Analyzing The 1769 Results

Back in England, the task of analyzing the available timings fell to Thomas Hornsby, the Savilian Professor of Astronomy at Oxford. His selection from the data available from Tahiti was peculiar, however. The observations from Cook himself, Green and Solander, all with their own telescopes, showed a scatter of ten seconds or more in some of the contact timings, as foreshadowed by the quote from Cook's journal given earlier. Hornsby seems to have selected the values from Tahiti that fitted in best with what he expected, based upon the calculations he had already completed using information over the shorter baselines. That is, because Cook did not arrive back until 1771, Hornsby had already made calculations using combinations of readings from Wardhus in Norway, Murmansk in Russia, and Hudson's Bay in Canada. Later came the timings from the French at San José del Cabo in Baja California, and finally the data from Cook's party in Tahiti. It seems that by the time that he received the final set of timings, Hornsby had already made up his mind what the answer should be.

This selection of data is dubious in itself, but also there were other observations available, which Hornsby ignored. One wonders how his report would be treated if subject to the rigorous perusal typical for modern-day scientific papers. Perhaps not by chance, Hornsby's final value for the Earth—Sun distance was much the same as that he had derived using the 1761 transit.

The matter did not sit there, though. The timings from Rodrigues in 1761 were misleading because the longitude of that island was imprecisely known. Cook and colleagues in Tahiti in 1769 determined the longitude of Fort Venus in two ways: from the eclipses of the satellites ofJupiter, and lunar observations. Both results differed from the true longitude, measured later when marine chronometers were carried to Tahiti, by tens of seconds. Even if the observations of Cook, Green, and Solander had agreed with each other, still there was another inherent source of error making the final result for the Earth—Sun distance incorrect: the site coordinates were wrong.

To that extent, one has to say that the expeditions mounted to observe the transit of Venus in 1769 overall were a failure; a failure that cost many lives. Of course there were many spin-offs, such as those accruing from Cook's sealed-envelope orders (I am a citizen of Australia, and previously lived in New Zealand for some years), but basically the science did not work.

The transit observations from 1761 and 1769, so eagerly recommended by Halley and others, did not lead to improvements in navigational capabilities, but within a handful of years that motive anyway had been surpassed by other developments. As aforementioned, on his second and third voyages James Cook carried accurate marine chronometers modeled on Harrison's clocks and fixed his longitude using those.

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