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Glossary of Physics Terms

Absolute Magnitude The magnitude that would be assigned to a star if it were placed at a distance of 10 parsecs from the observer. Stars closer to us than that distance would appear fainter. Stars farther from the solar system would appear brighter.


Achromatic Lens A lens that transmits white light without dispersing it into a color spectrum. It usually consists of two component parts, cemented together to form one unit.

Albedo Percentage of light reflected by a body, such as a planet, of total amount of light falling on it.

Altitude Angular distance between the horizon and a given object, measured along a vertical circle.


Annular Eclipse An eclipse of the central portion of the solar disk; an outer ring shows.

Aphelion The point on planet's orbit farthest from the sun.


Apogee Point on the moon's orbit farthest from the earth.

Apollo The name assigned to the U.S. project, whose mission was to land men on the moon. Also the name of the vehicles used. Apollo 11 landed Neil Armstrong and Edwin Aldrin on the moon on July 20, 1969.

Artificial Satellite A man-made object placed into an orbit about the earth or about another celestial body such as the sun or the moon.

Astronomical Unit The average distance between the earth and the sun 93 million miles or, more exactly, 92,955,700.

Aurora A diffused glow of light in the form of curtains, or bands, seen at high latitudes (70°N or 70°S). The glow is due to the interaction between the solar wind and particles in the earth's atmosphere. The aurora in the northern hemisphere is known as the Aurora Borealis, or northern lights; in the southern hemisphere it is known as the Aurora Australis, or southern lights.

Binary Star Two close stars held together by a gravitational force and revolving like a dumbbell about a common center of gravity. The center is closer to the more massive star.


Cassini's Division The empty space that separates the outer rings of Saturn from the bright inner rings.

Celestial Sphere An imaginary sphere of infinite radius surrounding the earth and serving as a screen against which all celestial objects are seen.

Cepheid A star the brightness of which varies periodically because of pulsations.

Chromatic Aberration (also called Color Defect) Blurring of image due to the separation of colors by a lens. A point of white light in the object appears as a complete spectrum of colored points in the image.


Collimator (also called Collimating Lens) A lens whose function it is to make rays of light parallel.


Colure, equinoctial (also called Prime Hour Circle) The hour circle that goes through the first point of Aries. The hour angles (same as longitude on earth) are measured from the equinoctial colure.

Conjunction Apparent line-up of sun, earth, and a planet. Inferior conjunction is when the planet is between the earth and the sun.


Superior conjunction is when the planet is on the opposite side of the sun.


Constellation A group of stars apparently close together in the sky. Modern astronomy recognizes 88 such groups (e.g., Cassiopeia, Leo, etc.). Actually, the individual stars of a constellation may be great distances apart and moving in different directions one from the other.

Copernican System The system that assumes that the sun is at the center, and the earth and the other planets move around it.

Culmination The position of a celestial body when it is on the meridian. A star is said to be at its “upper culmination” when it has reached its highest point for the day.

Declination Angular distance of an object from the celestial equator, measured in degrees, minutes, and seconds. Analogous to latitude in geography.


Diffraction of Light A phenomenon exhibited by light on passing through a narrow slit or a small aperture. The light is modified to form alternate dark and bright fringes.

Discrete Source A amall area in the sky—almost a point—from which very intense electromagnetic waves of radio frequency reach the earth. These points were formerly called radio stars.

Doppler Effect Change in frequency of light due to relative motion between observer and source of light.

Eccentricity Eccentricity indicates the degree of flatness of an ellipse, or its departure from a circle. It is denoted by e; its value is obtained from the formula image, when 2c is the distance between the foci, and 2a is the length of the major axis. When c is small (e.g., .05), the ellipse approaches a circle in shape; when it is large (e.g., .8), the ellipse is elongated.



A. Solar. The sun's light is cut off by the moon's interposition between the sun and the earth.

B. Lunar. The moon darkens because the earth intercepts the sunlight on its way to the moon.


Ecliptic Two equivalent definitions are possible.

A. The great circle on the celestial sphere formed by the intersection of that sphere with the plane of the earth's orbit.

B. The path described on the celestial sphere by the sun during its apparent annual motion around the earth.

Elongation Angular distance from the sun, measured in degrees, minutes, and seconds of angle.

Ephemeris A book of tables showing computed daily positions of heavenly objects.

Equinox One of the points of intersection between the ecliptic and the celestial equator. When the sun is at one of these two points, the length of day and night are equal everywhere on the earth. The sun is at these points every year on or about March 21 (vernal) and September 23 (autumnal).

Evening Star This is not a star, but a planet, especially Mercury or Venus when seen in the western sky just after sunset.

Extragalactic Beyond our galaxy.

Faculae Areas on the surface of the sun that appear brighter by comparison to surrounding regions.

Flyby A research mission in which the satellite collects data while passing close to the object of research.

Galaxy A large community of stars in space, such as the Milky Way (our galaxy), to which the sun belongs. Galaxies contain billions of stars. Many are shaped in the form of a spiral.

Gemini The name given to the U.S. program, as well as to the vehicles, designed to prepare man for landing on the moon. Gemini 3 to Gemini 12 (1965–66) carried crews of two astronauts each. The program included space walks, rendezvous with other space craft, as well as docking techniques.

Granules The smallest visible units on the sun's surface. Granules or granulations have diameters hundreds of miles long. They change in size and in structure continuously.

Heliocentric Parallax The apparent motion of nearby stars seen against the background of far away stars. The apparent motion is actually due to the revolution of the earth around the sun.

Helmholtz Contraction The theory that the energy of the light emitted by a star is derived from the gravitational potential energy (i.e., contraction) of the star.

Hertzsprung-Russell Diagram A diagram showing a scatter distribution of stars according to luminosity and temperature. The scatter distribution is related to the various ages of the stars.

Hour Angle This is analogous to longitude in geography: an angle between the local celestial meridian and the hour circle of a given object in the sky, measured westward from the meridian. It may be given in units of time (hours, minutes and seconds); 1 hour = 15 degrees of angle. Hour angles are easily visualized as either arcs along the celestial equator, or angles at the celestial poles.

Hour Circle This is similar to meridian in geography: a great circle passing through the two celestial poles.


Hubble's Constant The ratio of the velocity of recession to the distance of a galaxy. This ratio is 100 km/sec for every 1 million parsecs.

Infrared Radiation Invisible radiation of wavelength slightly longer than red light.

Ionosphere Several layers of ionized air high in the atmosphere. The ionosphere plays an important part in reflecting radio waves.

Libration Apparent “rocking” or “nodding” of moon or the planet Mercury. Due to this oscillation, some of the usually hidden sides are exhibited to the terrestrial observer.

Light Year The distance that light travels in a year.

Luminosity The ratio of the total light emitted by a celestial object to the total light emitted by the sun. Also, the total energy emitted by a star, per second.

Lunar Module The term for the vehicles that carried two men, in the Apollo project, from the command modules to the surface of the moon and back.

Magellanic Clouds These are not clouds; they are galaxies. Two relatively nearby galaxies visible from the southern hemisphere, of irregular shape, named after Magellan, the Portuguese explorer who first described them.

Magnitude (also called Apparent Magnitude) A number indicating the apparent brightness of a star. Bright stars are designated by small numbers (magnitude 1, say) while dim stars are designated by large numbers (e.g., magnitude 15).

Main Sequence A band in the scatter Hertzspung-Russell diagram. It includes more than 80 percent of all stars. The energy emitted by these stars is obtained from thermonuclear reactions in the core of the stars.

Mariner The name given to a U.S. series of space probes designated to obtain data from Venus, Mercury, and Mars. On December 14, 1962, Mariner 2 passed within 22,000 miles of Venus. Mariner 9 (launched on May 30, 1971) came within a distance of 900 miles of Mars. It was the first spacecraft to go into orbit around a planet other than earth. Mariner 10 (launched November 3, 1973) came within 3,600 miles of Venus and 450 miles of Mercury. This was the first probe of Mercury.

Mass-Luminosity Relationship This relationship, which applies to main sequence stars, states that luminosity is proportional to Ma, where M = mass and the power a = 4½, for stars whose mass is less than half the sun; a = 3½ for stars whose mass is more than 1½ that of the sun.

Meteor A meteoroid during the time it is giving off light. Also called a shooting star.

Meteorite A meteoroid that survived, because of its size, collision with the earth's atmosphere and reached the earth's surface. Meteorites can be seen on exhibit in many natural history museums.

Meteoroid A tiny solid object, usually the size of a sand particle, which the earth encounters in its orbit around the sun.

Micrometeorite A fine dust particle floating in space, too small to be seen with unaided eye and too small to become incandescent during its passage through the atmosphere.

Milky Way A luminous band across the sky, of which our galaxy is part. The light is due to the fact that the vast majority of stars in our (disk-shaped) galaxy are located along this narrow band on the celestial sphere.

Morning Star This is not a star, but a planet, e.g., Mercury, when seen in the eastern sky just before sunrise.

Neap Tide The lowest tide of the month.

Nebula A vast cloud of gas or gas and dust in space.

Node A point of intersection of one orbit (say, the moon's) with the plane of another orbit (say, the earth's).

Nova A star that suddenly increases in brightness and later returns to its original value of brightness.

Occultation The eclipse of a star or a planet by the moon. The term occulation also applies to any large astronomical body passing between a smaller body and the observer.

Parallax Apparent shift of position of an object with respect to its background due to a shift in position of the observer. An observer at A sees the star in region a of the sky. An observer at B sees the same star in region b.


Parsec A unit of stellar distance. It is the distance BD in the triangle ABD.

One parsec is equal to 19.2 million million miles.


Penumbra The outer and lighter part of a shadow cast by a planet or satellite, as in regions A and B.


Perigee The point in orbit of, say, the moon nearest the earth (point A).


Perihelion The point in orbit of a planet or a comet nearest the sun (point A).


Photosphere The visible surface of the sun or star. Below it is the interior of the sun; above it, the atmosphere.

Pioneer The name given to the first series of U.S. unmanned space-probe vehicles. They were instrumental in developing launching and guiding techniques for the Mariner, Lunar Orbiter, Ranger, and Surveyor series.

Planet One of the nine bodies revolving about the sun in almost circular orbits. Planets are made visible to us by reflected sunlight. It is reasonable to assume that many stars have planets revolving about them.

Planetary Nebula A nebula resembling a planet in shape.

Planetoid (also called Asteroid) A small irregularly shaped solid body revolving about the sun. Also considered to be a minor planet.

Plasma An ionized gas.

Poles, Celestial Points of intersection of extensions of earth's axis and the celestial sphere.

Precession The slow change in direction of the earth's axis due to the gravitational pull of the moon on the bulge at the earth's equator. The slow change in the axis causes the westward motion of the equinoxes among the constellations.

Proper Motion of Star The angular velocity (in seconds of angle per year) of a star in a direction perpendicular to the line of sight of a terrestrial observer.

Protostar The portion of a nebula that is about to become a star.

Pulsar A neutron star emitting pulsed radio signals. The first pulsar was discovered in 1967. Its pulse lasts ⅓ of a second and repeats with great regularity every 1⅓ second.

Pulsating Stars Stars that periodically vary in brightness because of periodic changes in volume.

Quadrature An elongation of 90° east or west of the sun.

Quasar The popular name for quasi-stellar object. These are extremely luminous objects (the most luminous known) at enormous distances (the most distant object known), which generate incredible amounts of energy. The true nature of quasars is still under study.

Radial Velocity of Star The velocity (in miles or kilometers per second) in line of sight of a terrestrial observer.

Radiant Point of Meteors A point in the sky from which meteors seem to come.

Radio Astronomy The branch of astronomy that deals with the radio waves emitted by various celestial bodies, as well as the theory of their emission.

Radio Star See Discrete Source.

Radio Telescope An instrument used for examination of celestial objects by means of the radio waves emitted by these objects.

Radio Window The transparency of the atmosphere to radio waves that range between .25 cm and 30 m in length.

Ranger The name given to a series of nine U.S. lunar probe vehicles, designed to transmit photographs before crashlanding on the moon. More than 17,000 photographs were obtained from the Rangers, the closest one taken .2 seconds before impact.

Red Giant A member of the giant sequence in the Hertzsprung-Russell diagram. They have radii fifteen to thirty times larger than the radius of sun and luminosities a hundred times that of the sun.

Red Shift The shift of all spectral lines toward longer wavelengths observed in all galaxies. Galactic red shift is due to the expansion of the universe. Gravitational red shift is due to the high value of the mass of the emitter.

Refraction A change in direction of light on entering a different medium (such as glass).


Resolving Power of Telescope The power to separate two close points into two distinct units.

Retrograde Motion Apparent backward (westward) motion of a planet, through a starfield.

Reversing Layer The lowest of the three solar atmospheric layers; it is responsible for most of the dark lines in the solar spectrum.

Right Ascension The angular distance from the prime meridian to a celestial body measured eastward along the celestial equator from 0° to 360°, or from 0 to 24 hours. Analogous to longitude in geography.

Rocket A tube designed to move through space that derives its thrust by ejecting hot, expanding gases—called a jet—that have been generated in its motor. The rocket contains within itself all the material needed for the production of the jet.

Saros The interval (about 18⅓ years) between two successive lunar or solar eclipses of the same series.

Satellite A celestial body revolving about one of the planets; e.g., the moon. Also any small body that revolves about a larger body, man-made or otherwise.

Sedimentary Rocks Rocks formed by precipitation from water or any other solution.

Service Module That part of the vehicle in the Apollo and other programs that contains the power, supplies, and fuel.

Sidereal Period The interval of time required by a planet to make one revolution (as seen from one of the fixed stars) about the sun.

Solar Constant The quantity of radiant solar heat, 1.94 calories per minute, received per square centimeter of the earth's surface.

Solar Wind The material, mainly protons and electrons, streaming out from the sun into space. The sun loses normally millions of tons per second of its mass because of this wind.

Solstice The point of maximum declination of the earth on the ecliptic. The solstices are halfway between the equinoctial points. In the northern hemisphere, the summer solstice occurs when the sun is farthest north from the equator; the winter solstice, when the sun is farthest south.

Space Probe An unmanned vehicle that is sent into space to obtain scientific data.

Spectrograph An instrument that (A) collimates (makes parallel rays of light), then (B) disperses the light (by means of a prism or grating) into a spectrum, and, finally, (C) produces a photograph of the spectrum.

Spectroheliograph An instrument that photographs the sun in monochromatic (single color) light.

Spectroscopic Binary A system of two stars that can be detected only with the aid of a spectroscope.

Spherical Aberration A shape defect of a lens. Light passing a spherical lens near its edge is converged more than light passing the center of the same lens, which causes the image to blur.

Spiral Nebula A galaxy of stars (not a nebula) in the form of a spiral.

Star A large globe of intensely hot gas, shining by its own light (e.g., the sun).

Sunspots Dark (by contrast with the surroundings) patches that appear from time to time on the photosphere of the sun.

Supernova A star that quite suddenly increases, perhaps a million times, in brightness. It is similar to a nova, but its increase is vastly greater. It never fully returns to its original brightness.

Surveyor The name given to a U.S. series of unmanned probes that landed on the moon to obtain data prerequisite for a manned landing.

Synodic Period The interval of time required by a planet or the moon to complete one revolution, as seen from the earth.

Telescope In astronomy, an instrument used to collect radiation from celestial objects.

Terminator The boundary between the illuminated and dark portion of the moon or a planet.


Transit The motion of a small body (e.g., Mercury) across the face of a larger body (e.g., the sun).

Tropical Year The ordinary year. The year used in everyday life.

Umbra The dark shadow cast by a planet or a satellite.


Universal Time Greenwich time. The mean solar time at the 0° meridian passing through Greenwich, England. Used as a world standard.

Van Allen Radiation Belts Two doughnut-shaped regions surrounding the earth in which high-energy particles are trapped.

Velocity of Escape The velocity that an object must acquire in order to escape from the gravitational pull exerted upon it by another body. The velocity of escape at the earth's surface is 7 miles per second. Any terrestrial body that can reach this velocity will permanently leave the earth.

Venera The name given a U.S.S.R. series of space probes made to collect data about Venus.

White Dwarf A white star of low luminosity, small size, and extremely high density.

Zeeman Effect A change in wavelength of emitted light when the source of light is placed in a magnetic field.

Zenith The point on the celestial sphere directly overhead.

Zodiac A band of the celestial sphere, 8° wide, on each side of the ecliptic. The zodiac is divided lengthwise into twelve equal sections; each 30° section is popularly identified by a “sign of the zodiac.” The signs are named for the constellations that were located in them at the time of Hipparchus more than 2,000 years ago. The sun, moon, and planets appear to “travel” within this belt. The sun passes through three signs each season of the year, e.g., during spring the sun passes through Aries, Taurus, and Gemini.

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