English astronomer, geophysicist, mathematician, meteorologist, and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena, Halley recorded a transit of Mercury across the Sun, and realised a similar transit of Venus could be used to determine the size of the Solar System. From his September 1682 observations, he used the laws of motion to compute the periodicity of Halley’s Comet in his 1705 Synopsis of the Astronomy of Comets; it was named after him upon its predicted return in 1758, which he did not live to see.