, of the same region of the sky. Stars remain fixed, while planets and asteroids move or ‘blink’.
1905–1907		Danish astronomer Ejnar Hertzsprung discovers that there is a relationship between the colour and absolute brightness of stars and classifies them according to this relationship. It is used to determine the distances of stars and forms the basis of theories of stellar evolution.
19 May 1910	USA	Halley's Comet – which comes near the Earth roughly every 75 years – returns, with the Earth passing through the comet's tail. In the USA, it is regarded by some as announcing the end of the world.
1920	USA	US physicist Albert Michelson, using a stellar interferometer, measures the diameter of the star Betelgeuse to be 386,160,000 km/241,350,000 mi, which is about 300 times the diameter of the Sun. It is the first time an accurate measurement of the size of a star other than the Sun has been made.
12 May 1922	USA	A 20.3 tonne/20 ton meteorite lands in a field near Blackstone, Virginia, leaving a 46 sq m/500 sq ft hole in the ground.
1924		US astronomer Edwin Hubble demonstrates that certain Cepheid variable stars are several hundred thousand light years away and thus outside the Milky Way galaxy.
1926	United Kingdom	English astrophysicist Arthur Eddington publishes Internal Constitution of the Stars, in which he shows that the luminosity of a star is a function of its mass.
1927	Belgium	Belgian astronomer Georges Lemaître proposes that the universe was created by an explosion of energy and matter from a ‘primaeval atom’ – the beginning of the Big Bang theory.
1929		English philosopher Alfred North Whitehead publishes Process and Reality: An Essay in Cosmology.
18 February 1930		US astronomer Clyde Tombaugh, at the Lowell Observatory, Arizona, discovers the ninth planet, Pluto.
1932	USA	US engineer Karl Jansky discovers that the interference in telephone communications is caused by radio emissions from the Milky Way. He thus begins the development of radio astronomy.
1932		US scientist Carl David Anderson, while analysing cosmic rays, discovers positive electrons (‘positrons’), the first form of antimatter to be discovered.
1933		British physicist Arthur Eddington publishes The Expanding Universe, in which he lays out his theory that the universe is constantly increasing in size.
1946	England	Cygnus A, the first radio galaxy (a galaxy that is a strong source of electromagnetic waves of radio wavelength), and the most powerful cosmic source of radio waves, is discovered by the English physicist James Hey.
1963	USA	An international team of astronomers discovers the first quasar (3C 273), an extraordinarily distant object brighter than the largest known galaxy yet with a star-like image.
7 July 1967	USA	British astronomers Jocelyn Bell and Anthony Hewish, at the Mullard Radio Astronomy Observatory, Cambridge, England, discover the first pulsar (announced in 1968).
1974	England	English physicist Stephen Hawking suggests that ‘black holes aren't black’ – they ‘evaporate’ by emitting subatomic particles.
1981	USA	The Very Large Array (VLA) radio telescope at Socorro, New Mexico, enters service. Its 27 25-m/82-ft diameter dishes can be steered and moved on railway tracks, and are equivalent to one dish 27 km/17 mi in diameter; together they provide high-resolution radio images.
1987	world	Objects the size of planets are found orbiting the stars Gamma Cephei and Epsilon Eridani.
23 February 1987	world	Astronomers around the world observe a spectacular supernova in the Large Magellanic Cloud, the galaxy closest to ours, when a star (SN1987A) suddenly becomes a thousand times brighter than our own Sun. It is the first supernova visible to the naked eye since 1604.
1993	USA	US astronomers identify part of the dark matter in the universe as stray planets and brown dwarfs. Known as MACHOs (massive astrophysical compact halo objects), they may constitute approximately half of the dark matter in the Milky Way's halo.
23 July 1995	USA	US astronomers Alan Hale and Thomas Bopp discover the Hale-Bopp comet. The brightest periodic comet, its icy core is estimated to be 40 km/25 mi wide.
24 March 1996	World	The comet Hyakutake makes its closest approach, passing within 15.4 million km/9.5 million mi of Earth. It is the brightest comet for decades, with a tail extending over 12 degrees of the sky.
23 March 1997	world, USA	The comet Hale-Bopp comes to within 190 million km/120 million mi of Earth, the closest since 2000 BC. NASA launches rockets to study the comet. Its icy nucleus is estimated to be 40 km/25 mi wide, making it at least ten times larger than that of the comet Hyakutake and twice the size of Halley's Comet.
27 January 1998	USA	Al Schultz of the Space Science Institute in Baltimore, Maryland, using the Hubble Space Telescope, announces the discovery of a giant planet, larger than the Sun, orbiting Proxima Centauri, the closest star to Earth. It is the first planet outside the Solar System to be directly observed.
4 July 1998	USA	Astronomers from the University of Hawaii discover the first asteroid entirely within the Earth's orbit; it is 40 m/130 ft in diameter.
9 January 1999	USA	Astronomers from San Francisco State University announce the discovery of three more planets orbiting around neighbouring stars, bringing the total number of known planets outside our Solar System to 17.
23 January 1999	USA	NASA scientists photograph light emitted by a gamma-ray burst for the first time.
11 August 1999	UK	A total solar eclipse occurs in England. The path of the eclipse passes over Cornwall, England – the first total solar eclipse visible there since 1927. The eclipse is marred by poor visibility due to clouds and rain.
November 2005		Astronomers detect a ring around a moon for the first time.

astronomy (redirected from Asstronomee)