Capt. Cook & The Transit of Venus

Photo of the 1882 transit, which revealed a precise distance for Venus's orbit.

Last month’s transit of Venus across the face of the sun was largely an astronomical curiosity, but a similar transit in 1769 held great potential for expanding our knowledge of the universe. By comparing observations made from several points throughout the world, scientists hoped to measure the true size of the orbit of Venus, and thus the orbits of the other planets, and thus the size of the Solar System. Playing a key role was a captain named James Cook.

Statue of Cook
in Greenwich

In 1716, Edmond Halley (he of comet fame) published a paper explaining how a transit of Venus could be used to make the necessary calculations. Halley was more than the scientific father of the 1769 expedition, though. He also commanded an expedition in 1689 to measure compass variations. Halley was so bad a commander that the Royal Navy refused to allow a scientist to command one of its ships ever again. Thus Cook, a gifted mathematician and cartographer as well as a naval officer, was selected to lead the expedition.

A civilian collier, the Earl of Pembroke, was selected as the vessel for the expedition. Its shallow bottom and sturdy construction made it ideal for the voyage in ways a traditional warship would not be. The ship was overhauled, armed, and commissioned Endeavour. In August 1768 the expedition set out for Tahiti, where Cook and two scientists on board would make independent observations. It arrived the following April, and Cook made good use of the weeks leading up to the June 3 transit to build an observatory.

The results were disappointing,. The three sets of measurements taken at Tahiti did not match up within the margin of error, due to an optical phenomenon called the “black drop effect.”  Combined with measurements from more than a dozen other sites around the world, scientists were able to refine the estimates for the size of Venus’s orbit, but not with the degree of precision they had hoped for.

Cook’s voyage of exploration was not over yet. After the transit, he opened sealed orders instructing him to find and claim Terra Australis Incognita, a large southern continent of supposed great riches. He never found it and, indeed, did not believe it existed in the first place. The Endeavour did visit New Zealand and Australia and Cook was able to determine that the latter was a separate continent (it was previously believed to be part of New Guinea).

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