- A dwarf stars
- A giant stars
- A stars
- A subdwarf stars
- A subgiant stars
- A supergiant stars
- Abell clusters
- Absolute magnitude
- Abundance ratio
- Accelerating universe
- Accreting binary stars
- Accretion
- Achondrites
- Active galactic nuclei
- Active galaxies
- Active sun
- Adaptive interferometry
- Ae stars
- Aerolites
- Aeronomy
- Alfven waves
- Algol variable stars
- All-sky cameras
- Almanacs
- Alpha2 Canum Venaticorum stars
- Alt-azimuth mounts
- Alternatives to cosmic inflation
- Altitude angle
- AM Canum Venaticorum stars
- AM Herculis stars
- Am stars
- Amateur astronomers
- Amateur astronomy
- Amor group
- Amorphous irregular galaxies
- Analytical mathematics
- Andromeda Galaxy
- Angular separation
- Annual equation
- Annual parallax
- Annular eclipses
- Anomalistic month
- Ansae
- Antapex
- Antarctic observatories
- Anthropic principle
- Antumbra
- Ap stars
- Apastron
- Aperiodic comets
- Aperture synthesis
- Apex
- Aphelion
- Apocenter
- Apogee
- Apollo group
- Apparent magnitude
- Apparent position
- Apparent superluminal motion
- Apsidal motion
- Apsis
- Archaeoastronomy
- Argument of perigee
- Argument of perihelion
- Armillary spheres
- Artificial satellites
- Ascending node
- Asteroid belt
- Asteroid occultation
- Asteroids
- Asteroseismology
- Astrobiology
- Astrochemistry
- Astrodynamics
- Astrographic catalogs
- Astroinformatics
- Astrometric binary stars
- Astrometry
- Astronomers
- Astronomical coordinate systems
- Astronomical databases
- Astronomical detectors
- Astronomical location
- Astronomical models
- Astronomical object identification
- Astronomical optics
- Astronomical radiation sources
- Astronomical reference materials
- Astronomical research
- Astronomical seeing
- Astronomical sextants
- Astronomical site protection
- Astronomical unit
- Astroparticle physics
- Astrophotography
- Astrophysical black holes
- Astrophysical dust processes
- Astrophysical explosive burning
- Astrophysical fluid dynamics
- Astrophysical magnetism
- Astrophysical masers
- Astrophysical processes
- Astrophysicists
- Astrosphere interstellar medium interactions
- Astrospheres
- Asymptotic giant branch
- Ataxites
- Aten group
- Atira group
- Atmospheric circulation
- Atmospheric effects
- Atmospheric extinction
- Atmospheric refraction
- Atmospheric science
- Atmospheric scintillation
- Atmospheric tides
- Atom interferometry
- Australites
- Automated telescopes
- Automatic patrol telescopes
- Autumnal equinox
- Azimuth
a community supported, open source project from the American Astronomical Society