Astronomy
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.
Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.
Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.
Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.
Etymology
Astronomy means study of celestial objects. Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. The two fields share a common origin but became distinct, astronomy being supported by physics while astrology is not.Use of terms "astronomy" and "astrophysics"
"Astronomy" and "astrophysics" are broadly synonymous in modern usage. In dictionary definitions, "astronomy" is "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties", while "astrophysics" is the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". Sometimes, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" means the qualitative study of the subject, whereas "astrophysics" is the physics-oriented version of the subject. Some fields, such as astrometry, are in this sense purely astronomy rather than also astrophysics. Research departments may use "astronomy" and "astrophysics" according to whether the department is historically affiliated with a physics department, and many professional astronomers have physics rather than astronomy degrees. Thus, in modern use, the two terms are often used interchangeably.History
Pre-historic
The initial development of astronomy was driven by practical needs like agricultural calendars. Before recorded history archeological sites such as Stonehenge provide evidence of ancient interest in astronomical observations.Evidence also comes from artefacts such as the Nebra sky disc which serves as an astronomical calendar, defining a year as twelve lunar months, 354 days, with intercalary months to make up the solar year. The disc is inlaid with symbols interpreted as a sun, moon, and stars including a cluster of seven stars. Megalithic structures located in Nabta Playa, Upper Egypt featured astronomy, calendar arrangements in alignment with the heliacal rising of Sirius and supported calibration the yearly calendar for the annual Nile flood.These practices have been linked with the emergence of cosmology in Old Kingdom Egypt.
Classical
Civilizations such as Egypt, Mesopotamia, Greece, India, China independently but with cross-cultural influences created astronomical observatories and developed ideas on the nature of the Universe, along with calendars and astronomical instruments. A key early development was the beginning of mathematical and scientific astronomy among the Babylonians, laying the foundations for astronomical traditions in other civilizations. The Babylonians discovered that lunar eclipses recurred in the saros cycle of 223 synodic months.Following the Babylonians, significant advances were made in ancient Greece and the Hellenistic world. Greek astronomy sought a rational, physical explanation for celestial phenomena. In the 4th century BC, Heracleides Ponticus was the first to proposed that the Earth rotates on its own axis.In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe. He also observed the small drift in the positions of the equinoxes and solstices with respect to the fixed stars that we now know is caused by precession. Hipparchus also created a catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy. The Antikythera mechanism was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.
Post-classical
After the classical Greek era, astronomy was dominated by the geocentric model of the Universe, or the Ptolemaic system, named after Claudius Ptolemy. His 13-volume astronomy work, named the Almagest in its Arabic translation, became the primary reference for over a thousand years. In this system, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. While the system would eventually be discredited, it gave the most accurate predictions for the positions of astronomical bodies available at that time.With the arrival of Hellenistic astronomy in India through trade and cultural contacts, Indian astronomy entered a new phase during the early centuries CE. Earlier indigenous traditions, such as those recorded in the Vedāṅga Jyotiṣa, provided calendrical foundations, while Greek astronomical models were later integrated by scholars including Āryabhaṭa, Varāhamihira, and Brahmagupta. Āryabhaṭa notably improved methods for calculating planetary motions and eclipses. In the later medieval period, the Kerala school contributed to astronomy through refined observational practices and more accurate planetary and eclipse calculations.
File:Al- Fargānī, Aḥmad ibn Muḥammad – Compilatio astronomica, 1493 – BEIC 13262685.jpg |thumb |upright |Portrait of Alfraganus in the Compilatio astronomica, 1493. Islamic astronomers collected and translated Indian, Persian and Greek texts, adding their own work.
Astronomy flourished in the medieval Islamic world. Astronomical observatories were established there by the early 9th century. In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars. The SN 1006 supernova, the brightest apparent magnitude stellar event in the last 1000 years, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Iranian scholar Al-Biruni observed that, contrary to Ptolemy, the Sun's apogee was mobile, not fixed. Arabic astronomers introduced many Arabic names now used for individual stars.
The ruins at Great Zimbabwe and Timbuktu may have housed astronomical observatories. In Post-classical West Africa, astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens and diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in 1583.
In medieval Europe, Richard of Wallingford invented the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes. Nicole Oresme discussed evidence for the rotation of the Earth. Jean Buridan developed the theory of impetus, describing motions including of the celestial bodies.
For over six centuries, the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.
Copernicus
During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. While his model maintained circular orbits, it was sufficient to calculate the size of planetary orbits and their period. The appealing simplicity of Copernican astronomy led to its adoption among astronomers even before it was confirmed by Galileo's telescopic observations in the 1600s.Early telescopic
Sometime around 1608 the telescope was invented and by 1610, Galileo Galilei observed phases on the planet Venus similar to those of the Moon, supporting the heliocentric model. Around the same time the heliocentric model was organized quantitatively by Johannes Kepler. Analyzing two decades of careful observations by Tycho Brahe, Kepler devised a system that described the details of the motion of the planets around the Sun. While Kepler discarded the uniform circular motion of Copernicus in favor of elliptical motion, he did not succeed in formulating a theory behind the laws he wrote down. It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope.Newton, in collaboration with Richard Bentley proposed that stars are like the Sun only much further away.
The new telescopes also altered ideas about stars. By 1610 Galileo discovered that the band of light crossing the sky at night that we call the Milky Way was composed of numerous stars. In 1668 James Gregory compared the luminosity of Jupiter to Sirius to estimate its distance at over 83,000 AU. The English astronomer John Flamsteed, Britain's first Astronomer Royal, catalogued over 3000 stars but the data were published against his wishes in 1712. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. Friedrich Bessel developed the technique of stellar parallax in 1838 but it was so difficult to apply that only about 100 stars were measured by 1900.
During the 18–19th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon 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 astrophotography. In 1814–15, Joseph von Fraunhofer discovered some 574 dark lines in the spectrum of the sun and of other stars. In 1859, Gustav Kirchhoff ascribed these lines to the presence of different elements.