A
- Ablation
- Absolute Magnitude
- Absorption Line
- Abundance Ratio
- Active Galactic Nuclei
- Airy Disk
- Albedo
- ALMA
- Alternative Cosmologies
- Altitude
- Amor Asteroids
- Amplitude
- Anaximander
- Anaximenes
- Andromeda galaxy
- Angle
- Angular Diameter
- Angular Frequency
- Angular Momentum
- Angular Velocity
- Anomalistic Year
- Anomalous Cosmic Rays
- Anomalous X-ray Pulsar
- Antarctic Circle
- Apastron
- Aperture
- APEX
- Aphelion
- Apoapsis
- Apogee
- Apollo Asteroids
- Apparent Diameter
- Apparent Magnitude
- Arcminute
- Arcsecond
- Arctic Circle
- Area
- Argument Of Perihelion
- Ascending Node
- Asterism
- Asteroid
- Asteroid Belt
- Astrolabe
- Astronomical Unit
- Astronomy