Solar System - Glossary

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SOLAR SYSTEM
Jiri W a g n e r , Jr., wag@volny.cz

Glossary

ACCRETION - Accumulation of dust and gas into larger bodies such as stars, planets and moons.
ALBEDO - The ratio of the amount of light reflected by an object and the amount of incident light; a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0).
APHELION - The point in its orbit where a planet is farthest from the Sun; when refering to objects orbiting the Earth the term apogee is used; the term apoapsis is used for orbits around other bodies. (opposite of perihelion).
ASTEROID - (also "planetoid") A medium-sized rocky object orbiting the Sun; smaller than a planet, larger than a meteoroid.
ASTRONOMICAL UNIT (AU) = 149,597,870 km; the average distance from the Earth to the Sun. 1 AU is a long way -- at 100 miles per hour (160 kph) it would take over 100 years to go 1 AU.
BOLIDE - A fireball that produces a sonic boom.
CRATER - Bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano.
DOPPLER EFFECT - The apparent change in wavelength of sound or light caused by the motion of the source, observer or both.
FIREBALL - A meteor brighter than magnitude -3.
KELVIN (K) - 0 Kelvin is absolute zero; water melts at 273 K (= 0° C = 32° F); water boils at 373 K (= 100° C = 212° F).
LIGHT-YEAR = 9.46053e12 km (= 5,880,000,000,000 miles = 63,239 AU); the distance traveled by light in a year.
LUNAR MONTH - The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes. Also called synodic month.
MAGNITUDE - The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude.
METEOR - A bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle (also "shooting star" or "falling star"). Very large, bright ones are called fireballs and bolides.
METEORITE - A rock of extra-terrestrial origin found on Earth.
METEOROID - A small rocky object orbiting the Sun; smaller than an asteroid.
PARSEC = 206265 AU = 3.26 light year.
PERIHELION - The point in its orbit where a planet is closest to the Sun. when refering to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion).
SIDEREAL MONTH - The average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.
SOLAR WIND - A tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are near 350 kilometers per second.
THEORY OF RELATIVITY - More accurately describes the motions of bodies in strong gravitational fields or near the speed of light than newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not specifically for Relativity but rather for his 1905 work on the photoelectric effect and "services to Theoretical Physics").

From The Nine Planets

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