- aplanatic system
- apoapsis
- apoastron
- apocenter
- apogalacticon
- apogee
- Apollo
- apparent area
- apparent libration
- apparent magnitude
- apparent place, apparent position
- apparent sidereal time
- apparent solar time
- apparent superluminal motion
- apsidal line
- apsis, apse
- arch filament system, AFS
- archaeoastronomy
- Argelander method
- argument of periapsis
- arm population
- armillary sphere
- artificial satellite
- ascending node
- asterism
- asteroid-like object
- asteroidal belt
- astrobiology
- astrobleme
- astrochemistry
- astrochronology
- astroclimate
- astrodynamics
- astrogeodynamics
- astrogeography
- astrogeology
- astrogeophysics
- astrograph
- astrolabe
- astrology
- astrometric binary
- astrometric distance
- astrometric position, astrometric place
- astrometry
- astronavigation, celestial navigation
- Astronomer Royal
- astronomical day
- astronomical ephemeris, astronomical almanac
- astronomical latitude
- astronomical longitude