Edmond Halley did not witness a transit of Venus but when he was in St Helena in the South Atlantic preparing tables of the southern stars be was able to observe the 1677 transit of Mercury, observations sent to the Astronomer Royal John Flamsteed, who wrote about them to Johannes Hevelius (CUL MS.RGO.1/43: ff.38v-40v). Satisfied that the observations were possible on a wide scale, Halley recommended that his astronomical descendants to observe the 1761/1769 events from as many geographically separated places as possible so that the Earth-Sun distance could be gauged accurately, which by the application of Kepler’s third law would in turn lead to an accurate scale of the whole Solar System.

Halley similarly behoved astronomers of the future to observe an event that he predicted. In the later 17th century there was much debate about the nature of comets, particularly stimulated by the Great Comet of 1680-1681. A bright comet with a long tail was seen to approach the Sun towards the end of 1680 and then at the beginning of 1681 a bright comet with a long tail appeared travelling away from the Sun. Were these two separate bodies coincidentally making consecutive appearances or a single object that was in orbit about the Sun? The Astronomer Royal, John Flamsteed, took the latter view; Isaac Newton was originally of the former opinion though he changed his mind and used the example of the1680 comet to illustrate elliptical orbits in the Principia mathematica.

Working more than ten years after the event it was from his observations, made from the grounds of his house in the village of Islington to the north of London, of a less prominent comet that appeared in 1682 later that Halley deduced orbital parameters which seemed to him familiar. Halley searched back in historical records and found that Kepler’s comet of 1607 followed a very similar path as did one observed in 1571 and he realised that these three comets were in fact one and the same, orbiting the Sun every 75 or 76 years, the period being influenced by the effects of Jupiter’s gravitation attraction on the comet. Knowing that he could not see the event he urged younger astronomers to hunt in the skies of 1758 for the return he predicted – and he was vindicated as was the celestial mechanics of Isaac Newton. Halley realised that depending on their speed of travel comets followed paths that were conic sections – ellipses, parabolas or hyperbolas. In this notebook of observations of the 1682 comet, Halley drew the sections speculating on the path of the comet – here as a parabola.