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.
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 (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "rule") 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 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 astronomical calendar arrangements in alignment with the heliacal rising of Sirius and supported calibration the yearly calendar for the annual Nile flood.
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 (c. 150–80 BC) 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.
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 (highest point in the heavens) 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 (1292–1336) 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 (1320–1382) discussed evidence for the rotation of the Earth. Jean Buridan (1300–1361) developed the theory of impetus, describing motions including of the celestial bodies. For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), 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.
Galaxies
In the late 1700s William Herschel mapped the distribution of stars in different directions from Earth, concluding that the universe consisted of the Sun near the center of disk of stars, the Milky Way. After John Michell demonstrated that stars differ in intrinsic luminosity and after Herschel's own observations with more powerful telescopes that additional stars appeared in all directions, astronomers began to consider that some of the fuzzy spiral nebulae were distant island Universes.
The existence of galaxies, including the Earth's galaxy, the Milky Way, as a group of stars was only demonstrated in the 20th century. In 1912, Henrietta Leavitt discovered Cepheid variable stars with well-defined, periodic luminosity changes which can be used to fix the star's true luminosity which then becomes an accurate tool for distance estimates.
Using Cepheid variable stars, Harlow Shapley constructed the first accurate map of the Milky Way. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.
Cosmology
Albert Einstein's 1917 publication of general relativity began the modern era of theoretical models of the universe as a whole. In 1922, Alexander Friedman published simplified models for the universe showing static, expanding and contracting solutions. In 1929 Hubble published observations that the galaxies are all moving away from Earth with a velocity proportional to distance, a relation now known as Hubble's law.
This relation is expected if the universe is expanding. The consequence that the universe was once very dense and hot, a Big Bang concept expounded by Georges Lemaître in 1927, was discussed but no experimental evidence was available to support it. From the 1940s on, nuclear reaction rates under high density conditions were studied leading to the development of a successful model of big bang nucleosynthesis in the late 1940s and early 1950s.
Then in 1965 cosmic microwave background radiation was discovered, cementing the evidence for the Big Bang.
Astrophysics predicted the existence of objects such as black holes and neutron stars. These have been used to explain phenomena such as quasars and pulsars.
Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. The LIGO project detected evidence of gravitational waves in 2015.
Observational astronomy
Observational astronomy relies on many different wavelengths of electromagnetic radiation and the forms of astronomy are categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Specific information on these subfields is given below.
Radio
Radio astronomy uses radiation with long wavelengths, mainly between 1 millimeter and 15 meters (frequencies from 20 MHz to 300 GHz), far outside the visible range. Hydrogen, otherwise an invisible gas, produces a spectral line at 21 cm (1420 MHz) which is observable at radio wavelengths. Objects observable at radio wavelengths include interstellar gas, pulsars, fast radio bursts, supernovae, and active galactic nuclei.
Infrared
Infrared astronomy detects infrared radiation with wavelengths longer than red visible light, outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies.
Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galactic protostars and their host star clusters.
With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space.
The James Webb Space Telescope senses infrared radiation to detect very distant galaxies. Visible light from these galaxies was emitted billions of years ago and the expansion of the universe shifted the light in to the infrared range. By studying these distant galaxies astronomers hope to learn about the formation of the first galaxies.
Optical
Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy. Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium.
Although visible light itself extends from approximately 380 to 700 nm that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.
Ultraviolet
Ultraviolet astronomy employs ultraviolet wavelengths which are absorbed by the Earth's atmosphere, requiring observations from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue OB stars that are very bright at these wavelengths.
X-ray
X-ray astronomy uses X-radiation, produced by extremely hot and high-energy processes. Since X-rays are absorbed by the Earth's atmosphere, observations must be performed at high altitude, such as from balloons, rockets, or specialized satellites. X-ray sources include X-ray binaries, supernova remnants, clusters of galaxies, and active galactic nuclei.
Since the Sun's surface is relatively cool, X-ray images of the Sun and other stars give valuable information on the hot solar corona.
Gamma-ray
Gamma ray astronomy observes astronomical objects at the shortest wavelengths (highest energy) of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory, or by specialized telescopes called atmospheric Cherenkov telescopes. Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.
Gamma-ray astronomy provides information on the origin of cosmic rays, possible annihilation events for dark matter, relativistic particles outflows from active galactic nuclei (AGN), and, using AGN as distant sources, properties of intergalactic space. Gamma-ray bursts, which radiate transiently, are extremely energetic events, and are the brightest (most luminous) phenomena in the universe.
Non-electromagnetic observation
Some events originating from great distances may be observed from the Earth using systems that do not rely on electromagnetic radiation.
