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G G is Newton's gravitational constant (also called the universal gravitational constant), a fundamental constant of nature that determines the strength of the force of the gravitational interaction between objects. G = 6.673 x 10-8 dynes cm2/gm2 |
G G is an asteroid's "magnitude slope parameter." It is specific to an asteroid and is used for the apparent magnitude computation (as the asteroid moves in relation to the Earth). |
g g is the acceleration of gravity at the surface of the Earth. It is equal to 9.8 m/sec2. |
 GAGARIN, YURI Yuri Gagarin (1934-1968) was a Soviet cosmonaut and the first man to orbit the Earth. He piloted the Vostok 1 mission which launched April 12, 1961 and orbited the Earth. The flight lasted 108 minutes. The spacecraft was recovered later that day in the Saratov region of the Soviet Union. Gagarin died years later in a plane crash. |
 GALATEA Galatea is a tiny moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 61,950 km from the center of Neptune and is about 158 km in diameter. |
 GALAXY A galaxy is a huge group of stars and other celestial bodies bound together by gravitational forces. There are spiral, elliptical, and irregularly shaped galaxies. Our Sun and solar system are a small part of the Milky Way Galaxy. |
GALILEAN MOONS The Galilean moons are the four largest moons of Jupiter. These moons, Ganymede, Io, Callisto, and Europa, were discovered by Galileo in 1610; Galileo was using his 20-power telescope. These satellites were the first bodies known to orbit another planet. |
GALILEI, GALILEO  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 |
 THE GALILEO The Galileo is a spacecraft launched in 1989 to study Jupiter and its moons, arriving in December, 1995. (Galileo was the first person to see these moons, using his telescope.) Galileo crashed into Jupiter on September 21, 2003. |
GALLE, GOTTFRIED 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). |
GAMMA RAY Gamma rays are very high energy electromagnetic radiation, like light or X-rays, but much higher in energy and frequency (and shorter in wavelength). |
GAMMA RAY BURST Gamma ray bursts are mysterious and powerful astronomical phenomenon that emit short-lived flashes of gamma rays (extremely high-energy radiation). These bursts last only up to a few seconds, occur every day, and come from different parts of the sky. |
GAMOW, GEORGE 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). |
 GANYMEDE Ganymede is the largest moon of Jupiter, a large, icy, outer moon that is scarred with impact craters and many parallel faults. It has a diameter of about 3,400 miles (5,268 km) and orbits Jupiter at a mean distance of 664,000 miles (1,070,000 km). It takes Ganymede 171.75 hours (7.15 Earth days) to orbit Jupiter. Its mass is 1.48x1023 kg. It was discovered by Galileo and Simon Marius (independently) in1610. Ganymede is the largest moon in the solar system; it is also larger than the planets Mercury and Pluto. |
GAS Gas is a phase of matter in which the molecules are widely separated, move around freely, and move at high speeds. |
GASEOUS Gaseous means existing as a gas (and having the characteristics of a gas). |
 GAS GIANTS The gas giants are the large outer planets of our Solar System: Jupiter, Saturn, Uranus, and Neptune (but not tiny, rocky Pluto). |
GAUSS The gauss is a unit of magnetic induction (denoted B) in the cgs system (centimeter-gram-second). A changing magnetic field produces an electric current - this is called magnetic induction. The gauss was named for the German mathematician Johann Carl Friedrich Gauss (April 30, 1777-Feb. 23, 1855), who did work in magnetism. Magnetic flux is a measure of flux density (a Gauss is the magnetic flux per square centimeter). |
GEGENSCHEIN Gegenschein (meaning "counterglow") is a very faint glow in the sky that appears exactly opposite to the Sun. |
 GEMINI [Abbreviation: Gem] Gemini (also known as "The Twins") is one of the constellations of the zodiac, located along the ecliptic between Taurus and Cancer. The brightest stars in Gemini are Castor (a sextuple star - three double stars) and Pollux (a 1st magnitude yellow star). The open cluster M35 is located in Gemini. The Geminid meteor shower seems to radiate from Gemini. |
 GEMINID METEOR SHOWER The Geminids are a meteor shower that occur each year from Dec. 6-19, with a maximum on Dec. 13-14. This meteor shower occurs each year as the Earth passes through the orbit of the asteroid #3200 Phaeton, and dusty remnants of the asteroids burn up as they enter the Earth's atmosphere. The meteors in this shower seem to emanate from the constellation Gemini (but they do not). |
GENERAL RELATIVITY General Relatiity is a theory formulated by Albert Einstein. General Relativity expands the theory of Special Relativity to include acceleration and gravity, both of which are explained via the curvature of space-time. |
GEOCENTRIC Geocentric means Earth-centered. In a geocentric model of the universe, everything revolves around the Earth; this, of course, is a false model. |
GEOLOGICAL TIME The history of the earth is described in geological time, which is measured in millions of years and billions of years. The divisions used are: eon, era, period, and epoch. |
 GEOLOGY Geology is the study of the Earth's structure. |
  GEOLOGIST A geologist is a scientist who studies geology. |
GEOSTATIONARY ORBIT A satellite is in geostationary orbit (GSO) when it remains over the same spot on the Earth (somewhere along the equator). The orbital period is 24 hours (equal to the earth's rotational period). A geostationary orbit is geosynchronous, circular and has zero inclination. There is only one geostationary orbit, a belt encircling the earth's equator at an altitude of about 22,236 miles (35,786 km). It is impossible to have a satellite in geostationary orbit over a point that is not along the equator. Many communication and weather satellites are in geostationary orbit, but their number is limited, sine they 're all in the same orbit and must be spaced enough to avoid interference. |
GEOSYNCHRONOUS ORBIT A satellite is in geosynchronous orbit (GEO) when its orbital period is 24 hours (equal to the earth's rotational period). The satellite remains near the same spot on the Earth (somewhere along the equator). Many communication and meteorological satellites are geosynchronous. |
GEYSER A geyser is a jet of hot liquid or gas that shoots from a crack in the crust of a planet or moon. |
 GIBBOUS MOON A gibbous moon is between a full moon and a half moon, or between a half moon and a full moon. |
 GLENN, JOHN John Glenn (1921- ) piloted the first American manned orbital mission on February 20, 1962. He flew NASA's Friendship 7, a Mercury-Atlas 6 spacecraft, to about 162 miles in altitude, going at a maximum orbital velocity of about 17,500 miles per hour. This mission orbited the Earth 3 times and lasted 4 hours, 55 minutes, and 23 seconds, from launch to impact in the Atlantic Ocean. In 1998, 36 years later, Glenn flew a 9-day mission on the Space Shuttle (STS-95). Glenn was the US senator from Ohio from 1974 to 1998. |
 GLOBULAR CLUSTER A globular star cluster is a spherical group of up to a million stars held together by gravity. These remote objects lie mostly around the central bulge of spiral galaxies. The brightest globular cluster is Omega Centauri (in the constellation Centaurus); it is easily seen by the naked eye and is magnitude 4. |
GLOBULE A globule is an interstellar cloud of dust and hydrogen gas that contracts and undergoes gravitational collapse to form a star. |
 GLYCOLALDEHYDE Glycolaldehyde (C2H402) is a type of simple sugar that was recently found in a giant interstellar cloud of gas and dust [called Sagittarius B2 (North)], located near the center of the Milky Way Galaxy, about 26,000 light-years from Earth. This amazing find is the first sugar (and one of the most complex molecules) found in space. This molecule was detected by Jan M. Hollis (NASA/Goddard), Frank J. Lovas (University of Illinois), and Philip R. Jewell (NRAO/Green Bank) using the National Science Foundation's 12 Meter Telescope, a radio telescope on Kitt Peak, Arizona. Hollis said, "Finding glycolaldehyde in one of these interstellar clouds means that such molecules can be formed even in very rarefied conditions. We don't yet understand how it could be formed there." |
googolplex A googolplex is the number 10 raised to the googol power: 10(googol) or 10(10100) (1 followed by a googol of zeros. A googolplex is much larger than the number of atoms in the Universe. |
 GRANULATION Granulation consists of solar granules together with intergranular lanes (dark, cool areas between granules where solar material is descending into the surface). Granulation covers the visible surface (the photosphere) of the Sun. |
GRANULE Granules are regions of the Sun where hot solar material comes to the solar surface. Granules are about 600 miles (1,000 km) across and only exist for about 5 to 10 minutes before they fade away. It is almost as though the surface of the Sun is bubbling like a pot of boiling water. |
 GRAPH A graph is a diagram that represents a series of points or lines. |
GRAVITATIONAL CONSTANT The gravitational constant (abbreviated G) is the constant of proportionality in Newton's equation (formulated in 1666) that describes the gravitational attraction between objects; their gravitational attraction (F) depends only on their masses and the distance between them, according to the formula |
GRAVITATIONAL LENS A gravitational lens is a massive object in space (like a galaxy) that warps space and bends light that passes by it, due to the gravitational forces of the massive object. |
GRAVITATIONAL LENSING Gravitational lensing is the displacement of light due to the warping of space by a gravitational lens (a massive object in space that bends light that passes by it, due to the gravitational forces). |
 GRAVITY Gravity is a physical force that pulls objects together. Every bit of mass produces a gravitational force; this force attracts all other masses. The more massive an object, the stronger the gravitational force. Newton formulated the laws of gravity. |
GRAVITY ASSIST MANEUVER A gravity assist maneuver (or gravity assist fly-by) is the use of a large celestial object's gravity (such as a planet) in order to change a spacecraft's trajectory. In a close encounter with a large object, a tiny bit of the object's orbital energy is transferred to the spacecraft. This idea was discovered in the 1960's by Michael A. Minovitch and by Gary Flandro (who were at the Jet Propulsion Laboratory). The first time that this gravitational slingshot maneuver was used was in the 1970's, when the spacecraft Voyager used multiple gravity assist flybys of the aligned planets Jupiter, Saturn, Uranus and Neptune, to boost the spacecraft beyond the gas giants. |
GREAT ANNIHILATOR The Great Annihilator is a powerful, high-energy x-ray source located near the center of the Milky Way Galaxy. The Great Annihilator may be a black hole with a mass of roughly 10 to 1000 times that of the Sun. This object produces anti-matter (positrons, which are anti-electrons) which, when they collide with ordinary electrons, will annihilate the matter, resulting in the production of energy. |
GREAT ATTRACTOR The Great Attractor is a giant group of roughly 100,000 galaxies beyond our Local Group of galaxies. No one has yet seen the galaxies of the Great Attractor, but the effect on nearby galaxy clusters has been detected. |
GREAT CIRCLE A great circle is an imaginary circle on the surface of a sphere whose plane passes through the center of the sphere. |
 GREAT RED SPOT Jupiter's Great Red Spot is a huge, long-lasting storm in the atmosphere of the Southern Hemisphere of Jupiter (near the equator). Its color is actually pink to orange. This whirlwind varies in size and color from year to year. It is about 17,000 miles (28,000 km) long and 9,000 miles (14,000 km) wide; it is the biggest storm in this solar system. It is so big that the Earth would fit in it with room to spare. It was discovered in 1664 by Robert Hooke. |
 GREAT DARK SPOT Neptune's Great Dark Spot is an Earth-sized hole in the thick cloud cover of the methane atmosphere of Neptune. The size, shape, and location of the spot vary greatly over time; it even disappears and reappears occasionally. Horrendous winds near the spot were measured by Voyager 2 to be about 1,500 miles per hour (2,400 kph). These are the strongest recorded winds in our Solar System. |
GREAT RIFT The great rift is a series of dark, obscuring dust clouds in the Milky Way galaxy. These clouds stretch from the constellation Sagittarius to the constellation Cygnus. |
GREEK ALPHABET The Greek alphabet has 24 lettters, alpha, beta, gamma, delta, epsilon, zeta, eta, theta, iota, kappa, lambda, mu, nu, xi, omicron, pi, rho, sigma, tau, upsilon, phi, chi, psi, and omega. The Bayer system in astronomy uses Greek letters to denote stars by their relative brightness in each constellation (in order of decreasing brightness). The brightest star in a constellation is alpha, the second-brightest is beta, the third is gamma, etc. |
GREEN FLASH Sometimes, at sunrise or sunset, there are flashes of green light visible near the sun. This effect only happens when there are no clouds nearb the sun. The green flash (also called green ray) is caused mostly by refraction (blue light from the sun bends more than other wavelengths) of light from the sun by the Earth's atmosphere. This phenomenon is also caused by sunlight scattering (blue light is Rayleigh scattered away from the sun) and being differentially absorpted by the Earth's atmosphere when the sun is low on the horizon. |
 GREENHOUSE EFFECT The greenhouse effect is an increase in the temperature of a planet as heat energy from sunlight is trapped in the atmosphere. Excess carbon dioxide and water vapor increase this effect. The greenhouse effect is strong on Earth and Venus, maintaining warm temperatures. |
GREENWICH OBSERVATORY The Royal Greenwich Observatory is athe national astronomical observatory of England. It is located in Greenwich, England (near London). |
 GREGORY, JAMES 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. |
GUNPOWDER The ancient Chinese invented gunpowder (made of saltpeter, sulfur, and charcoal dust) roughly 1,000 years ago. It was used for making fireworks (which were used to celebrate important occasions), medicine, alchemy and later, to power early rockets. |
GUTENBERG, BENO 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). |
 GUTENBERG DISCONTINUITY The Gutenberg discontinuity separates the outer core and the mantle of the Earth. |
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Virgil "Gus" Ivan Grissom (April 3, 1926 - January 27, 1967) was one of the original seven NASA astronauts. Grissom flew the second manned US space flight, commanded the first two-man Gemini mission, and was killed along with Edward White II and Roger Chaffee in a launch pad fire during a test for the upcoming first Apollo flight, which Grissom was scheduled to command.