Franklin and the Transit of Venus

May 05, 2012 19:43

"The improvement of geography and astronomy is the common concern of all polite nations".

—Benjamin Franklin (1753)

Franklin’s interest in astronomy began when he was a boy. One of the earliest books he read was The Surprising Miracles of Nature and Art which discussed meteors, comets and eclipses. By 1748 he was a minor expert on astronomy, and every year Poor Richard listed the upcoming eclipses of the sun and moon.

The Transit of Mercury, 1753{EZFOOTNOTE: Adapted from The Life of Benjamin Franklin: Soldier, Scientist and Politician, Joseph Lemay, ISBN 0812241215, pp. 140 ff.}

Astronomers knew there would be a transit of Mercury in 1753, and Poor Richard gave the exact Greenwich times for the transit. Thus local astronomers who knew the latitude and longitude of their towns could then determine when the eclipse would occur. Comparisons of the results could confirm the earth’s distance from the moon, but the main purpose of making accurate observations in 1753 was to practice for the transit of Venus which would occur in 1761 and 1769. If made in the East Indies as well as in America, the results of measuring the transit of Venus could establish the distance of the earth both from the sun and from other planets in the solar system. In particular, more accurate measurements of the distance of the earth from the sun would improve the accuracy of navigation—fuzzy navigation still causing significant loss of ships, lives and cargo in that day. (Mercury is too close to the sun for navigationally useful measurements to have been made.)

Mercury’s transit caused international cooperation. The French astronomer, geographer and mapmaker Joseph-Nicolas Delisle sent directions for observing the transit of Mercury to New York’s governor, asking him to forward them overland to Joseph-Pierre de Bonnéchamps in Quebec. Delisle also wrote to the Jesuits in the East Indies to make observations.

Franklin’s Poor Richard printed an illustration of the course of Mercury across the sun. And he printed 50 pamphlets at his own expense explaining its importance and how to measure it, and sent them to those he knew would be interested. And, “weather permitting,“ Franklin planned on observing the transit “at our Academy“.

Alas, Franklin’s hopes were dashed. All up and down the East Coast, clouds hid the transit. Not a single observation was made. But his efforts were not entirely in vain. He had also sent a half-dozen copies to his nephew Benjamin Mecom in Antigua, who gave one to the Reverand William Shervington. With a clear sky in Antigua on May 6, Captain Richard Tyrell, with Shervington in attendance, made observations of the transit. He forwarded his results to Franklin, and they found their way to the Royal Society in London.

Transit of Venus, 1761{EZFOOTNOTE: Adapted from Benjamin Franklin's Science, I. Bernard Cohen, ISBN 0674066596, pp.190 ff.}

After declaring in Poor Richard that the distance of the earth from the sun was at least “80 millions of miles“, Franklin continues “but it is not certainly known, whether it is not a great deal more. In the year 1761 the distance of all the planets from the sun will be determined to a great degree of exactness by observations on a transit of the planet Venus over the face of the sun, which is to happen 6th of May, O.S. in the year.“ When the transit of Venus actually occurred, the most notable American feature of that event was the Harvard professor John Winthrop’s voyage to Newfoundland aboard the province sloop, furnished for that purpose by Massachusetts Governor Bernard—the first scientific expedition sponsored by an American college—which provided the only American observation of the transit in 1761.

(Franklin was in London in 1761. I have been unable to determine what direct involvement in the 1761 transit Franklin was able to manage at this time.)

Transit of Venus, 1769{EZFOOTNOTE:}

On 11 March 1769 Benjamin Franklin wrote a letter from London to his friend John Winthrop at Harvard University, announcing that he was finally dispatching a telescope that Winthrop had ordered a year earlier. Winthrop who had observed the first transit in Newfoundland was planning to view the second transit from Cambridge, Massachusetts but after a devastating fire at Harvard University had destroyed the telescopes there, he had asked Franklin to purchase a new one in London from instrument-maker James Short.

‘At length after much Delay and Difficulty I have been able to obtain your Telescope that was made by Mr. Short before his Death’, Franklin wrote. When Short had died in June 1768, Winthrop’s telescope got stuck in the complicated probate of Short’s estate. Not only that Winthrop’s order for equal altitude and transit instruments from instrument-maker John Bird had also been delayed. There was such a “great and hasty demand on him from France and Russia, and our society,“ Franklin had reported in July 1768, that Bird had not even started to work on the American’s order. But with the instruments for the European expeditions dispatched, Bird had promised to finish Winthrop’s by the end of following week. ‘Possibly he may keep his word’, Franklin wrote, but also warned, ‘we are not to wonder if he does not’ – instruments-makers in London were working around the clock. Bird’s instruments were sent in September but Winthrop was still waiting for his telescope.

‘I hope every thing will be found right’, Franklin wrote to Winthrop, because with the transit only three months away ‘I have no time left to get any philosophical or astronomical Friends to examine it as I intended, the Ship being on the Point of sailing, and a future Opportunity uncertain.’ There was not much time left.

But observations were made in the American colonies after all. In 1769 Benjamin Franklin published an article in the prestigious journal of the Royal Society of London presenting the transit of Venus observations of Messrs. Biddle and Bayley. Some historians credit this account from pre-revolutionary America as the first occasion on which American science went on display before the international community, an occasion made all the more propitious for involving a natural phenomenon that galvanized international attention in the scientific community.

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