atom - Balm
- atomic clocks
- atomic lines
- atomic physics
- atomic processes
- atomic spectra
- atomic time
- atoms
- attenuation
- Auger showers
- auroral arcs
- auroral jets
- auroras
- australites
- autoguiders
- automated telescopes
- automatic patrol telescopes
- autumnal equinox
- auxiliary instruments
- avalanche photodiodes
