| Cartagena astronomical observatory | España | Canteras | — |
 Carter Observatory | Nueva Zelanda | Región de Wellington | — |
| Casal Lumbroso observatory | Italia | Roma | — |
| Casalecchio di Reno Observatory | Italia | Bolonia | — |
| Casasco observatory | Italia | Casasco | — |
| Cascade Mountain observatory | Estados Unidos | Cascade Mountain | — |
| Casellina observatory | Italia | Scandicci | — |
| Casey Station Magnetic Observatory | — | — | — |
| Casselman observatory | Canadá | Casselman | — |
| CAST Observatory | Italia | Talmassons | — |
| Castellon observatory | España | Castellón de la Plana | — |
| Castelmartini observatory | Italia | Castelmartini | — |
 Castillo de Abbadia | Francia | Hendaya | — |
 Castillo de Blackrock | Irlanda | County Cork | — |
| Castle Fields Observatory | Reino Unido | Calne | — |
| Castlemaine observatory | Australia | Castlemaine | — |
| Castrofilippo observatory | Italia | Castrofilippo | — |
| Cat's Eye observatory | Italia | Manciano | — |
| Catalina 40" Telescope | Estados Unidos | Pima County | 2.791 m |
 Catania Astrophysical Observatory | Italia | Catania | 1.735 m |
| Catania Astrophysical Observatory "A. Riccò" | Italia | Catania | — |
| Catholic University Observatory | Estados Unidos | Washington D. C. | — |
| Cavriana observatory | Italia | Cavriana | — |
 Cawthron Atkinson Observatory | Nueva Zelanda | Nelson | — |
| CBA Concord observatory | Estados Unidos | Tucson | — |
| CBA-East Observatory, Laurel | Estados Unidos | North Laurel | — |
| CBA-MM Observatory | Estados Unidos | Bass Lake | — |
| CBA-NOVAC Observatory, | Estados Unidos | Front Royal | — |
| CCAT Trieste observatory | Italia | Trieste | — |
| CEAM observatory | España | Caimari | — |
| CEAMIG-REA Observatory | Brasil | Belo Horizonte | — |
| Cedar Drive Observatory | Estados Unidos | Hofa Park | — |
| Cedar Glen Observatory | Estados Unidos | Lake Arrowhead | — |
| Cedar Knolls observatory | Estados Unidos | Municipio de Hanover | — |
| Celado, observatory | Italia | Castello Tesino | — |
| Celbridge Observatory | Irlanda | Celbridge Community School | — |
| Celestial Observatory | Estados Unidos | Arkansas | — |
| Celico observatory | Italia | Celico - Centro | — |
| Centennial Observatory | Estados Unidos | Twin Falls | — |
| Central Aerological Observatory | Rusia | Dolgoprudny | — |
| Centre d'astronomie de La Couyère | Francia | La Couyère | — |
| Centre for Basic Space Science and Astronomy | Nigeria | Nsukka | — |
 Centro Astronómico de Tiedra | España | Tiedra | — |
 Centro Astronómico de Toruń | Polonia | Torún | 94 m |
 Centro de Astronomia Espacial | España | — | — |
 Centro de Investigación Atmosférica de Izaña | España | Tenerife | — |
 Centro de Investigación en Astronomía, Astrofísica y Geofísica de Argel | Argelia | Argel | — |
 Centro de investigación en geodinámica y astrometría | Francia | Caussols | — |