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 | |
|---|---|---|---|
| Pickering, Dr. Edward Charles, OPM | USA | 1894 | 1919 |
| Pickering, Mr. William Henry | USA | 1894 | 1938 |
| Plaskett, Prof. Harry H. | Canada/United Kingdom | 1961 | 1979 |
| Proctor, Miss Mary | USA | 1896 | 1957 |
| Proctor, Mrs. Richard A. | USA/Ireland | 1891 | 1896 |
| Reber, Mr. Grote | USA/Australia | 1988 | 2002 |
| Robinson, Mr. Leif J. | USA | 2000 | 2011 |
| Rogers, Prof. William A. | USA | 1894 | 1898 |
| Russell, Rev. Norman H. | Canada/India | 1898 | 1902 |
| Russell, Prof. Henry Norris | USA | 1946 | 1957 |