This week, the magnificent Australian-built replica HMB Endeavour could be visited at the Waterfront City Marina in Melbourne. On 26 August 1768, the real HMB Endeavour, commanded by Lieutenant James Cook, sailed from Plymouth to the South Pacific Ocean. The aim of the expedition was twofold: to observe the 1769 transit of Venus across the disc of the Sun and, as contained in the Secret Instructions given to Cook, to explore, map, and eventually take possession of the hypothesized Terra Australis Incognita.

Cook's chart of New Holland East coast (1770)

By observing the passage of Venus across the Sun astronomers had long expected to resolve what Wayne Orchiston has defined as “the most pressing problem in world astronomy” at that time, which was to determine the length of the astronomical unit, or the mean distance from Earth to the Sun.  As Donald A. Teets referred in his 2003 paper:

Though the idea of using a transit of Venus or Mercury to determine the solar parallax dates back at least to the Scottish mathematician James Gregory in 1663, it was Edmond Halley who became its greatest advocate. Halley observed a transit of Mercury from the southern hemisphere in 1677, and in his report on the observations, he discussed the possibility of using transits of Mercury or Venus to determine the solar parallax. Of the two, he believed that the geometry of Venus transits was far more likely to produce accurate results. Halley proposed the Venus transit idea in papers presented to the Royal Society in 1691, 1694, and most importantly, in 1716.  (…).

Halley’s paper called for observers to be stationed far and wide across the globe, a monumental task in 1761. Despite the obvious difficulties involved in sending observers on distant locations, not to mention the fact that Great Britain and France were in the midst of the Seven Years’ War at the time, the response of his call was overwhelming. In all, when the transit took place, there were at least 122 observers at sixty-two separate stations, from Calcutta on the Siberian city of Tobolsk, from the Cape of Good Hope to St.John’s in Newfoundland, and of course, at a large number of locations throughout Europe. Many had traveled weeks or even months to reach their destinations. (Donald A. Teets. 2003. Transits of Venus and the Astronomical Unit. Mathematics Magazine, Vol. 76, No. 5: 335-348).

Both Halley’s call for global observers to the two scheduled transits of Venus in 1761 and 1769 and the response it received are astounding examples of scientific crowdsourcing.  The data gathered after the two observations drew the length of the astronomical unit “ever closer to the values accepted today” (Teets 2003: 347) so that 19th century astronomers using these data “. . . knew the distance to the Sun much better than present-day astronomers know the distance to the centre of the galaxy.” (Wolley 1970: 135; quoted in Orchiston (2004: 62)).

Drawings of the Transit of Venus by Captain James Cook and Charles Green

What is new to present-day scientific endeavours, nevertheless, is that astronomers of the 21st century can benefit from the unprecedented global effort of hundreds of thousands of volunteers who are able to deal with ever-growing datasets. In this line, the Citizen Science Alliance hosts one of the most popular collections of projects where citizens can contribute to classify galaxies (galaxyzoo.org), help to measure our Milky Way (milkywayproject.org), explore the Moon using high-resolution NASA imagery (moonzoo.org), discover new planets around other stars (planethunters.org), and spot explosions on the Sun (solarstormwatch.org), among many others. Citizen science leverages the fact that humans excel in their pattern recognition abilities, being more reliable than machines in most cases, especially when a decision is to be made in a grey area. But, all in all, citizen science relies on the passion and cooperative behavior that is inherent to great discoveries.

As a reminder, the next transit of Venus across the Sun is scheduled on June 6, 2012. Mark your calendars and enjoy it!

Marta