page_title: Transit of Venus
section_title: 1761 and 1769
hierarchical_section_title: Transit of Venus / History of observation / 1761 and 1769
caption_attribution_description: English: Diagram from Edmund Halley 1678 paper to the Royal Society ("Dr. Halley's Dissertation of the Method of Determining the Parallax of the Sun by the Transit of Venus, June 6, 1761") showing how the Venus Transit could be used to calculate the distance between the Earth and the Sun.
context_page_description: A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black dot moving across the face of the Sun. The duration of such transits is usually several hours. A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is more than three times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth.
  Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities.
  The last transit of Venus was on 5 and 6 June 2012, and was the last Venus transit of the 21st century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882.
context_section_description: In 1663 Scottish mathematician James Gregory had suggested in his Optica Promota that observations of a transit of the planet Mercury, at widely spaced points on the surface of the Earth, could be used to calculate the solar parallax and hence the astronomical unit using triangulation. Aware of this, a young Edmond Halley made observations of such a transit on 28 October O.S. 1677 from Saint Helena but was disappointed to find that only Richard Towneley in Burnley, Lancashire had made another accurate observation of the event whilst Gallet, at Avignon, simply recorded that it had occurred. Halley was not satisfied that the resulting calculation of the solar parallax at 45" was accurate.
  In a paper published in 1691, and a more refined one in 1716, he proposed that more accurate calculations could be made using measurements of a transit of Venus, although the next such event was not due until 1761 (6 June N.S., 26 May O.S.). Halley died in 1742, but in 1761 numerous expeditions were made to various parts of the world so that precise observations of the transit could be made in order to make the calculations as described by Halley—an early example of international scientific collaboration. This collaboration was, however, underpinned by competition, the British, for example, being spurred to action only after they heard of French plans from Joseph-Nicolas Delisle. In an attempt to observe the first transit of the pair, astronomers from Britain, Austria and France traveled to destinations around the world, including Siberia, Newfoundland and Madagascar. Most managed to observe at least part of the transit, but successful observations were made in particular by Jeremiah Dixon and Charles Mason at the Cape of Good Hope. Less successful, at Saint Helena, were Nevil Maskelyne and Robert Waddington, although they put the voyage to good use by trialling the lunar-distance method of finding longitude.
  That Venus might have an atmosphere was widely expected (because of the plurality of worlds belief) even before the transit of 1761.  However, few if any seem to have predicted that it might be possible to actually detect it during the transit.  The actual discovery of the  atmosphere on Venus has long been attributed to the Russian Academician Mikhail Lomonosov on the basis of his observation of the transit of Venus of 1761 from the Imperial Academy of Sciences of St. Petersburg. At least in the English-speaking world, this attribution seems to have been owing to comments from the multi-lingual popular astronomy writer Willy Ley (1966), who consulted sources in both Russian and German, and wrote that Lomonosov observed a luminous ring (this was Ley’s interpretation and was not indicated in quotes) and  inferred from it the existence of an atmosphere “maybe greater than that of the Earth” (which was in quotes). Because many modern transit observers have also seen a threadlike arc produced by refraction of sunlight in the atmosphere of Venus when the planet has progressed off the limb of the Sun, it has generally, if rather uncritically, been assumed that this was the same thing that Lomonosov saw.  Indeed, the term “Lomonosov’s arc” has frequently been used in the literature.  In 2012, Pasachoff and Sheehan consulted original sources, and questioned the basis for the claim that Lomonosov observed the thin arc produced by the atmosphere of Venus. A reference to the paper was even picked up by the Russian state-controlled media group RIA Novosti on January 31, 2013, under the headline “Astronomical Battle in US Over Lomonosov’s discovery." An interesting attempt was made by a group of researchers to experimentally reconstruct Lomonosov’s observation using antique telescopes during the transit of Venus on 5-6 June 2012.  One of them, Y. Petrunin, suggested that the telescope Lomonosov actually used was probably a 50 mm Dolland with a magnifying power of 40x. It was preserved at Pulkova Observatory but destroyed when the Germans bombed the observatory during World War II. Thus, Lomonosov’s actual telescope was not available, but other pr
caption: Diagram from Edmund Halley's 1761 paper to the Royal Society showing how the Venus transit could be used to calculate the distance between the Earth and the Sun
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