Aberration of starlight. As light does not move infinitely fast, but at a rate of practically 300 000 km s–1, and as the Earth is moving round the Sun at an average velocity of 25 km s–1, the stars appear to be shifted slightly from their true positions. The best analogy is to picture a man walking along in a rainstorm, holding an umbrella. If he wants to keep himself dry, he will have to slant the umbrella forward; similarly, starlight seems to reach us ‘from an angle’. Aberration may affect a star's position by up to 20.5 seconds of arc.

Ablation. The erosion of a surface by friction or vaporisation.

Absolute magnitude. The apparent magnitude that a star would have if it could be observed from a standard distance of 10 parsecs (32.6 light-years).

Absolute zero. The coldest theoretically possible temperature: –273.16 °C.

Accretion disc. A disc structure which forms round a spinning object when material falls on to it from beyond.

Achromatic object-glass. An object-glass which has been corrected so as to eliminate chromatic aberration or false colour as much as possible.

Aerolite. A meteorite whose main composition is stony.

Airglow. The light produced and emitted by the Earth's atmosphere (excluding meteor trains, thermal radiation, lightning and auroræ).

Albedo. The reflecting power of a planet or other non-luminous body. The Moon is a poor reflector; its albedo is a mere 7% on average.

Alfvén wave. A low-frequency travelling oscillation of the ions and the magnetic field of a plasma.