The Transit of Venus

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The Transit of Venus


A Celestial Event. In 1716 Englishman Edmund Halley (for whom the comet is named) described a procedure for using observations of the transit of the planet Venus across the face of the Sun to determine the solar parallaxan angle calculated from two positions on Earthand hence the distance from the Earth to the Sun. Astronomers were eager to know this distance because it would enable them to calculate more accurately the distances of the other known planets from the Sun. They knew the relative distances of the planets from one another, but they could not determine accurately the solar systems size without learning the Earths distance from the Sun. And there was a problem: transits occurred rarely; before 1761, the last transit of Venus had been in 1639. The transit was expected again in 1761 and 1769, but after that not for another 105 years. Clearly the opportunity to make precise observations had to be siezed, for the chance would not come again for anyone then living. The entire scientific world focused on the approaching events.

The Requirements. A proper calculation of the parallax required at least three observation points on the Earth, each with precisely measured longitude and latitude. Each observer had to know the exact local time of day when the transit began (as well as the time at that same moment at the Royal Observatory in Greenwich, England), and then had to measure the length of time of the transit from start to finish with the same pinpoint accuracy. The most powerful telescopes and the most exact timekeeping intruments would be required for the transit when it came. Clear weather was also a necessity.

American Science. Those colonials interested in developing the sciences recognized the advantages of American involvement in these celestial events: It would be a great honour, wrote one, to our young Colleges in America if they forthwith prepared themselves with a proper apparatus for that Observation Scmade it. However, almost nothing was done in America for the 1761 transit. John Winthrop of Harvard was the only American astronomer with a European reputation at the time; equipment was lacking; and few provincial legislatures wished to spend public money to fund scientific observations. Massachusetts was the only province that made any real effort, and the results had not been entirely satisfactory.


The planet Venus moves in an orbit closer to the Sun than that of the Earth, and it completes its orbit sooner. This means that Venus periodically passes between the Earth and the Sun: for a few hours these three bodies are almost directly aligned. When this happens, Venus can be seen from the Earth, appearing as a small, dark disc moving across the face of the Sun. When this transit is viewed from different points on earth, careful calculations yield the Suns parallaxthe angle of the Sun made by a change in the position of the observer on earth. The parallax is a key factor for determining the Earths distance from the Sun at the time the transit was observed. In turn that distance helps resolve the size of the entire solar system, and the relative distances of the other planets in it from the Sun and from each other.

Source: Silvio A. Bedini, Thinkers and Tinkers: Early American Men of Science (New York: Scribners, 1975).

One Last Opportunity. The last chance for any living person came eight years later in 1769. For the newly revitalized American Philosophical Society, and for the reputation and promotion of scientific endeavor in America generally, the 1769 transit was an opportunity that the scientific community could not afford to miss. And this time American observations were considered vital. Only the beginning of the transit would be visible in Europe, but most of it would be observable in the North American colonies; in fact, astronomers calculated that the area around Lake Superior was one of the few spots on Earth where the full transit could be seen. Due to its changed political situation, Massachusetts would not spend the money to support the observations this time. The American Philosophical Society in Philadelphia raised public money by somewhat untruthfully declaring the observations to be an Object, on which the Promotion of Astronomy and Navigation, and consequently of Trade and Commerce so much depends. In fact the transit was of only marginal importance to navigation and trade, but some people who would willingly support efforts to boost commerce would not contribute to purely scientific knowledge.

Preparations. The American observers employed the most sophisticated instruments available to them. Near Philadelphia, master clockmaker David Rittenhouse used his own specially built timepiece, a 144-power refracting telescope, and an equal-altitude instrument. In Cambridge, Massachusetts, John Winthrop had the use of Harvards eight-foot telescope and exceptionally accurate pendulum clock and an astronomical quadrant he borrowed from Bostons collector of customs. Other observations points were set up in Rhode Island, New York, New Jersey, and Virginia. In all, at least twenty-two official observations were made in North America. Popular enthusiasm grew; nonacademics gathered smoked glasses to use with their less powerful telescopes and pocket watches. They knew little about what was going on but realized that it was big and wanted to be part of it.

Success. When the day came that June, the weather was clear in virtually every place where an observation was made. In the cities crowds gathered in hushed respect to watch the observers. Rittenhouse, peering expectantly through his telescope, became so excited at the moment of contact that he actually forgot to report it for several seconds (in a situation when every second made a huge difference in the final calculations). When it was all over, the American Philosophical Society printed several observations. The quality of these varied considerably, but all were sought out, for it was felt that a greater number of variations would ultimately yield a more accurate mean. The European scientists seemed pleased with the American efforts; one Swedish scientist declared that they had given infinite satisfaction to our astronomers. The data gained was a triumph in itself; although several American astronomers and mathematicians tackled the painstaking math, only one attempt to calculate the parallax was actually published in America, and that was after some of the European results were already known. Comparing Pennsylvania and Greenwich observations, Rittenhouse and a compatriot reckoned the distance from the Earth to the Sun at about 93 million milesclose to todays accepted figure. The transit of Venus gave not only public attention and a much-needed stimulus to scientific pursuit in America, the observations brought international attention. One enthusiast undoubtedly spoke for many when he declared that the transit of Venus hath done a Credit to our Country which would have been cheaply purchased for twenty times the Sum!


Brooke Hindle, The Pursuit of Science in Revolutionary America 1735-1789 (Chapel Hill: University of North Carolina Press, 1956);

Harry Wolfe, The Transits of Venus: A Study of Eighteenth-century Science (Princeton, N.J.: Princeton University Press, 1959).

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The Transit of Venus

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The Transit of Venus