The Significance of the Transit of Venus

By W.D. McIlveen

About two years ago, there was an event that made a notable item in the news. This was the phenomenon where the planet Venus traces a course across the face of the sun. This happened on June 5, 2012. A similar transit took place on June 8, 2004. Transits of Venus are rare events, occurring in pairs about eight years apart but separated by periods of over a century. As notable as this recent event might have been, it was rather minor in significance by comparison with the attention paid in 1769 as described below.

Venus (small dark circle) entering face of sun, Acton Library, June 5, 2012. Photo made with hand-held digital camera and telescope. Photo by W.D. McIlveen.
Venus (small dark circle) entering face of sun, Acton Library, June 5, 2012. Photo made with hand-held digital camera and telescope. Photo by W.D. McIlveen.

During the 18th Century and onward, marine shipping around the world was increasing in importance, whether for commercial or military purposes. A critical part of this activity was the dependence upon the ability of ships to navigate the seas. At this time, ship captains could determine their latitude reasonably well based on the technology that existed at the time. The matter of knowing their longitude was an entirely different matter. By not knowing both the latitude and longitude, it was very difficult to know their true position and as a result, a great many ships were lost in various mishaps at sea.

In 1716, Edmund Halley realized that the problem of longitude could be resolved by understanding the direction, angular separation of planets and stars and the distance between them. A critical factor is knowing the distance of the earth from the sun. Halley realized that an ideal opportunity for making this distance measurement would occur by accurately observing the transit of Venus across the face of the sun and using this information in some complex mathematical calculations. He knew that the next such opportunities for making the needed observations would happen in 1761 and 1769. Although he never lived long enough to witness the results of the work carried out by astronomers making the measurements, he did set in motion an international process to collect the critical information.

Based on sea voyages to St. Helena and Barbados in 1764 and celestial observations made there, Nevil Maskelyne, Astronomer Royal of England, was able to produce a book of tables, The Nautical Almanac, in which the position of the moon for every noon and midnight was forecast for several years into the future. From these tables, navigators could estimate a longitude to a greater degree of accuracy than they could before that date. The required accuracy was still not sufficient and detailed observations on the next transit of Venus were needed.

Some early measurements of the transit of Venus were undertaken at Tobolsk, Siberia by Jean-Baptiste Chappe d’Auteroche on June, 6, 1761. Despite support from the empress, his trip from Paris to the viewing site was difficult to say the least, but he did manage to obtain the needed data.

Various competing but cooperative ventures were launched to observe the transit of Venus on June 3, 1769 at different points around the globe. A number of these failed completely because the skies were covered in cloud at the critical moment or because of travel logistics and political conditions. For example, Le Gentil from the French Academy of Science could not land at the intended outpost of Pondicherry in the Indian Ocean because the port had fallen into the hands of the British. He could not operate the telescope properly on the deck of a floating ship so his efforts produce nothing of consequence. Also, in 1769, the surveyors Charles Mason and Jeremiah Dixon (whose names live on in the form of the Mason-Dixon Line in the United States) were commissioned to set up an observatory at Bencoolen, Sumatra. A series of delays beyond their control caused them to shorten their trip so that they were only able reach Cape Town. They set up their observatory there but were able to collect only partial details due to clouds on the critical day.

Three teams of observers did collect the required data. One team consisted of Father Maximilian Hell and Joannes Sajnovics from Hungary. After a harrowing trip along the edge of fiords, stormy fall seas and spending the previous long dark winter on an island at Vardos off the north coast of Norway, they set up an observatory and made the needed observations. By comparison, Chappe was sent to collect the data San Jose del Cabo on the tip of Baja California. This required a trip on horseback across Mexico in the company of Spanish co-observers Vincente de Doz and Salvador de Medina followed by a boat trip across the Gulf of California. Chappe had the misfortune to arrive at his destination in the midst of a serious outbreak of typhus. Although he fell victim to the disease himself, sheer will power saw Chappe complete his task. He survived until August 1 when he too died. Fortunately, his assistant was able to bring the data back to Europe where the information could be utilized. Perhaps the most famous observer team that successfully collected transit information was that of Captain James Cook and his senior observer Charles Green. They made their way to the island of Tahiti in the Pacific Ocean after rounding Cape Horn. On that trip, the young botanist/naturalist Joseph Banks was a paying passenger. He was out to collect botanical and other natural history information along with his own team of servants and assistants. Fortunately for the expedition, Banks was more diplomatic than Cook and so he was able to retrieve the critical instruments stolen by the Tahitian natives who were enthralled by any metal objects. But Cook’s expedition experienced good weather for the observation date and they obtained all of the required data.

The data collected by all of the observers eventually was to pass through the clearing house set up under the auspices of the Royal Academy of Science in Paris. At the time, Jerome Lalande was in charge of processing the data. He was somewhat arbitrary in selecting the data that he accepted and rejected. This led to a public and heated spat with Father Hell. Lalande’s approach led to a measurement of the distance to the sun that was less accurate than some of the collected data would have indicated. Since then, more accurate measurements have been made using other techniques but the technology used in 1769 was the most advanced at that time.

The next pair of Venus transits will occur on 10–11 December 2117 and in December 2125. Not many people alive for the most recent transit will be alive to witness the next.

Anderson, Mark, 2012. The Day the World Discovered the Sun. DaCapo Press, Philadelphia. 280 pp.

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