In 1986 he won the Amateur Achievement Award of the Astronomical Society of the Pacific.
Brennan was a member of several scientific societies, including the British Astronomical Association, the Astronomical Society of the Pacific, the Astronomy and Astrophysical Society of America, the Saint Louis Academy of Science, the American Mathematical Society, and the National Geological Society.
His imaging and image-processing abilities brought him the Amateur Achievement Award of the Astronomical Society of the Pacific in 2004.
He was president of the American Forestry Association in 1890-91, president of the Astronomical Society of the Pacific in 1898, and also served as president of the California Academy of Schools.
Pacific Ocean | Pacific | Canadian Pacific Railway | Society of Jesus | Royal Society | National Geographic Society | Pacific War | American Cancer Society | South Pacific | Union Pacific Railroad | Pacific Islands | Pacific Northwest | Northern Pacific Railway | Royal Television Society | International Astronomical Union | American Physical Society | Pacific Islander | American Chemical Society | International Society for Krishna Consciousness | American Society of Civil Engineers | Royal Society of Canada | Royal Geographical Society | American Philosophical Society | Theosophical Society | Royal Society of Edinburgh | Society of Antiquaries of London | Southern Pacific Transportation Company | Society of the Cincinnati | Society for the Prevention of Cruelty to Animals | Justice Society of America |
Eggen's professional memberships and honors include the American Astronomical Society's Russell Lectureship (1985), membership in the Royal Astronomical Society (vice president 1961-1962), Pawsey Memorial Lectureship of the Australian Institute of Physics, member of the Australians Society of Astronomers (president 1971-1972), and member of the Astronomical Society of the Pacific.