His work has included studies of the ring systems of Saturn, Uranus, and Neptune via Voyager observations and ground-based stellar occultations; Earth-based observations with the 5-meter Hale Telescope at Palomar Observatory of several small moons of Jupiter and Saturn discovered by the Voyager spacecraft; dynamical investigations of the planetary system around the pulsar PSR 1257 + 12,[2] and of the rotational evolution of natural satellites; and studies of the zodiacal dust bands discovered by the Infrared Astronomical Satellite in 1983.
Together with colleagues in Canada and at Harvard, he has been involved in the discovery of numerous outer satellites of Uranus, Saturn and Neptune.
[3][4][5] Nicholson was a member of the Visual Infrared Mapping Spectrometer science team on the NASA/ESA Cassini–Huygens mission to Saturn, and was the leader of a team of Cornell and Caltech astronomers studying the impact of Comet Shoemaker-Levy 9 into Jupiter in July 1994 using the Hale Telescope.
The inner main-belt asteroid 7220 Philnicholson, discovered by Edward Bowell at Anderson Mesa in 1981, was named in his honor.
[7] Nicholson won the Harold Masursky Award in 2019, "for meritorious service to planetary science" in his role as editor-in-chief of Icarus.