1891 - appointed assistant at the Royal Observatory at Greenwich; collaborated at Greenwich with Philip Cowell to develop a more accurate way of calculating the orbit of a comet when subject to the gravitational perturbations of the planets. They predicted Halley's comet would pass the sun in 1910; by extrapolating backwards in time, they identified returns of the comet to 240 BCE. For their studies of Halley's comet, the two were jointly awarded the Lindemann prize of the Astronomische Gesellschaft; 1919 - led the British astronomical expedition to Sobral, Brazil to view the total eclipse