While Astronomer at the Cape (in modern South Africa), he derived a solar parallax of 8″.8 from observations of three asteroids, a value that remained the standard until 1941. Photographs taken by Gill of the Great September Comet of 1882 showed so many stars that he conceived the idea of producing a star catalogue based on photography. The result was the Cape Photographic Durchmusterung, compiled by J. C. Kapteyn from Gill’s photographic plates. A further consequence was the mammoth Carte du Ciel photographic project.