| Amanecer de Arrakis observatory | España | Alcalá de Guadaíra | — |
| Ametlla del Valles observatory | España | Can Reixac (Sant Gregori) | — |
 Amphi Ashdod Observatory | Israel | Ashdod | — |
| An Carraig Observatory | Reino Unido | Newry | — |
| Ananjev observatory | Ucrania | Raión de Ananiv | — |
 Ancien observatoire astronomique de la Marine de Nantes | Francia | Nantes | — |
| Andean Northern observatory | Finlandia | Nummi-Pusula | — |
| Andromeda Astronomical Observatory | Bulgaria | Stolichna obshtina | — |
| Andromeda Observatory | Hungría | Miskolc | — |
| Angel Peaks observatory | Estados Unidos | Creek | — |
 Angell Hall Observatory | Estados Unidos | Míchigan | — |
 Aniane astronomical observatory | Francia | Aniane | — |
| Antananarivo Magnetic Observatory | Madagascar | — | — |
| Antananarivo Observatory | — | — | — |
 Antares Astronomical Observatory | Brasil | Feira de Santana | — |
| Antares MTM Observatory | Italia | San Donato Val di Comino | — |
| Antares observatory | Argentina | Provincia de Buenos Aires | — |
| Antelope Hills Observatory | Estados Unidos | Bennett | — |
| Antibes observatory | Francia | Antibes | — |
 Antiguo Observatorio de Moscú | Rusia | Presnensky District | — |
| Antikythira Bird Observatory | Grecia | Kythira Municipality | — |
 Antonín Bečvář observatory | República Checa | Most | 395 m |
| Anunaki Observatory | España | Rivas-Vaciamadrid | — |
| Anwar Damha observatory | Brasil | São Paulo | — |
| Anysllum observatory | España | Ager | — |
| Anzio observatory | Italia | Lavinio Lido di Enea | — |
| Aorangi Iti Observatory | Nueva Zelanda | — | — |
| Apex observatory | Estados Unidos | Carolina del Norte | — |
| Apia Magnetic Observatory | Samoa | — | 2 m |
| Apollo Observatory | Estados Unidos | Dayton | — |
 Apollo Telescope Mount | — | — | — |
| Appledorne Observatory | Reino Unido | Farnsfield | — |
| Aqaba Bird Observatory | Jordania | Gobernación de Aqaba | — |
| Aravaca Observatory | España | Madrid | — |
| Arbonne la Foret observatory | Francia | Arbonne-la-Forêt | — |
 ARCADE | — | — | — |
| Arcadia observatory | Australia | Arcardia Gas Field | — |
| Arcangel observatory | España | Las Zocas | — |
 Archeops | — | — | — |
| Arcturus Observatory | Estados Unidos | Municipio de Manchester | — |
| Area 52 Observatory | Estados Unidos | Nashville | — |
| AREA31 Radio Observatory | Canadá | Shelburne | — |
| Arenal-Miravalles Seismological and Volcanological Observatory | Costa Rica | — | — |
| Argelander Institute for Astronomy observatory | Alemania | Bonn | — |
| Argentine Island Magnetic Observatory | — | — | — |
| Argus Array | Estados Unidos | Texas | — |
| Arizona Radio Observatory | Estados Unidos | Arizona | — |
| Arkansas Sky Observatory, Petit Jean Mountain South | Estados Unidos | Petit Jean Mountain | — |