ETYM Old Eng. astronomie, French astronomie, Latin astronomia, from Greek, astronomer; aster star + nomos law, regulation. Related to Star, and Nomad.
The branch of physics that studies celestial bodies and the universe as a whole; SYN. uranology.
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.
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 catalogs 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 center 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 about AD 180, and little progress was made for some centuries.
The Arabs revived the science, developing the astrolabe and producing good star catalogs. 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 center.
The dawn of a new era came 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 center 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, for 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 England’s Isaac Newton had established celestial mechanics.
Galileo and the telescope.
The 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.
Theoretical researches continued, and astronomy made rapid progress in many directions. Uranus was discovered in 1781 by German-born English astonomer 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 German astronomer 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 correct 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.
William Herschel, probably the greatest observer in the history of astronomy, 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 center of the system; in fact, it is well out toward 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 “starry 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 toward 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 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 1965 of microwave background radiation suggested that a residue survived the tremendous thermal power of the giant explosion, or Big Bang, that brought the universe into existence.
Although the practical limit in size and efficiency of optical telescopes has apparently been reached, the siting of these and other types of telescope 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. When the US launched the Hubble Space Telescope into permanent orbit 1990, it was the most powerful optical telescope yet constructed, with a 2.4 m/94.5 in mirror. It detects celestial phenomena seven times more distant (up to 14 billion light-years) than any land telescope.
See also black hole and infrared radiation.