William Pickering. William Henry Pickering (1858- 1938), American astronomer. Pickering predicted the existence of seven extra hypothetical planets in the solar system during 1908-32 using graphical analysis of perturbations in the orbits of known planets. He named them O, P, Q, R, S, T and U. He revised his results several times to fit observed data better, but planets have never been found that matched his predictions. He obtained some of the first photographs of Mars & used these as evi- dence that the supposed canals on Mars were actu- ally natural features. In 1898 he found Saturn's ninth moon, Phobe; this was the first satellite to be discovered using photographic means.