Transits of Venus

Credit: National Library of Australia The view from Point Venus, Tahiti, where Cook and his men observed the transit of Venus. Oil on canvas, William Hodges, 1744-1797.

No wonder Cook and Banks had so little to say about the transit when it finally happened on June 3rd, 1769. Venus' little black disk, which could only be seen gliding across the blinding sun through special telescopes brought from England, couldn't compete with Tahiti itself.

Banks' log entry on the day of the transit consists of 622 words; fewer than 100 of them concern Venus. Mostly he chronicled a breakfast-meeting with Tarróa, the King of the Island, and Tarróa's sister Nuna, and later in the day, a visit from "three handsome women." Of Venus, he says, "I went to my Companions at the observatory carrying with me Tarróa, Nuna and some of their chief attendants; to them we shewd the planet upon the sun and made them understand that we came on purpose to see it. After this they went back and myself with them." Period. If the King or Banks himself was impressed, Banks never said so.

Cook was a little more expansive: "This day prov'd as favourable to our purpose as we could wish, not a Clowd was to be seen … 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 Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the contacts particularly the two internal ones."

The "dusky shade round the body of the Planet" was a problem. Intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit. For this reason, his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds.

Cook and Green also observed the "black drop effect." When Venus is near the limb of the sun - the critical moment for transit timing - the black of space beyond the Sun's limb seems to reach in and touch the planet. You can recreate the black drop effect with your thumb and index finger: Hold the two in front of one eye and narrow the distance between them. Just before they touch, a shadowy bridge will spring across the gap. According to John Westfall, writing for Sky & Telescope magazine in June 2004, "this is simply the result of how two fuzzy bright-to-dark gradients add together." The black drop effect, like the fuzziness of Venus' atmosphere, made it hard to say just when the transit began or ended.

This was a problem for observers elsewhere, too, not only Cook in Tahiti. In fact, when all was said and done, observations of Venus' 1769 transit from 76 points around the globe, including Cook's, were not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits.