Honorary Membership was limited to 10 from 1890-1900, and to 15 from 1901 onwards. Corresponding Membership was a lesser class of Honoris causa membership in the early years. A few honorary members, such as Barnard, were first made corresponding members for a short time before being recommissioned as honorary members.
| Nation(s) | Start | End | |
|---|---|---|---|
| Strömgren, Prof. Bengt G. | Denmark | 1968 | 1987 |
| Struvé, Dr. Otto | USA | 1959 | 1963 |
| Swift, Dr. Lewis A. | USA | 1903 | 1913 |
| Terby, M. François J. | Belgium | 1894 | 1911 |
| Turner, Prof. Herbert Hall | United Kingdom | 1916 | 1930 |
| Underhill, Dr. Anne | Canada | 1996 | 2002 |
| Unsöld, Dr. Albrecht | Germany | 1956 | 1995 |
| van Biesbroeck, Dr. Georges-Achille | Belgium/USA | 1962 | 1974 |
| van de Hulst, Prof. Hendrik | Netherlands | 1968 | 2000 |
| Vogel, Dr. Hermann C. | Germany | 1894 | 1907 |