It was a sunny morning in Jamestown, St Helena, on 7 November 1677. Given its extreme remoteness – St Helena is a tip of volcanic land in the South Atlantic Ocean equidistant between South America and Africa – Jamestown was surprisingly urbane in the 1670s. For over a hundred years, the port had been a supply outpost of the British East India Company and, before that, was used by the Portuguese. Far south of the equator, usually cloudless, formerly uninhabited and now English-civilized, St Helena was a perfect place to observe the southern night sky. In November 1677 one aspiring astronomer, a well-off twenty-one-year-old Englishman named Edmond Halley, was here to map the southern stars and to observe a relatively rare transit of Mercury. That is what he was doing on the sunny morning of 7 November, at around 9 o'clock. Halley had his telescope (fitted with a smoked-glass filter) pointed at the sun. A small black notch appeared at the outer edge of the solar disc, creeping into the limb of the sun. The notch grew into a small black semicircle. It was the silhouette of the planet Mercury, cast onto the sun as seen from Earth. Halley watched with special interest as, at approximately 9.30, the shadow's entire shape, a circular silhouette, finally completed itself.