Credit: THE ART ARCHIVE/HARPER COLLINS At intervals, Venus passes directly between the Earth and the Sun, making a transit across the Sun's bright disk. On 8 June, an entire transit of Venus will be visible from most of Asia, Africa and Europe. Australia will experience the transit at around sunset, and across the eastern side of the Americas it will occur around sunrise.

Johannes Kepler, as he plotted the motions of the heavens, was the first to realize that such transits would occur. However, his predicted transit of 1631 was not visible in Europe. The first recorded sighting of a transit was made, in north-west England, by Jeremiah Horrocks, whose own calculations had pinpointed a repeat of the spectacle on 4 December 1639. The intervals between transits follow an unusual pattern: 8 years, then 121.5, then 8, then 105.5 years, and will do so until 2984.

Seventeenth-century astronomers hoped that observations of a transit of Venus would allow them to obtain an accurate measure of the distance from the Earth to the Sun. The concept was eventually formally published in 1716 by Edmund Halley, who predicted from where the transit of 1761 should be viewed so that measurements of the transit times could be used to calculate the Sun's distance. But his predictions were somewhat awry: Halley mistakenly used a plus sign in his calculation where there should have been a minus.

Mathematical fallibility aside, in fact the transit of Venus has never yielded an accurate measure of the Earth–Sun distance. The calculation is hampered by what is known as the ‘black drop problem’. The exact moment at which the dark disk of Venus is fully on the disk of the Sun is hard to ascertain, because the planet's disk seems to stay connected to the dark edge of the Sun: the disk appears as a black drop rather than a perfect circle. Only last year was this effect attributed to the way in which the Sun's brightness lessens at its visible edge, in combination with the natural blurring caused by a telescope.

The difficulty of timing a transit was also noted by Captain James Cook, on his first round-the-world voyage on the Endeavour. In 1769, before reaching the east coast of Australia, Cook was at Tahiti. On Saturday 3 June, he wrote in his logbook: “This day proved as favourable to our purpose as we could wish. Not a Cloud was to be seen the whole day, and the Air was perfectly Clear, so that we had every advantage we could desire in observing the whole of the Passage of the planet Venus over the Sun's Disk. We very distinctly saw an Atmosphere or Dusky shade round the body of the planet, which very much disturbed the times of the Contact, particularly the two internal ones. Dr. Solander observed as well as Mr. Green and myself, and we differ'd from one another in Observing the times of the Contact much more than could be expected.”

By 1882, transit fever had captured both the public imagination and the newspaper headlines. The San Francisco Chronicle reported that “Many of the residents of San Francisco were noticed yesterday with a piece of smoked glass to their eye, looking curiously at the sun”. William Harkness, then director of the US Naval Observatory in Washington DC, had warned on the eve of that transit that “there will be no other till the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004”.

The transit begins shortly after 5:00 Universal Time on 8 June. To view it, the simplest of pinhole cameras will suffice — weather permitting, of course.

Authors Alison Wright View author publications You can also search for this author in PubMed Google Scholar

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About this article Cite this article Wright, A. Captain Cook and the black drop. Nature 429, 257 (2004). https://doi.org/10.1038/429257a Download citation Issue Date: 20 May 2004

DOI : https://doi.org/10.1038/429257a