Historical Notes

No record exists of a transit having been observed before the invention of the telescope. The possibility that the inferior planets might occasionally pass in front of the Sun was not considered until the early part of the seventeenth century, when Kepler predicted a transit of both Mercury and Venus for 1631. The transit of Mercury was observed by Gassendi but Venus crossed the Sun during the European night and eluded observers. Kepler believed that the next transit of Venus would not occur until 1761, but the young English astronomer Jeremiah Horrocks calculated another transit for 1639 which he observed from Much Hoole, a village near Preston in Lancashire. William Crabtree, a colleague whom Horrocks had notified of the event, saw the transit from Manchester. A popular account of Horrocks’ observation of this ‘first’ transit has been given by Allan Chapman and there is also a more comprehensive study of Horrocks’ achievements set against the background of seventeenth century science by the same author. It is remarkable that two transits occurred during Horrocks’ short life.

Of the five transits which are known to have been observed, it is believed that the first was seen by just Horrocks and Crabtree. The others have each been the subject of scientific investigation, principally in the attempt to determine the solar parallax, and hence the Astronomical Unit. The results of observations made by expeditions during the transits in 1761 and 1769 have been thoroughly documented by Woolf. In anticipation of the most recent pair of transits, a popular review of past and coming transits was published by Proctor. The transit in 1874 was the first which could be studied by photography, and expeditions were despatched to several locations. An interesting account of the American expedition has been given by Janiczek and Houchins.

Even with the aid of photographic plates the results of the observations proved to be largely unsuccessful, and well before the next transit was due consideration was being given to finding alternative ways in which to obtain the Sun’s distance. Of the 1882 transit, just the early stages were visible in the British Isles, the ingress taking place during the early afternoon and the Sun setting before mid-transit. Although conditions were generally unfavourable, some observers were fortunate in seeing Venus on the Sun, though with varying degrees of clarity, as indicated in their reports.

Of course, as Venus presents a sizeable disk it is quite possible that the planet had already been seen in transit during historical times, especially if the Sun was low or behind a thin layer of cloud; indeed Johnson suggests that a record of such an observation may survive from ancient times: ‘The reader’s attention may just be called to a broken Assyrian tablet in the British Museum, concerning Venus. The following succession of broken lines occurs: ‘the planet Venus’ − ‘it passed across’ − ‘the Sun’ − ‘across the face of the Sun.’ It may be just possible to fill up each hiatus in a manner not to refer to a transit. But it reads somewhat like the record of one, which, it seems, must be before the 16th century B.C.’

Peter Macdonald (From the JBAA Paper "The transit of Venus on 2004 June 8". See the Resources page for the full paper.)