Transit Trivia: Preparing for 1769

In 1663, Scottish mathematician James Gregory proposed using the transits of Mercury and Venus to determine the solar parallax and, from that, determine the distance from Earth to the sun, a measurement known today as the Astronomical Unit. In a 1716 issue of the Philosophical Transactions of the Royal Society, Edmund Halley repeated this proposal and explained it further. In his report, Halley suggested places that a full transit should be viewed due to a "cone of visibility." The places Halley recommended included the Hudson Bay, Norway and the Molucca Islands in the Pacific.


The viewing of the 1761 transit involved the effort of 120 observers from nine nations. But British astronomer and mathematician Thomas Hornsby (1733 - 1810) later reported that the 1761 observations were on the whole unsuccessful, primarily due to poor weather conditions. Hornsby alerted the Royal Society in 1766 that preparations needed to begin for the 1769 transit. Hornsby's publication in the Philosophical Transactions of the Royal Society in 1766 focused attention on the "cone of visibility" indicating, like Halley, some of the better places to observe the transit. The Royal Society boasted that the British "were inferior to no nation on earth, ancient or modern" and were eager to make another attempt.


When choosing locations for viewing the 1769 transit, The Royal Society basically chose those Halley suggested in his 1716 article. The committee recommended that the transit be observed from three points: the North Cape at the Arctic tip of Norway, Fort Churchill at Hudson Bay, Canada and a suitable island in the South Pacific. They stated that two competent observers were to be sent to each location. King George III approved of the project and arranged for the Navy to provide ships. He allocated to the society 4,000 pounds to help fund the British expeditions.


The Czech Jesuit, astronomer and teacher Christian Mayer (1719 - 1783) was invited by Empress of Russia Catherine the Great to observe the transit in Saint Petersburg along with Swedish-born Russian astronomer, mathematician, and physicist Anders Johan Lexell (1740 - 1784). Other members of the Russian Academy of Sciences went to eight other locations in the Russian Empire.


In Philadelphia, the American Philosophical Society erected three temporary observatories and appointed a committee led by the renowned American astronomer, inventor, clockmaker, mathematician, surveyor, scientific instrument craftsman and public official, David Rittenhouse (1732 - 1796). The results of these observations would later be printed in the first volume of the Society's Transactions, published in 1771.


In addition, scientists traveled to San José del Cabo (Baja California, then under Spanish control). Regardless of the expedition and the location, all observers had high hopes for results that were better than the 1761 transit...

The next transit of Venus will occur over June 5-6. To learn more, visit these links.

2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html

Transit of Venus, Sun-Earth Day 2012, NASA: sunearthday.nasa.gov/transitofvenus/

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