A
accretion
Accumulation of dust and gas into larger bodies.
Adams, John Couch 1819-1892
English astronomer and mathematician. At the age of 24, Adams was the first person to predict the position of a planetary mass beyond Uranus. After Johann Gottfried Galle confirmed the existence of Neptune based on independent calculations done by Urbain Jean Joseph Le Verrier, the two became embroiled in a dispute over priority.
albedo
Reflectivity of an object; ratio of reflected light to incident light.
albedo feature
A dark or light marking on the surface of an object that might not be a geological or topographical feature.
allocthonous
(1) Material that is formed or introduced from somewhere other than the place it is presently found. (2) Fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.
Alpha Centauri
The closest bright star to our solar system.
angstrom
A unit of length = 1.0E-08cm.
antipodal point
The point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole.
aphelion
The point in its orbit where a planet is farthest from the Sun.
apoapsis
The point in orbit farthest from the planet.
apogee
The point in orbit farthest from the Earth.
ash
The fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of less than 2 millimeters.
asteroid number
Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
astronomical unit (AU)
The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).
atmosphere
One atmosphere is 14.7 pounds per square inch; standard atmospheric pressure at sea level on Earth.
aurora
A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.
aurora borealis
The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.