He became an assistant at the Royal Greenwich Observatory on 17 January 1873. He was first in charge of the manuscripts and library, followed by the time department, and then the circle computations. One of his areas of study was that of the altazimuth or horizontal coordinate system, as well as bring one of the four observers of the transit circle and altazimuth. In 1875, Downing was elected as a fellow of the Royal Astronomical Society, contributing 75 papers dealing primarily with correcting errors in the star catalogues and the computation of the motions of astronomical bodies. In one paper, he worked with George Johnstone Stoney on the perturbations of the Leonid meteors, predicting and explaining the spareness of the 1899 shower.