Glossaire beginning with A

a b c d e f g h i j k l m n o p q r s t u v w x y z
Albedo

Fraction of sunlight reflected or scattered by a planet or a comet. The albedo of cometary nuclei, which are very dark, is generally less than 0.1.

Aphelion
The point of the orbit of a planet or comet where it is farthest from the sun.
Asteroid
Small solid body belonging to the Solar system; most asteroid orbits are located between that of Mars and that of Jupiter.
Astrometry
Measurement of position, of apparent movements and of distance of celestial objects (comets, planets, satellites, stars, etc.).
Astronomical Unit (AU or a.u.)
Unit of length equal to the semi-major axis of Earth's orbit (150 million kilometers).