| 150-foot solar tower | — | — |
| 7300 observatory | Mayhill | — |
 A.C. Barnes Observatory | Ithaca | — |
 Academia Naval de los Estados Unidos | Maryland | — |
| Adena Brook Observatory | Columbus | — |
| Adolphson Observatory | Monmouth | 232 m |
 Advanced Composition Explorer | — | — |
| Advanced X-Ray Imaging Satellite | — | — |
| AHS Observatory | Castaic | — |
 Air Force Maui Optical and Supercomputing observatory | Maui County | — |
| Alder Springs observatory | Alder Springs | — |
| Allen observatory | Bob Dylan | — |
| Altimira observatory | Coto de Caza | — |
| Angel Peaks observatory | Creek | — |
 Angell Hall Observatory | Míchigan | — |
| Antelope Hills Observatory | Bennett | — |
| Apex observatory | Carolina del Norte | — |
| Apollo Observatory | Dayton | — |
| Arcturus Observatory | Municipio de Manchester | — |
| Area 52 Observatory | Nashville | — |
| Argus Array | Texas | — |
| Arizona Radio Observatory | Arizona | — |
| Arkansas Sky Observatory, Petit Jean Mountain South | Petit Jean Mountain | — |
| Arkyd-6A | — | — |
 Ashton Observatory | Iowa | — |
| Assonet observatory | Assonet | — |
 ASTERIA | — | — |
 Astronaut Memorial Planetarium and Observatory | Florida | — |
| Astronomical League Alpha observatory | Portal | — |
 Astronomical Observatory | Urbana | — |
| Astronomical Research Observatory, Charleston | Charleston | — |
| Astronomical Research Westfield observatory | Westfield | — |
| Astrophysical Observatory of the College of Staten Island | Nueva York | — |
| ATLAS-MLO Auxiliary Camera | Waiākea-Uka | — |
| ATLAS–HKO observatory | Maui County | 3.030 m |
| ATLAS–MLO observatory | Hawaii County | 3.408 m |
| ATU Astronomical Observatory | Russellville | — |
 Badlands Observatory | Quinn | — |
| Bagdad observatory | Bagdad | — |
| Baker Observatory | Misuri | — |
| Bakersfield observatory | Bakersfield | — |
| Ball State University Observatory | Indiana | — |
| Bar J Observatory | Lake Ashby | — |
 Barnard Observatory | Oxford | — |
| Barrow Magnetic Observatory | — | — |
| Battle Point Observatory | Bainbridge Island | — |
| Behlen Observatory | Nebraska | — |
| Berkeley Heights observatory | Berkeley Heights | — |