Thirty-eight years after the Horrocks observation there was another planetary transit. This time it was the planet Mercury, and another precocious twenty-year-old English astronomer; Edmond Halley (1656 – 1742). Halley had dropped out of Oxford University to travel to the South Atlantic island of St Helena to observe the southern stars and the predicted transit of Mercury. On 28 October 1677 he recorded both the ingress and egress of Mercury onto the solar disc. (Ingress and egress are astronomical terms describing the planets ‘moving onto’ and ‘moving off’ the solar disc.) Jean-Charles Gallet also saw the egress from Avignon in France. All observers in England were defeated by clouds. Halley realised that an accurate timing of the transit by two widely spaced observers would “give a demonstration of the Sun’s Parallax, which hitherto was never proved, but by probable arguments” (see p. 40, Correspondence and papers of Edmond Halley, ed E. F. MacPike, Taylor & Francis, 1937). Unfortunately the 1677 observations were incomplete, the timings were inaccurate, and the final result was useless. But this observation sowed the seeds of a greater endeavour.

By 1716 Professor Halley (as Edmund had now become), was describing to the Royal Society (see Philosophical Transactions, Volume 29, pp 454 – 464) how the distance between the Earth and the Sun (a quantity known as the Astronomical Unit, or AU) could be calculated, if the time it took Venus to transit the Sun was measured accurately from two known sites separated by a large north-south distance. Subtracting one transit time from the other Halley noted that “if we have this difference true to two seconds it will be certain what the sun’s parallax is … to within its 500th part at least.”

The importance of the AU is underlined by the fact that our knowledge of the size of planets, and the distances, masses, sizes and energy output of stars depends on it. The accurate measurement of the AU became a major goal of renaissance astronomy. To quote Giovanni Antonio Rocca, writing in 1651, “the problem of solar distance and parallax was one of the most important in astronomy, well worth a lifetimes work by any astronomer.” Halley’s “500th part at least” galvanised eighteenth century astronomy and was a clarion call to adventurous observers who scurried to the four corners of the Earth to observe the Venus transits of June 1761 and 1769.