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Astronomy Dictionary

Click on an underlined word for more information on that subject.


An astronomer is someone who studies astronomy. The following night-owls are important astronomers, astrophysicists, mathematicians, and other scientists who have contributed greatly to our knowledge of the universe.

John Couch Adams (1819-1892) was an English astronomer and mathematician who, at 24 years old, predicted the existence of the planet Neptune (Le Verrier also predicted its existence, independently).


Sir George Bidell Airy (1801-1892) was the director of Greenwich Observatory/Astronomer Royal of England from 1835 to 1881. Airy installed a transit (a precise surveying device) at Greenwich, England, which was used to define the zero degree meridian of the Earth (zero-degrees longitude). A crater on Mars about 5 degrees south of the equator and on what is defined as Mars' prime meridian (zero-degrees longitude) is call Airy. A small crater within this crater (which is called Airy-0) is where the meridian line (zero-degrees longitude) crosses. A crater on the moon is also named for him (latitude 18.1 degrees, longitude 354.3 degrees, diameter 36 km). Airy is supposed to have stated incorrectly that Charles Babbage's new "analytical engine" (the predecessor of the computer) was "worthless," effectively ending Babbage's government funding.
Aristotle (384-322 BC) was a Greek philosopher who theorized about astronomy. Using only philosophical speculation (he did no scientific observations), Aristotle believed that the universe is spherical, finite, and centered around the Earth. Aristotle, like many others of his time, believed that the circle was the "perfect" shape, so the universe must be spherical, and all the orbits in it must also be circular. He also believed that celestial bodies were composed of ether (in addition to the four other basic elements believed to exist at that time, earth, air, fire, water). Aristotle's ideas were adopted by the Church and were not tested for over a thousand years, until Galileo, who was tried for heresy when his experimentation showed Aristotle to be wrong.
Heinrich Louis d'Arrest (1822-1875) was a Danish astronomer and the co-discoverer of Neptune (in 1846), with Galle.
Edward Emerson Barnard (1857-1923) was an American astronomer who discovered Barnard's star (the star system second-closest to us) in 1916, 16 comets, and Amalthea, a moon of Jupiter, in 1892.
Johannes Bayer (1564-1617) was a Bavarian (German) astronomer who first named stars by assigning them to constellations and giving them Greek letters (alpha, beta, gamma, delta, etc.), in magnitude classes (by decreasing brightness). Bayer published Uranometria (a detailed star chart/catalog) in 1603.
Ulugh Beg (1359-1449) was a Persian astronomer who cataloged 1012 stars and made detailed observations of the moon and planets. He also determined the inclination of the plane of the ecliptic.
Susan Jocelyn Bell Burnell (1943- ) is an astronomer who discovered the existence of pulsars in 1967, while she was a graduate student at Cambridge University. A pulsar is a rapidly spinning neutron star that emits energy in pulses. Bell's graduate advisor (Anthony Hewish) was given a share of the 1974 Nobel Prize, but Bell was ignored. No one had any idea what these unusual objects were at the time, so the name little green men (LGM) was used. Soon, Thomas Gold suggested that pulsars were rapidly-spinning neutron stars, the remnants of a supernova.
Friedrich Wilhelm Bessel (July 22, 1784-March 17, 1846) was a German astronomer and mathematician who cataloged about 50,000 stars, mathematically predicted the existence of a planet beyond Uranus (1840), was the first person to see the "motion" of a star due to parallax (observing 61 Cygni), was the first person to calculate the distance to a star (observing 61 Cygni - 10.3 light-years from Earth), realized that there were dark stars, devised the famous Bessel function (a mathematical function), and made many other contributions to science.
William Cranch Bond (1789-1859) was an American astronomer who, with William Lassell, discovered Saturn's moon Hyperion in 1848. He was the first director of the Harvard College Observatory.
Tycho (Tyge) Brahe (1546-1601) was a Danish astronomer who made extensive and seminal calculations of the orbits of the planets. His work (done without a telescope) was the basis upon which Kepler made his revolutionary orbital formulas. He worked with Kepler for a few years before his death, and Kepler edited his principal work, Astronomiae Instauratae Progymnasmata, ("Exercises Toward a Restored Astronomy"). He also observed the "new star" (really a nova) in Cassiopeia in 1572. He observed a comet in 1577 and was realized that it was not in the atmosphere, but was in space. He corrected most astronomical quantities. Although he incorrectly believed that the Earth was at the center of the universe and that the sun and the stars revolved around the Earth, he did accept part of the Copernican theory, that the other planets orbit the sun. He had a nose made of gold; he lost most of his real nose in a duel about mathematics!
Giordano (Filippo) Bruno (1548-1600) was an Italian philosopher, poet, and priest who spread the ideas of Copernicus as well as his own ideas that there were an infinity of worlds in the universe and that the stars were other suns. He was burned at the stake for heresy.
Callipus of Cyzicus (370 - 300 BC) was an ancient Greek who accurately measured the length of the seasons. Callipus improved the Greek calendar, reconciling the lunar month with the solar year, by introducing a unit of time called the Callippic cycle (it was an improvement on the Metonic cycle of 6939.6 days or 19 solar years or 235 lunar months; the Callippic cycle was 4 Metonic cycles). He also added to the (incorrect) theory of the motions of the plants, as spheres within spheres, adding 7 more spheres to Eudoxus' system.
Annie Jump Cannon (1863-1941) was an American astronomer who cataloged 225,300 stars in the HD (Henry Draper) catalog; every star is classified by its stellar spectrum. Cannon and Edward C. Pickering (director of the Harvard Observatory) published the original HD catalog (9 volumes) from 1918 to 1924. The catalog was later expanded by Cannon and Margaret W. Mayall in 1949.
Giovanni Domenico Cassini (1625-1712) was an astronomer born in Italy who later became a naturalized French citizen. He discovered four of Saturn's moons (Tethys, Dione, Rhea, and Iapetus) and a dark division in Saturn's rings (called the Cassini Division).


Henry Cavendish (1731-1810) was an English chemist and physicist. Cavendish discovered that hydrogen gas was a substance different from ordinary air (whose components he analyzed), he described the composition of water (hydrogen and oxygen) and made other important discoveries. Cavendish was the first person to determine Isaac Newton's gravitational constant and accurately measured the Earth's mass and density.
Anders Celsius (1701-1744) was a Swedish professor of astronomy who devised the Celsius thermometer. He also ventured to the far north of Sweden with an expedition in order to measure the length of a degree along a meridian, close to the pole, later comparing it with similar measurements made in the Southern Hemisphere. This confirmed that that the shape of the earth is an ellipsoid which is flattened at the poles. He also cataloged 300 stars. With his assistant Olof Hiorter, Celsius discovered the magnetic basis for auroras.
Thomas Chrowder Chamberlain (Sept. 25, 1843 - Nov. 15, 1928) was an American geologist and teacher who proposed the planetesimal hypothesis of the formation of the Solar System. In this theory, a star is supposed to have passed near the Sun, pulling matter away from the Sun. Later, this matter is to have condensed into larger masses, forming the planets.
Subrahmanyan Chandrasekhar (Born Lahore, India on Oct. 19, 1910 -Died Chicago, USA in 1995) was an Indian-American astrophysicist who studied stellar physics, evolution, and black holes. He realized that the fate of dying stars depended upon their mass, and above a certain point (1.4 times the mass of the Sun, now known as the "Chandrasekhar limit"), a star will undergo extreme collapse and not simply becomes a white dwarf. He won the Nobel prize in physics in 1983. The orbiting X-ray Observatory Chandra was named to honor S. Chandrasekhar.
Carl Vilhelm Ludvig Charlier (1862-1932) was a Swedish astronomer who studied celestial mechanics, the calibration of photographic photometry, and the theory of lenses. Charlier also worked in statistics, including the theory of errors; he studied the distribution and motions of stars. Charlier showed that hotter stars and galactic clusters formed flattened systems. In Charlie's hierarchical model of the Universe, he argued that the Universe has infinite mass, and that the density of matter approaches zero as one goes farther into space (resolving Olber's paradox). A 100 km wide crater on Mars (at 68.6 south, 168.4 west, shown above) was named for Charlier in 1973.
Nicolaus Copernicus (1473-1543) was an amateur Polish astronomer who developed the revolutionary Copernican system, a model of the solar system in which all the planets orbit the sun. His ideas overturned the old Ptolemaic System. His seminal work was De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orb"), published in 1543.


Frank Drake (1930-) is an American radio astronomer who, in 1961, formulated an equation for very-rough estimates of the possible number of intelligent civilizations in our galaxy (the Drake equation).
Draper, Henry (1837-1882) was an American astronomer. He was also an early astronomical photographer. In September, 1880, Draper took the first photograph of a distant astronomical object (the Orion Nebula). He also studied stellar spectra and was the first person to photograph stellar spectral lines. The Henry Draper system of star identification (HD) system of stellar classification was named for Draper. His father, John William Draper (1811-1882), who was a chemist, took the first photographs (using a five-inch reflector telescope) of the moon in 1839-1840.
Arthur Eddington (1882-1945) was an English astronomer who first described the internal structure of a star.
Albert Einstein (1879-1955) was a German/American physicist. He revolutionized our conception of the universe with his Theories of Special and General Relativity.

Special relativity supplanted Newtonian mechanics, yielding different results for very fast-moving objects. The Theory of Special Relativity is based on the idea that speed has an upper bound; nothing can pass the speed of light. The theory also states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called space-time. Energy and momentum are similarly linked. As a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc2.

General Relativity expands the theory to include acceleration and gravity, both of which are explained via the curvature of space-time.

Einstein won the Nobel Prize in 1921 for explaining the photoelectric effect. His theories explained the perturbations in the orbits of Mercury.
Johann Encke discovered the Encke Division in 1837. This division splits the A Ring, the outermost of the major rings of Saturn. This gap is 200 miles (325 km) wide and is 83,000 miles (133,570 km) from the center of Saturn.
Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away ). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stasia is is unknown, so his accuracy is uncertain. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon, and devised a method for determining the prime numbers up to a given number (the Sieve of Eratosthenes). Eratosthenes made numerous contributions to the sciences and arts in many fields, including geography, mathematics, astronomy, chronography (calendars), music, and literature. Eratosthenes was a brilliant all-around scholar; although not the top expert on any topic, he was well-versed in all subjects, and therefore nicknamed "Beta" (which is the second letter of the Greek alphabet).
Eudoxus of Cnidus (408-355 B.C.) was a Greek scholar (perhaps a student of Plato) who theorized that the Earth was at the center of the universe and that the other celestial objects (stars and planets) were set into geometric spheres around the Earth. His major contribution was inventing the modern notion of real numbers.
Armand Hippolyte Louis Fizeau Fizeau (1819-1896) was a French physicist who was the first person to measure the speed of light on the Earth's surface. He measured the speed of light in 1849 using a device that consisted of a light, a toothed wheel and a distant mirror. He calculated light's speed by adjusting the speed of the wheel (the distance between wheel and mirror was 5 mi/8 km) so that the time it took the wheel to move the width of one tooth was equal to the time it took the light to travel from the wheel to the mirror and back again. He also measured the speed of light in other media, and found that light travels faster in air than in water. Fizeau also realized that the motion of a star affects its spectrum. He also did early work in daguerreotype photography.
Jean Bernard Léon Foucault (1819-1868) was a French physicist who was the first to demonstrate how a pendulum could track the rotation of the Earth (the Foucault pendulum) in 1851. He also invented the gyroscope (1852), showed that light travels more slowly in water than in air (1850), and improved the mirrors of reflecting telescopes (1858).
Joseph Fraunhofer (1787-1826) was a German physicist who first studied the Sun's spectra (these dark lines are now called Fraunhofer lines). His work with the spectra and also with diffraction gratings was seminal in the science of spectroscopy.
Galileo Galilei (1564-1642) was an Italian mathematician, astronomer, and physicist. Galileo found that the speed at which bodies fall does not depend on their weight and did extensive experimentation with pendulums.

In 1593 Galileo invented the thermometer.

In 1609, Galileo was the first person to use a telescope to observe the skies (after hearing about Hans Lippershey's newly-invented telescope). Galileo discovered the rings of Saturn (1610), was the first person to see the four major moons of Jupiter (1610), observed the phases of Venus, studied sunspots, and discovered many other important phenomena.

For more information on Galileo, click here.

Johann Gottfried Galle (1812-1910) was a German astronomer who discovered the crepe ring of Saturn (in 1838) and was a co-discoverer (with d'Arrest) of Neptune (in 1846).


George Gamow (March 4, 1904-Aug. 19, 1968) was a nuclear physicist, cosmologist, and writer who formulated the Big Bang Theory (with Ralph Alpher in 1948), worked on quantum theory, stellar evolution, and did work on genetic theory (proposing the existence of DNA - deoxyribonucleic acid in 1954). Gamow's popular books included: Mr. Tomkins in Wonderland (1936), the "Mr. Tomkins" series (1939-67), One, Two, Three ... Infinity (1947), The Creation of the Universe (1952; revised edition 1961), A Planet Called Earth (1963), and A Star Called the Sun (1964).
Robert Hutchings Goddard (October 5, 1882-August 10, 1945) was an American physicist and inventor who is known as the father of modern rocketry. In 1907, Goddard proved that a rocket's thrust can propel it in a vacuum. In 1914, Goddard received two U.S. patents: for liquid-fueled rockets and for two- to three-stage rockets that use solid fuel. In 1919, Goddard wrote a scientific article, "A Method of Reaching Extreme Altitudes," describing a high-altitude rocket; it was published in a Smithsonian report. Goddard's many inventions were the basis upon which modern rocketry is based.

After many years of failed attempts and public ridicule, Goddard's first successful rocket was launched on March 16, 1926 from a relative's farm in Auburn, Massachusetts. It was a liquid-fueled 10-ft. rocket that he called Nell. The flight lasted 2 1/2 seconds; the rocket flew a distance of 184 feet and achieved an altitude of 41 feet.

Goddard soon moved to Roswell, New Mexico, where he developed more sophisticated multi-stage rockets, rockets with fins (vanes) to steer them (1932), a gyro control device to control the rocket (1932), and supersonic rockets (1935). In 1937, Goddard launched the first rocket with a pivotable motor on gimbals using his gyro control device. Altogether, Robert Goddard had 214 patents.

For more information on Goddard, click here.

James Gregory (1638-1675), a Scottish mathematician, invented the first reflecting telescope in 1663. He published a description of the reflecting telescope in "Optica Promota," which was published in 1663. He never actually made the telescope, which was to have used a parabolic and an ellipsoidal mirror.


Beno Gutenberg ( June 4, 1889 - 1960) was a German geophysicist. In 1913, he accurately determined the size of the core of the Earth. Gutenberg discovered that the Earth has a low-velocity zone in the upper mantle; this zone is now called the Gutenberg discontinuity. Gutenberg published a series of papers with Charles Richter (they were titled "On Seismic Waves" and published between 1931 and 1939) and Seismicity of the Earth (published in 1941).
John Hadley (1682-1744) was an English mathematician and inventor who built the first reflecting telescope and invented an improved quadrant (known as Hadley's quadrant).
George Ellery Hale (June 9, 1868 - February 21, 1938) was an astronomer who founded the Yerkes Observatory (1892), the Mt. Wilson Observatory (in 1904), and the Palomar Observatory. Hale invented the spectroheliograph (a device used to analyse the Sun's spectrum) when he was an undergraduate at the Massachusetts Institute of Technology (Cambridge, Massachusetts, USA). Later, Hale discovered that sunspots were low-temperature areas on the sun and that they had high magnetic fields.
Asaph Hall (1829-1907) was an American astronomer who discovered Mars' two moons, Phobos and Deimos, on August 12, 1877, at the U. S. Naval Observatory's 26-inch refracting telescope.
Edmund Halley (1656-1742) was an English astronomer who predicted the return of a spectacular comet in 1758 (after his death, this was confirmed by Johann G. Palitzsch). This comet had previously been seen in 1531, 1607, and 1682. This comet is now known as Halley's Comet.
Johannes Franz Hartmann (1865-1936) was a German astrophysicist who, in 1904, discovered clouds of interstellar calcium gas (he detected the absorption lines of ionized calcium atoms using spectrography while studying binary stars). He also developed a theory about novas, studied the asteroid #433 (Eros) and developed a method of testing telescope lenses, which is still named for him.
Stephen Hawking (1942- ) is a British physicist and cosmologist. His work centers on the physics of black holes and singularities in space-time. Hawking (1971) proposed that early after the Big Bang, mini-black holes existed, obeying quantum-mechanical laws due to their sub-atomic size. Hawking (1974) hypothesized that black holes emit subatomic particles until they explode.
Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was a German astrophysicist who studied solar energy production and star formation. See Helmholtz contraction.
Sir William Herschel (1738-1822) was a British astronomer and organist who built an improved reflecting telescope and used it to discover the planet Uranus (March 13, 1781) and moons of Uranus and of Saturn. Herschel cataloged over 2500 discoveries, mostly deepsky objects. Herschel's sister Caroline Lucretia Herschel (1750-1848) helped him in his discoveries and discovered many clusters and nebulae (and 8 comets) herself.
Ejnar Hertzsprung (1873-1967) was a Danish astronomer who, independently of H. N. Russell, realized the relationship between a star's temperature (color) and its brightness, and designed a diagram illustrating this relationship in 1911, later called the Hertzsprung-Russell Diagram.
Johannes Hevelius (1611-1687) was a German astronomer who published the first moon map. He also published a celestial atlas introducing many constellations (including Canes Venatici, Lacerta, Lynx, Sextans, etc.).
Hipparchus (190-120 B.C.) was an ancient Greek astronomer who compiled first-known catalogue of stars and first map of the skies. He listed 850-1,000 stars, organized by constellation. He noted each star's position and brightness (he rated the brightness on a scale from 1 to 6, the brightest being 1). Hipparchus also devised the system of epicycles, an Earth-centered system in all celestial objects moved in perfect circles around the Earth. He also founded trigonometry.
Edwin Powell Hubble (1889-1953) was an American astronomer who was very influential in modern cosmology. He showed that other galaxies (besides the Milky Way) existed and observed that the universe is expanding (since the light from almost all other galaxies is red-shifted).
Sir William Huggins (February 7, 1824-May 12, 1910) was an amateur English astronomer who was the first person to use spectroscopy to determine the compositions of astronomical objects (in 1861). He determined that the Sun and the stars are composed mostly of the element hydrogen. He also examined the spectra of nebulae and comets. Huggins' wife (they were married in 1875), Margaret Lindsay Murray Huggins (1848-1915), was a self-taught astronomer who did extensive work in spectroscopy and photography. Margaret studied the Orion Nebula extensively. William and Margaret were the first people to realize that some nebulae, like the Orion Nebula, consisted of amorphous gases (and were not a congregation of stars, like the nebula Andromeda). A lunar crater, a Martian crater, and an asteroid (#2635 Huggins) have been named for William Huggins.
Christian Huygens (1629-1695) was a Dutch physicist and astronomer who developed new methods for grinding and polishing glass telescope lenses (about 1654). With his new, powerful telescopes, he identified Saturn's rings and discovered Titan, the largest moon of Saturn in 1655. Huygens also invented the pendulum clock in 1656 (eliminating springs), wrote the first work on the calculus of probability (De Ratiociniis in Ludo Aleae, 1655), and proposed the wave theory of light (Traité de la lumiere, 1678).
Hypatia of Alexandria (AD 370(?)-415) was a Greek mathematician, astronomer, teacher, and head of the Platonist school at Alexandria about AD 400. Hypatia wrote commentaries on the astronomical canon of Ptolemy and did work on conic sections . Her works are lost, but are referred to in the Suda lexicon. She was the daughter of the mathematician and philosopher Theon of Alexandria (he was also the last head of the Museum at Alexandria). A pagan, she was murdered in 415 by Christian monks in a religious/political struggle. The lunar Crater Hypatia and Rimae Hypatia were named for her.
Karl G. Jansky (1905-1949) was an American radio engineer who pioneered and developed radio astronomy. In 1932, he detected the first radio waves from a cosmic source - in the central region of the Milky Way Galaxy. Jansky's work was continued by Grote Reber.
Lord Kelvin, William Thomson (1824 - 1907) designed the Kelvin temperature scale in which 0 K is defined as absolute zero and the size of one unit is the same as the size of one degree Celsius. Water freezes at 273.16 K; water boils at 373.16 K.
Johannes Kepler (1571-1630) was a German mathematician who realized that the planets go around the sun in elliptical orbits. He formulated what we now call "Kepler's Three Laws" of planetary motion that mathematically describe the elliptical orbits of celestial objects. For a few years he worked with Tycho Brahe.
Gustav Kirchoff (1824-1887) was a German physicist who realized that each element gave off a characteristic color of light when heated to incandescence. When separated by a prism, the light for each element had a specific pattern of wavelengths. Kirchoff, together with Bunsen, used his techniques to discover two new elements, cesium (1860) and rubidium (1861). Kirchoff found that when light shines through a gas, the gas absorbs some of the light, the same wavelengths of light that it would emit when heated. He applied his techniques to the Sun, explaining Fraunhofer lines.
Daniel Kirkwood (1814-1895) was an American astronomer who discovered the radial gaps in the asteroid belt in 1866 (now known as the Kirkwood gaps). Kirkwood also hypothesized that Saturn's moon Enceladus creates the Cassini division with its gravitational attraction (but astronomers today think that Mimas causes it).
Gerard P. Kuiper was a Dutch-American astronomer who who predicted the existence of the Kuiper belt in 1951. The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects orbit. This belt is located from 30 to 50 (?) A.U.'s and was discovered in 1992. It is a region where the planet-building process was stopped in before any large objects were formed; there are only primitive remnants from the early accretion disk of the solar system, 4.5 billion years ago. The Kuiper belt may be the source of the short-period comets (like Halley's comet).
William Lassell was an amateur English astronomer (a brewer by trade) who discovered Triton, the largest moon of Neptune (in 1846) and Ariel, the brightest moon of Uranus in 1851. With W.C. Bond, he discovered Saturn's moon Hyperion in 1848.
Henrietta Swan Leavitt (1868-1921) was an American astronomer who first described the relationship between the period and the brightness (luminosity) of Cepheid variable stars. She also discovered 1,777 variable stars in the Magellanic Clouds.
Georges LeMaitre (1894-1966) was a Belgian mathematician who developed the Big Bang Theory of the formation of the universe.
Urbain Jean Joseph Le Verrier (1811-1877) was a French astronomer who predicted the existence and position of the planet Neptune using orbital calculations.
Bertil Lindblad (1895 - 1965) was a Swedish astronomer who theorized that the areas around the center of a galaxy revolves. Oort proved that this does indeed happen. He studied the structure and dynamics of star clusters, estimated the Milky Way's galactic mass, the period of our Sun's orbit, confirmed Harlow Shapley's direction and approximate distance to the center of the Galaxy, and developed spectroscopic means of distinguishing between giant and main sequence stars.
Hans Lippershey (1570?-1619) was a German-born Dutch lens maker who demonstrated the first refracting telescope in 1608, made from two lenses; he applied for a patent for this optical refracting telescope (using 2 lenses) in 1608, intending it for use as a military device.


Percival Lowell (1855-1916) was an American astronomer who founded the Lowell Observatory at Flagstaff, Arizona in 1894. Lowell studied Mars extensively, especially its surface markings, which he thought were canals. He also thought that the bright areas were deserts and the dark ones were areas containing vegetation (this was not true). Lowell published three books on Mars: Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908). Lowell also calculated that an unknown planet, dubbed Planet X, must orbit beyond Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916, before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto). Tombaugh did his observations at the Lowell Observatory.
Simon (Mayr) Marius (1570-1624), was a German astronomer and physician who studied with Kepler and attended Galileo's lectures. He claimed to have discovered the four largest moons of Jupiter in 1610, the same year that Galileo discovered them (independently).
Charles Messier (1730-1817), was a French astronomer who searched the skies for comets. He compiled a list of 103 fuzzy objects (nébuleuse sans étoile, or starless nebulosities) in space in order not to mistake star clusters, galaxies, and nebulae for comets (for which he was searching). The Messier list has been added to and now consisted of 35 galaxies, 30 open clusters, 29 globular clusters, 4 planetary nebulae, 7 diffuse nebulae, and two unconfirmed objects (which were mistaken for nebulae by Messier).
Maria Mitchell (August 1, 1818 -June 28, 1889 ) was the first woman Professor of Astronomy of the United States. In 1865, Maria Mitchell became a professor of astronomy at Vassar College in Poughkeepsie, New York (she had previously been a librarian) . She discovered a comet (in 1847), studied the planets Jupiter and Saturn, and photographed many stars. Despite her accomplishments, when she visited the Vatican Observatory in Italy, she was only allowed to enter the observatory during the day. Maria Mitchell was the first woman accepted into the American Academy of Arts and Sciences (1848), the Association for the Advancement of Science (1850), and the American Philosophical Society (1869). Mitchell was one of the founders of the American Association for the Advancement of Women (1873).


Andrija Mohorovicic (1857 - 1936) was a Yugoslavian geophysicist. After examining seismic waves from the 1909 Kulpa Valley earthquake, Mohorovicic theorized that a boundary between the Earth's crust and the upper mantle existed (about 50 km beneath the surface) in which the speed of earthquake waves became very rapid. This region is now called the Mohorovicic discontinuity. A crater on the far side of the moon was also named for Mohorovicic.
Patrick Moore (1923- ) is an English astronomer who has written over 60 books on astronomy and made regular BBC television appearances popularizing astronomy. He has done work on lunar mapping.
Johann Müller, also known as Johann Regiomontanus (1436-1476) was a German astronomer and mathematician. He studied trigonometry, translating Ptolemy's Almagest, from the original Greek. Ironically, his translation helped overthrow the Ptolemaic view of the universe (in which the Earth was thought to be at the center of the universe). He also did work on plane and spherical trigonometry. Muller also obsesrved the motion of the moon, planets, and comets. A 108 km diameter lunar crater, called Regiomontanus (Latitude: -28.3 degrees, Longitude: 1.0 degrees), was named for Muller.
Isaac Newton (1642-1727) was an English mathematician and physicist who invented calculus (simultaneously, but independently of Leibniz), formulated the laws of gravitation, investigated the nature of light (he discovered that sunlight is made of light of different colors), and the laws of motion: 1. An object in uniform motion tends to remain in that state of motion unless an external force is applied to it (the Law of Inertia). 2. A force causes a change in the velocity (acceleration) of an object (F=ma). 3. For every action there is an equal and opposite reaction. Newton also improved the design of the refracting telescope (using an objective mirror, instead of a lens), and it is now called a Newtonian telescope.
Heinrich Wilhelm Matthäus Olbers (1758-1840) was a German astronomer and physician who published Olbers' paradox (Why is the sky dark at night? or Why doesn't starlight make the night sky bright?) (1823), determined that Uranus is a planet, not a comet (1781), discovered Olbers's comet (1815), the asteroids #2 Pallas (1802) and #4 Vesta (1807), and formulated a method for calculating comet orbits.
Jan Hendrik Oort (1900-1992) was a Dutch astronomer who calculated the distance to the middle of the Milky Way galaxy, mapped our galaxy, proved that the areas around the center of a galaxy revolves, and proposed the existence of the Oort Cloud in the 1950's. The Oort Cloud is a cloud of rocks and dust that may surround our solar system. This cloud may be where long-period comets originate. It has been hypothesized that the Oort Cloud is responsible for the periodic mass extinctions on Earth.
Giuseppe Piazzi (1746-1826) was an Italian astronomer who discovered and named the first (and largest) asteroid, Ceres, on January 1, 1801. Asteroids are large, rocky objects that orbit the Sun in a belt between Mars and Jupiter.
Claudius Ptolemaeus or Ptolemy (about 87-150) was a Greek astronomer and mathematician who wrote about his belief that all celestial bodies revolved around the Earth. His writings influenced people's ideas about the universe for over a thousand years, until the Copernican System (with a heliocentric solar system) was accepted.
Pythagoras of Samos (569-475 BC) was a Greek philosopher, mathematician, and astronomer who founded a philosophical and religious school, the Pythagorean school in Croton, Italy. Pythagoras believed that the Earth was a sphere at the center of the Universe. He correctly realized that the morning star and the evening star were the same object, the planet Venus. Pythagoras (or the Pythagoreans) made a number of fundamental mathematical discoveries: that for a right triangle, the sum of the squares of the two shorter sides is equal to the square of the hypotenuse (known as the Pythagorean theorem); that the sum of the angles of a triangle is equal to two right angles; and that irrational numbers exist. A 142 km wide lunar crater was named for Pythagoras (Latitude 63.5°, Longitude 63.0°).
Grote Reber (Dec. 11. 1911-Dec. 20. 2002) was a radio engineer and pioneering amateur astronomer who was the first person to follow up Karl Jansky's discovery of radio waves coming from space. Reber built a 9-meter parabolic reflector dish radio antenna in his yard in Illinois - it was the first radio telescope used for astronomy. He detected the first signals (at a frequency of 160 megahertz, about 2m wavelangth) in 1939, using his third receiver. Reber's work led to many developments in radio astronomy; he made the first radio maps of the sky and showed that the brightest areas corresponded to the center of the Milky Way. Reber started the field of very long-wavelength/low-frequency (1-2 MHz, 150-300 m wavelength) radio astronomy, moving to Tasmania (an island off the southeastern coast of Australia) where these radio waves can be received (because the long-wavelength radio waves can get through the Earth's ionosphere over that part of the globe due to a hole in the ozone layer).
Ole Romer (1644-1710) was a Danish astronomer who, in 1675-1676, was the first person to demonstrate that the speed of light is finite. Romer did this by observing eclipses of Jupiter's moon Io as Jupiter's distance from Earth varied through the year. He noticed that the observed period of Io's orbit differed by about 20 minutes; he concluded that this difference was due to the extra distance that the light had to travel to Earth. His calculations put the speed of light at about 225,000 kilometers per second (it is really a bit faster, at 299,792 kilometers per second).
Henry Norris Russell (1873-1967) was an American astronomer who, independently of E. Hertzsprung, realized the relationship between a star's temperature (color) and its brightness, and designed a diagram illustrating this relationship in 1913, later called the Hertzsprung-Russell Diagram.
Johann Müller, also known as Johann Regiomontanus (1436-1476) was a German astronomer and mathematician. He studied trigonometry, translating Ptolemy's Almagest, from the original Greek. Ironically, his translation helped overthrow the Ptolemaic view of the universe (in which the Earth was thought to be at the center of the universe). He also did work on plane and spherical trigonometry. Muller also obsesrved the motion of the moon, planets, and comets. A 108 km diameter lunar crater, called Regiomontanus (Latitude: -28.3 degrees, Longitude: 1.0 degrees), was named for Muller.
Carl Sagan (1934-1996) was an American astronomer who discovered that the surface of Venus was extraordinarily hot and noxious (contrary to previous models of a mild Venusian surface). Sagan also showed that the universe has many organic (carbon-based) chemicals and that life is likely to exist throughout the cosmos. He was a great popularizer of astronomy, was also involved in many NASA flights and SETI, and he pioneered the field of exobiology.
Allan Rex Sandage (1926-) is an astronomer who, in the 1950's, measured the rate of the expansion of the Universe, Hubble's constant (H), which he calculated to be 50 km/sec/mpc. From this, Sandage estimated of the age of the Universe (T) to be 19.2 billion years [T = 2/3 x (1/H) ]. These calculations have changed through the years and now, H=~ 75 (T=12.9 billion years) is more generally accepted. Sandage also discovered quasars in 1964.
Giovanni Schiaparelli (1835-1910) was an Italian astronomer (and director of the Milan Observatory) who first mapped Mars (in 1877) and brought attention to the network of "canali" (Italian for canals or channels) on Mars. These "canals" were later found to be dry and not to be canal at all. A Martian impact crater (Crater Schiaparelli, 461 km = 277 mi in diameter) and a hemisphere of Mars have been named after Schiaparelli. His late niece Elsa was an influential fashion designer.
Heinrich Schwabe was an an amateur German astronomer discovered that sunspots appeared in an 11-year cycle. Schwabe was a pharmacist who observed the sun daily and published his observations, "Solar Observations During 1843," in 1843.
Harlow Shapley (1885-1972) was an American astronomer who was the first person to accurately estimate the size of the Milky Way Galaxy and our position in it.
Bengt Georg Daniel Strömgren (January 21, 1908 - July 4, 1987) was a Danish astronomer who studied the structure of stars, including stellar atmospheres, their composition (what elements they contain), and ionization in stars. He theorized that ionized hydrogen gas clouds surround hot stars (the "Strömgren sphere"). Stromgren also determined the relationship between the gas density of a star, its luminosity, and the size of the "Strömgren sphere" of ionized gas that surrounds it. He also did work on photoelectric photometry (measuring the intensity of the photoelectric effect from an astronomical object).
Joseph John Thomson was a British scientist who discovered the existence of the electron in 1897. Electrons are tiny, negatively-charged atomic particles. In an atom, they orbit around the nucleus.
Timocharis was a Greek scientist who observed stars around 290 BC and was an apparent influence on Hipparchus.
Clyde Tombaugh (1906-1997) was an American astronomer who discovered Pluto in 1930. He also correctly predicted (in 1950) that the surface of Mars was covered with craters.
James A. Van Allen was an American physicist who discovered doughnut-shaped belts of radiation that circle the Earth (the van Allen Belt).
Thomas Wright (1711-1786) was a British cosmologist. Wright was one of the first people (along with Johann Lambert (1728-77) and Immanuel Kant (1724-1804) ) who in 1750 speculated about the structure and origin of our solar system and galaxy. Using religious and philosophical arguments, Wright hypothesized that the Milky Way was a thin flat system of stars with our solar system near the center and that there were other similar but distant star systems (which he called nebulae).

Astronomy Dictionary

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