| HCO | 1640 | Recitations in astronomy by Harvard seniors |
| HCO | 1664 | Comet of 1664 observed in New England |
| HCO | 1672 | First Harvard telescope donated by John Winthrop |
| HCO | ca. 1715 | Robie's telescopes on roof of Massachusetts Hall |
| HCO | 1764 | Fire destroys Harvard science instruments in Harvard Hall |
| HCO | 1767 | 19-cm reflector donated by Ben Franklin |
| HCO | 1839 Oct 26 | HCO founded, William Cranch Bond hired as first HCO director |
| HCO | 1839 Dec | Dana-Palmer House (then on present site of Lamont) is HCO's first home |
| HCO | 1839 Dec 31 | Bond makes his first observation from Cambridge |
| HCO | 1842 | HCO moves to Garden St site |
| HCO | 1847 | Great Refractor, 0.38m telescope - equal biggest refractor w. Pulkovo |
| HCO | 1847 | Early daguerrotypes of the Moon |
| HCO | 1848 | Bond discovers Hyperion (Saturn VII) |
| HCO | 1849 | Nautical Almanac moves to HCO |
| HCO | 1849 | Gould starts the AJ in Cambridge |
| HCO | 1850 | First daguerrotype of a star in America (Vega, by Bond) |
| HCO | 1856 | Gould and AJ move to Albany |
| HCO | 1859 | George Bond becomes 2nd director |
| HCO | 1866 | Joseph Winlock is 3rd director |
| HCO | 1866 | Nautical Almanac leaves HCO for USNO |
| HCO | 1875 | Joseph Winlock dies |
| HCO | 1875 | Anna Winlock is first female staff member |
| HCO | 1877 | EC Pickering 4th director of HCO |
| HCO | 1882 | Harvard Photometry published |
| HCO | 1886 | Gould and AJ return to Cambridge |
| HCO | 1887? | Harvard plate collection begins |
| HCO | 1889 | Expedition to Peru to begin southern hemisphere studies |
| SAO | 1889 Nov | Langley starts building observatory, SAO's 1st director |
| SAO | 1890 Mar 1 | Langley's Astrophysical Observatory (APO) founded |
| HCO | 1890 | Draper Catalog of stellar spectra |
| HCO | 1890 | Boyden Stn. est. at Arequipa, Peru |
| SAO | 1895 | Abbot becomes Langley's assistant |
| HCO | 1898 | W Pickering discovers Phoebe (Saturn IX) |
| HCO | 1898 | W Pickering est. HCO Jamaica station |
| HCO | 1898 | W Fleming presents results on O stars at first AAS, |
| HCO | 1903 | First photographic all sky map |
| HCO | 1906? | Revised Harvard Photometry published |
| SAO | 1906 | Abbott is SAO's 2nd director |
| HCO | 1910 | Harvard spectral types adopted as world standard |
| HCO | 1918 | AAVSO created at HCO under Leon Campbell |
| HCO | 1919 | EC Pickering dies |
| HCO | 1922 | Shapley is 5th director |
| HCO | 1927 | Boyden Stn. transferred to Bloemfontein, S Africa |
| HCO | 1934 | "The Telescope" magazine taken over by HCO |
| HCO | 1940 | HAO Colorado site established |
| HCO | 1941 | "The Telescope" merges with "The Sky" |
| HCO | 1941 | "Sky and Telescope" in building C |
| HCO | 1949 | Mayall takes over AAVSO from Campbell |
| HCO | 1951? | HAO transferred to U of Colorado? |
| HCO | 1952 | Menzel is director |
| HCO | 1953 | AAVSO is ejected from HCO amid controversy |
| HCO | 1955 | Boyden Stn transferred to joint ownership |
| SAO | 1955 | FLW is SAO director |
| HCO | 1958 | Sky and Tel moves out of HCO to Bay State Rd |
| HCO | 1960 | Goldberg is director |
| SAO | 1960 | Last solar observing stations closed |
| HCO | 1966 | Sky and Tel severs links with HCO |
| HCO | 1971 | Goldberg steps down as director |
| SAO | 1973 | FLW steps down as SAO director |