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Created 10 Sep 99 |
Tangents |
Modified 02 Dec 02 |
A
Layman's
Brief Guide to
Astronomy Terms
albedo: the measure of a non-luminous object's reflectivity. The surface of a light, icy object has a higher albedo than a dark, rocky one. A (theoretical) object that reflects all light has an albedo of 1, while the albedo of an object that absorbs all light (and therefore reflects none) is 0.
aphelion: the point in its elliptical orbit about the Sun at which a planet or other object is farthest from the Sun. [Also see perihelion.]
apogee: the point in its elliptical orbit about the Earth at which the Moon or other object is farthest from the Earth. [Also see perigee.]
asteroid: a relatively small carbonaceous, silicate, or metallic body (up to a few km across). Most asteroids are in orbit about the Sun, occupying a belt between the orbits of Mars and Jupiter. Some other asteroids hold very elliptical orbits, some of which cross the orbits of planets, including that of Earth.
astronomic unit: the mean distance (center-to-center) between Earth and the Sun; the mean radius of Earth's orbit about the Sun, about 92,500,000 miles or 150,000,000 km.
astronomical distance: Because distances in space are so great, it is impractical to measure them in terrestrial units such as miles or kilometers. Therefore, larger units, such as astronomic units (au), light-years (ly), and parsecs (pc) are used to measure interplanetary, interstellar, and intergalactic distances.
black hole: a stellar remnant compressed to a tiny volume of such enormous density that even light is trapped by its intense gravitational field. Black holes are believed to be the result of the collapse of massive stars.
Cepheid variable star: a variable star with an extremely regular frequency of pulsation, whose period is directly related to the star's absolute luminosity. This characteristic has made Cepheid variables important in measuring the distances of galaxies, since the apparent luminosity of objects with the same absolute luminosity varies inversely with the square of their distance from the observer.
cluster: a grouping of a few thousand stars gravitationally bound to each other.
collapsar, collapsed star: See stellar remnant.
comet: a cometary body in an orbit which brings it close enough to the Sun that the solar heat boils off some of its volatile material and drives the ejected material backward, forming a visible "tail."
cometary body: an icy body in orbit about the Sun. The source of cometary bodies is believed to be the Kuiper belt and / or the Oort cloud.
conjunction: an apparent close grouping of two or more celestial objects relative to our line of sight from Earth. (Objects so grouped may in fact be very distant from each other along the line of sight.) Objects grouped on the far side of the Sun (relative to Earth) are said to be at superior conjunction; objects grouped in the night sky opposite to the direction of the Sun are said to be at inferior conjunction.
eclipse: the shadow cast by one non-luminous body (such as the Earth or the Moon) upon another, as it passes between the other body and a self-luminous body (such as the Sun). The shadow consists of two parts, the umbra and the penumbra.
ecliptic: the geometric plane of Earth's orbit about the Sun. Except for Pluto, whose eccentric orbit is inclined by several degrees, the orbital planes of all the planets are very close to the ecliptic, i.e., the orbital plane of Earth.
ellipse, elliptical: the oval shape of the closed orbit of one body gravitationally bound to another.
equinox: a seasonal phenomenon caused by the tilt of the Earth's rotational axis relative to the ecliptic, defined as the time at which the angle of the Sun is directly above the Earth's Equator. There are two equinoxes each year, one in spring (vernal equinox) and one in fall (autumnal equinox), occurring at opposite times of the calendar year in the Northern and Southern Hemispheres. [Also see solstice.]
galaxy, galactic: a large grouping of several billion stars gravitationally bound to each other.
globular cluster: a stellar cluster having an approximately spherical shape.
hyperbola: the curved, open orbit of a non-periodic object. [Also see ellipse.]
Jupiter (adj. Jovian): 5th planet of our solar system, 1st in size and mass.
Kuiper belt: a region beyond the orbit of Neptune containing cometary bodies.
light-year (ly): the distance which light travels through a vacuum in one year, about 9,460,000,000,000 kilometers or 63,000 astronomic units.
luminosity: the amount of light or other radiation which a star or other object gives off.
Luna (adj. Lunar): Moon, Earth's only natural satellite.
lunar: of or pertaining to Luna, the Moon.
lunar eclipse: the darkening of the Full Moon's surface caused by the Earth's shadow when the Earth passes directly between the Sun and the Moon. Lunar eclipses are of three types: (1) penumbral, in which Earth's penumbra only dims the moon; (2) umbral, in which Earth's umbra darkens part of the moon; and (3) total, in which Earth's umbra darkens the moon's entire surface.
lunar phase: the varying pattern of light reflected by the Moon as observed from the Earth, as the Moon revolves around the Earth during the (approximately monthly) lunar cycle. During each cycle, the lunar phases progress in the following sequence: New - Waxing Crescent - First Quarter - Waxing Gibbous - Full - Waning Gibbous - Last Quarter - Waning Crescent - New.
Magellanic clouds: two minor galaxies in the vicinity of our own Milky Way galaxy. The Magellanic clouds are visible only from the Southern Hemisphere.
Mars (adj. Martian): 4th planet of our solar system, 7th in size and mass.
Mercury (adj. Mercurian): 1st planet of our solar system, 8th in size and mass.
meteor: an object which enters Earth's atmosphere from space at such a speed that air friction causes it to heat to incandescence. Most meteors are smaller than a grain of sand, and completely vaporize upon entering the atmosphere.
meteor shower: a marked increase in meteor strikes occasioned by the Earth's passage through the debris trail of a comet. Some meteor showers, particularly the Eta-Aquarids (April), Perseids (August), Leonids (November), and Geminids (December), often reach spectacular levels.
meteorite: a meteor which is not completely vaporized in Earth's atmosphere, and which therefore strikes the Earth's surface.
meteoroid: an object in space whose path may cause it to enter Earth's atmosphere as a meteor. Meteoroids are of four main types: ice (ammonia or water), carbonaceous (organic compounds), silicate (rock), and metallic.
Milky Way: the galaxy in which our own solar system is located.
Neptune (adj. Neptunian): 8th planet of our solar system, 4th in size and mass.
neutron star: a stellar remnant, the subatomic particles of whose core have been gravitationally compressed into neutrons.
nova: a stellar explosion, in which a star blows off its outer layers of gas.
observable universe: that portion of the universe which can be detected with available technology.
Oort cloud: a roughly spherical array of of cometary bodies orbiting the Sun at a distance of from 1 to 2 light-years.
orbit: the curved path through which one celestial body moves as it revolves about another body. An orbit may be elliptical (closed, or periodic), as with planets, satellites, and periodic comets; or hyperbolic (open, or single-pass), as with transient comets.
parabola: the open, curved trajectory of a free-falling object under the influence of gravity. (This is a convenient approximation for objects propelled from and returning to the surface, such as artillery shells; the true trajectory of an object in free-fall is either an ellipse or a hyperbola).
parallax: the apparent shift in position, relative to a fixed background, exhibited by an object when viewed from different angles.
parallax-second / parsec (pc): the distance at which an object exhibits a stellar parallax of one arc-second (1/3600 of a degree of arc), relative to the fixed stars, as viewed from opposite ends of the Earth's orbit about the Sun. One parsec equals 3.258 light-years.
penumbra: the lighter part of a shadow cast by a non-luminous body, where it only partially obscures the light from a self-luminous body. [Also see umbra, eclipse.]
perigee: the point in its orbit about the Earth at which the Moon or other object is closest to the Earth. [Also see apogee.]
perihelion: the point in its orbit about the Sun at which a planet or other object is closest to the Sun. [Also see aphelion.]
period, periodic, periodicity: the time required for a repeating event, such as an orbit or a pulsation, to complete each cycle.
planet: a major non-luminous body in orbit about the Sun or another star. The known planets of our Sun are (in order of their distance from the Sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
plasma: a phase of matter in which atomic nuclei and their electrons become unbound.
Pluto (adj. Plutonian): 9th planet of our solar system, 9th in size and mass.
primary: the massive body about which a less massive body is in orbit. The Earth's primary is the Sun; the Moon's primary is the Earth.
proper motion: the movement of a celestial object across the observer's line of sight. [Compare radial motion.]
pulsar (pulsating star): a rapidly spinning neutron star emitting a beam of radiation which comes in line with the Earth with each rotation (whose period may be on the order of a fraction of a second), thereby producing a pulsating effect.
quasar, quasi-stellar object: a very distant and extremely radiative object of unknown characteristics.
radial motion: movement of a celestial object along the observer's line of sight (either approaching or receding). [Compare: proper motion.]
revolution, revolve: the orbital movement of one object around another, as Earth around the Sun. [Compare: rotation.]
rotation, rotate: the movement of an object spinning about its own axis. [Compare: revolution.]
satellite: a minor non-luminous body in orbit about a planet; a moon is a natural satellite.
Saturn (adj. Saturnian): 6th planet of our solar system, 2nd in size and mass.
Sol (adj. Solar): our star, the Sun, central and most massive object of our solar system.
solar: of or pertaining to Sol, the Sun, our star.
solar eclipse: the darkening of part of the Earth's day-lit surface caused by the New Moon's shadow when the Moon passes directly between the Sun and the Earth. Solar eclipses are of three types: (1) partial, in which the Moon obscures only a part of the Sun's visible disk; (2) annular, in which the Moon obscures the center, but not the circumference, of the Sun's disk; and (3) total, in which the Moon obscures the Sun's entire disk.
solar system: the grouping of planets, satellites, asteroids, and cometary bodies gravitationally bound to and in orbit about the Sun; also, any similar grouping of bodies in orbit about another star.
solstice: a seasonal phenomenon caused by the tilt of the Earth's rotational axis relative to the ecliptic, defined as the time that the Sun's angle from Earth's Equator is greatest, above either the Tropic of Cancer to the north, or the Tropic of Capricorn to the south. There are two solstices each year, one in summer, the other in winter, occurring at opposite times in the calendar year for Northern and Southern Hemispheres. [Also see equinox.]
star: a massive, self-luminous celestial body, such as the Sun.
stellar: of or pertaining to a star, or stars.
stellar parallax: the angular difference in the perceived position, relative to fixed stars, of an object when viewed from opposite ends of Earth's orbit.
stellar remnant (a.k.a. collapsar): the collapsed core of a star whose gravity is no longer balanced by the fusion processes of normal stars. A stellar remnant may take the form of a white dwarf, a neutron star, or a black hole, depending on the mass of the star which formed it.
Sun: the star (named Sol) at the center of our own solar system.
Terra / Tellus Mater (adj. Terran, Terrestrial, Tellurian): Earth, 3rd planet of our solar system, 5th in size and mass. These are Latin names for our planet. Terra means land; Tellus Mater means Mother Earth.
trajectory: the path of a moving object through space.
umbra: the darkest part of the shadow cast by a non-luminous body, where it completely obscures the light from a self-luminous body. [Also see penumbra, eclipse.]
universe: all physical matter and energy everywhere, taken as a whole. [Compare: observable universe]
Uranus (adj. Uranian): 7th planet of our solar system, 3rd in size and mass.
variable star: a star whose luminosity increases and decreases over a period ranging from a few days to several years.
Venus (adj. Venerian): 2nd planet of our solar system, 6th in size and mass.
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