In 1716 astronomer Edmond Halley calculated that you can quantify the distance from the sun to the earth (solar parallax) by having observers across the globe time the passage of Venus across the sun. Knowing he would not live to see the next transit, Halley predicted global sites that would be suitable for viewing a transit and called upon future generations to pursue his plan. …. The quest to time the transit of Venus in 1761, during the Seven Years War, marked the first time the international community cooperated to answer one of the leading scientific questions of the day.”

— NASA website

Halley’s admonition to the future

“We therefore recommend again and again, to the curious investigators of the stars to whom, when our lives are over, these observations are entrusted, that they, mindful of our advice, apply themselves to the undertaking of these observations vigorously. And for them we desire and pray for all good luck, especially that they be not deprived of this coveted spectacle by the unfortunate obscuration of cloudy heavens, and that the immensities of the celestial spheres, compelled to more precise boundaries, may at last yield to their glory and eternal fame.”

— Edmund Halley (1656-1742)

By quantifying the distance from the sun to the earth, a simple application of Keller’s Third Law would give the distances of all the planets from the sun, and thus the scale of the solar system.
Global Expeditions

In response to Halley’s call ships were sent to observe the Transit of Venus from precise areas across the globe. The 1761 Transit was observed from 62 sites around the world with 120 observers-French, British, Danish, German, Italian, Dutch Swedish and Portuguese. These efforts were renewed for the 1769 Transit when there were 63 sites and 138 observers. By 1769, although hostilities between the French and British had only just ended, the French were reported to have instructed their forces not to obstruct the British ships coming into their territories to observe the event since they were on a mission of service to all mankind.

Captain James Cook in Tahiti

It was under this banner that James Cook’s Endeavour arrived in Tahiti to observe the 1769 Transit. Cook was charged with sealed orders for a second commission,which he was commanded not to open until he had observed the Transit. This charge was to go in search of the Great South Land, Terra Australis Incognita.

“This was a voyage of scientific discovery, carrying trained observers: artists, astronomers, and naturalists. The ship’s botanists collected so much exotic flora that they expanded the number of known plant species in the West by a quarter. This seeded the modern notion of biodiversity…”

— From Blue Latitudes, Tony Horwitz

Cook carried onboard Royal Astronomer Charles Green. They observed and drew the Transit of Venus. Cook went on to chart southern waters including New Zealand and Australia where the Endeavour was almost lost when it hit the Great Barrier Reef at Cape Tribulation. Green died in Batavia on the journey home but his drawings were carried back to the Royal Observatory where they were added to others from around the globe. Cook survived to make two more major expeditions to the South Seas before he died in Hawaii. The calculations from those Transits brought us closer to knowing the astronomical unit but it was not precisely measured for another century.

A Global Effort

“Finally, the problem of the transits of Venus produced an intensity and breadth of effort on the part of 18th-Century scientists that was unmatched by any other single problem. It brought to a common focus men of almost every national background with an abiding concern for the advancement of knowledge. In doing so, it helped to shape the growing international community of science and to demonstrate with striking clarity what co-operation and goodwill might achieve in the peaceful pursuit of truth.”

— H Woolf, Author of The Transits of Venus (1959)