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aberration of starlight - A difference between the observed position and true position of a star, an effect of the earth's motion.

Alexandrian Library (300 B.C.- 300 A.D.) - The most famous library of antiquity; under the Ptolemies, was the center of Hellenistic culture, containing at least 700,000 volumes.

Amalthea - A tiny potato-shaped moonlet of Jupiter.

amino acids - A constituent of all proteins; the fundamental building blocks of life.

Anaxagoras 500-428 B.C. - Greek philosopher and astronomer who studied the phases of the moon, the size of the sun, and the behavior of meteors; was among the first to postulate that the heavenly bodies were composed of ordinary matter.

Anaximander (611-547 B.C) - Greek philosopher and scientist who invented the sundial, made a map of the known world, and determined the length of the year and the seasons.

Arecibo Observatory - Located in Arecibo, Puerto Rico, the site of a giant, disc-shaped radio telescope.

Aristarchus (310-230 B.C.) - One of the last of the old Ionian scientists; best known for his belief that the sun, rather than the earth, was at the center of the planetary system.

artificial selection - The process whereby a new variation of plant or animal is produced by the repeated crossing of desirable genotypes.

asteroid - A small planet, less than 500 miles in diameter and orbiting in space between Mars and Jupiter.

asthenosphere - The hot, amorphous and sometimes molten layer of a planet lying just below the lithosphere.

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Big Bang - A now widely accepted theory that hypothesizes that the universe was "born" and began expanding some 15 to 20 billion years ago when all space, time, matter, and energy were compressed into a gigantic, high-density fireball.

bit - A unit of computer information representing a choice between two alternatives.

black hole - A region of extremely warped space-time caused by an intense gravitational field.

blue star - A young, vigorous, short-lived star.

Brahe, Tycho (1546-1601) - Danish astronomer who studied the motions of the planets, sun, and moon, observed the locations of stars, and was one of the first to reject heliocentric cosmology.

brainstem - The part of the brain linking the spinal cord with the forebrain and cerebrum.

Bruno, Giordano (1548-1600) - Italian philosopher who believed in the infinity of the universe and was burned at the stake by the Inquisition.

Callisto - An outer moon of Jupiter.

Cambrian explosion - The period in terrestrial evolution, which began some 600 million years ago, characterized by an enormous proliferation of new and more complex lifeforms.

catastrophism - A philosophy, emphasizing the importance of sudden and violent events in causing changes, particularly in evolutionary processes.

cerebral cortex - In humans, the convoluted surface layer of gray matter covering the cerebral hemispheres; the seat of coordination of high nervous activity, including thought and intuition.

Champollion (1790-1832) - French Egyptologist who deciphered the Rosetta Stone, thus unlocking the secrets of Egyptian hieroglyphics.

chaos - From the Greek, the state of things when chance is supreme.

chloroplast - The chlorophyll-bearing body in cells that is the site of photosynthesis.

Chryse - The region on the Martian surface chosen for the Viking 1 landing.

comet - an object with a nucleus generally considered to be "dirty ice," which travels in an eccentric orbit around the sun and when near the sun shows a coma and a tail.

contact binaries - Two proximal stars whose atmospheres are elongated by their mutual gravity and whose star stuff is intermingled.

Copernicus, Nicolaus (1473-1543) - Polish astronomer whose heliocentric (sun- centered) model of the planetary system revolutionized astronomy.

corpus callosum - A band of fibers uniting the cerebral hemispheres of the brain.

cosmic background radiation - Remnants of radiation from the Big Bang, detected by radio telescopes.

cosmic ray - A stream of atomic nuclei that enter the earth's atmosphere at very high speeds.

cosmology - The study of the physical universe as a whole.

cosmos - Derived from the Greek word meaning order of the universe, refers to a harmonious and systematic universe whose constituent objects are interconnected and obey the same physical laws.

crater - A depression on the surface of an object in space, usually formed by an impact.

Cydonia - The region on the Martian surface chosen for the Viking 2 landing.

Cygnus X-1 - An X-ray source and member of a binary system; possibly a black hole.

Darwin, Charles (1809-1882) - English naturalist best known for his formulation of the theory of evolution by natural selection.

Democritus (460-370 B.C.) - Greek philosopher who invented the word "atom" and conceived the idea that everything in the universe is composed of atoms that move about in space and form into bodies.

DNA (deoxyribonucleic acid) - A long chain of molecules, found in the nuclei of cells, that contain the genetic information of life.

Doppler effect - The change in wavelength that is caused by the motion of an object or some other source of waves, relative to an observer.

Einstein, Albert (1879-1955) - German-Swiss-American physicist whose theory of quantum mechanics won him the Nobel Prize in 1922.

electron - A subatomic particle that has a negative charge and normally moves about the nucleus of an atom.

element - A fundamental substance that consists of atoms of only one kind. At present, more than 100 elements have been discovered. Singly or combined, they constitute all matter.

Empedocles (fl. 444 B.C.) - Greek philosopher and scientist who identified four immutable elements in the universe - earth, air, fire, and water - and made observations about the nature of the physical world, particularly about the behavior and motion of light and air, that anticipated modern physics.

epicycle - The orbit in which a planet moves and which has a center that is itself carried around at the same time on the circumference of a larger orbit.

Eratosthenes (273-192 B.C.) - Greek geographer and philologist who determined that the earth was round, measured its circumference, and calculated the approximate distance of the sun from the earth; served as a librarian and encyclopedist at the Alexandrian Library (235-195 B.C.).

Europa - An inner moon of Jupiter.

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galaxy - A collection of gas, dust, and stars, usually containing millions of solar systems; there are estimated to be some 100 billion galaxies in the universe.

Galilean satellites - Four moons of Jupiter first observed by Galileo: Io, Europa, Ganymede, and Callisto.

gamma ray - A form of light composed of high-energy photons with short, high- frequency wavelengths.

Ganymede - An outer moon of Jupiter.

genetic code - The self-reproducing record of the specific protein pattern of an organism.

geocentric - Earth-centered.

Goddard, Robert 1882-1945) - American physicist and rocket pioneer.

gradualism - A philosophy emphasizing the importance of steady, slow changes, particularly an evolutionary process.

greenhouse effect - The process by which a solid body traps and absorbs the sun's radiation, leading to higher temperatures. On earth, the warming of lower layers of the atmosphere tends to increase with greater concentrations of carbon dioxide.

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Halley, Edmond (1656-1742) - English astronomer who discovered that the "comets" which appeared in 1531, 1607, and 1682 were actually one comet, later names "Halley's comet."

heliocentric - Sun-centered.

heliopause - Outer boundary of the solar system.

Hubble, Edwin - American astronomer whose discovery that light from distant galaxies is increasingly redshifted depending on the distance of the galaxy from earth helped lay the groundwork for the Big Bang theory.

Humason, Milton - Assistant to Hubble who helped discover the Big Bang.

Huggins, William (1824-1910) - English astronomer and pioneer in the use of the spectroscope to analyze stellar light.

Huygens, Christian (1629-1695) - Dutch mathematician, physicist, and astronomer.

inverse square law - A law formulated by Newton that explains why the planets orbit the sun in regular elliptical orbits.

Io - Inner moon of Jupiter.

Ionians - A group of Greek scientists and natural philosophers living in the six centuries before the birth of Christ who made seminal discoveries in astronomy, physics, geography, medicine, biology, mathematics, and other sciences.

isotropy - Exhibiting properties (such as the velocity of light transmission ) with the same values when measured along axes in all directions.

Jupiter - The largest of the planets in our solar system, and the fifth planet from the sun.

Kepler, Johannes (1571-1630) - German astronomer whose formulation of the laws of planetary motion helped establish the validity of the Copernican model of the solar system.

law of interia - A law formulated by Newton; the tendency of a moving object to continue moving in a straight line unless something influences it, and moves out of its path.

law of universal gravitation - Newton's law which explains the force in nature by which two masses attract each other.

lenticular cloud - A cloud shaped like a lens, sometimes mistaken for an extraterrestrial spaceship.

light year - The distance light travels in a year, at a speed of 300,000 kilometers per second.

limbic system - A group of structures of the brain concerned with motivation and emotion.

lithosphere - The relatively rigid and solid outer layer of a planet.

Local Group - The collection of two dozen or so galaxies, of which our own Milky Way is a member, that forms a subcluster.

Lowell, Percival (1855-1916) -American astronomer who predicted the planet Pluto and was an active proponent of the theory that Mars was inhabited.

Lucretius (98-55 B.C.) - Roman poet who outlined a complete science of the universe based on the philosophies of Emocritus and Epicurus.

Mariner 9 - American spacecraft that photographed Mars and its two moons in 1971.

Mars - The fourth planet from the sun.

Mars Jar - A container in which the environment of Mars is simulated.

Mars 3 - Soviet spacecraft which landed on Mars in 1971.

Mars 6 - Soviet spacecraft which landed on Mars in 1974.

Mercury - The planet closest to the sun.

meteor - A track of light observed when extraterrestrial matter enters the earth's atmosphere and burns up; considered by some astronomers to be a fragment of a comet.

meteorite - A piece of interplanetary matter that has impacted on earth; considered by some astronomers to be a fragment of an asteroid.

Michell, John (1724-1793) - English astronomer, physicist, and geologist who founded seismology and created the notion of black holes.

Milky Way - A rotating, spiral galaxy made up of dust, gas, and some 250 billion suns, of which our own sun is one, estimated to be about 100,000 light years in diameter.

Miller, Stanley - American biochemist who, with Harold Urey in the 1950s, recreated the essential building blocks of life in a laboratory setting.

Mitochondria - Enzyme-rich organelles found in the cytoplasm of cells that play an important part in cell metabolism.

mutation - An abrupt change in a gene or chromosome that accounts for differences between one generation of species and the next; plays an essential role in the process of evolution.

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natural selection - Outlines by Darwin, a process whereby only those forms of plant and animal life that are best adapted to reproduction and the environment survive the evolutionary process.

Neptune - The eighth planet from the sun.

neuron - Nerve cell.

neutrino - A massless, chargeless elementary particle that travels with the speed of light and can pass through matter.

neutron - Subatomic particle having no charge and whose mass is approximately equal to that of the proton.

neutron star - A dead star of very high density composed entirely of neutrons.

Newton, Isaac (1642-1727) - English mathematician and natural philosopher who invented calculus and discovered the law of the composition of light and the law of universal gravitation.

nova - A star, possibly a member of a binary star, that suddenly increases in luminosity hundreds or thousands of times.

nucleic acid - Any of two types of acid - RNA and DNA - that carry genetic information.

nucleosynthesis - In stellar evolution, the production of a chemical element from hydrogen nuclei.

nucleotide - A molecule that is the basic component of DNA and RNA.

nucleus - In atoms, the heaviest part of the atom, composed mostly of protons and neutrons. Electrons revolve around the nucleus.

Parmenides - Eleatic philosopher (c.500) who argued that what is inconceivable is impossible, even if the senses tell one that it has, in fact, happened.

particulate ring - A ring of particles in orbit around a planet.

photon - A unit of electromagnetic energy.

photosynthesis - The biologigical process in which plants convert carbon dioxide and water into carbohydrates by absorbing light and energy from the sun into chlorophyll.

Pioneer Venus - Pioneer 11 and 12 space probes that studied the atmosphere of Venus in 1978.

planetary nebulae - A shell of gas around an extremely hot star.

Pluto - The ninth planet from the sun.

principle of covariance - At the heart of Einstein's special theory of relativity, the concept of ultimate equality of all observers.

proton - a heavy subatomic particle with a positive energy charge; one of the two main constituents (the other being the neutron) of the atom.

ptolemies - A succession of Alexandrian rulers who supported the development of the Alexandrian Library.

Ptolemy (second century A.D.) - Alexandrian astronomer and geographer who formulated the rules of astrology, named the stars, discovered how to predict eclipses, and developed a geocentric model to explain the motions of the planets.

pulsar - A small but powerful radio source in space transmitting signals at regular, short intervals.

Pythagoras (fl. 540-510 B.C.) - Greek philosopher and mathematician best known for his so-called Pythagorean theorem and for his quasi-mystical view of the harmonious nature of the universe.

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quark - A subatomic particle having a small electrical charge; may be a constituent of known elementary particles.

quasar - A highly redshifted, star-like object that has a high energy output and that emits blue and ultraviolet light and radio waves; an acronym for quasi-stellar radio source.

radiation - The emission and transmission of energy in the form of particles and waves.

radioactivity - A property of some elements (for example, uranium) of emitting alpha or beta, and sometimes gamma, rays through the disintegration of nuclei.

radio astronomy - Making astronomical observations through the use of radio wavelengths.

R complex - The part of the brain "capping" the brainstem; the seat of aggression, ritual, territoriality, and social hierarchy.

red giant - A large, cool, highly luminous elderly star.

red shift - The shift of a spectrum, particularly of spectral lines, to longer wavelengths.

ring galaxy - A ring-shaped galaxy, probably resulting from the collision of two galaxies.

Rosetta Stone - A stone bearing inscriptions in Egyptian hieroglyphics, demotic characters, and Greek. Discovered in 1799, it gave the first clue to the decipherment of hieroglyphics.

Rutherford, Ernest (1871-1937) - British physicist who developed a model of the atom.

Saturn - The sixth planet from the sun.

Schiaparelli, Giovanni (1835-1910) - Italian astronomer who observed canali (lines) on Mars.

solar wind - A continuous flow of atomic particles ejected from the sun into interplanetary space.

Soviet Venera expeditions - A series of eight expeditions sent by the Soviets to Mars.

spiral galaxy (spiral arms) - a type of galaxy characterized by arms that unwind like a pinwheel.

supernova - A stellar explosion in which a star suddenly increases in luminosity by hundreds of thousands or more times.

terraforming - The process of changing the environment of a previously uninhabitable extra-terrestrial body so that it can support life.

Thales (624-546 B.C.) - Greek philosopher regarded by some as the founder of Greek philosophy and science; measured the height of a pyramid and believed that the world was made not by gods but by the interaction of material forces in nature.

Titan - Moon of Saturn.

trilobites - Tiny, insect-like organisms that flourished in vast herds on the ocean floors some 500 billion years ago.

Tsiolkovsky, Konstantin (1856-1935) - Soviet rocket pioneer who anticipated solutions to problems of spaceflight.

Uranus - The seventh planet from the sun.

Urey, Harold - With Stanley Miller, recreated the basic building blocks of life in the laboratory.

Venus - The second planet from the sun.

Viking 1 and 2 (1976) - U.S. expeditions to Mars which conducted important biological and other experiments.

viroids - The smallest-known living organisms, composed of less than 10,000 atoms.

Voyager 1 and 2 - Two interstellar probes launched in 1977, which encountered Jupiter in 1979.

Wallace, Alfred Russel (1823-1913) - English naturalist who debunked Lowell's Mars theories and formulated a theory of evolution bynatural selection independently of Charles Darwin.

white dwarf - A star in a final stage of its evolution that has exhausted all or almost all of its nuclear fuel and collapsed to small size with high surface temperature and density.