Section II - Jeremiah Horrocks, Edmund Halley and the importance of the Transits of Venus.

With the new and improved tables of planetary motion, Kepler was able to predict the motions of the inferior planets, Mercury and Venus, with greater accuracy than was previously possible. Whilst so engaged, he discovered that both planets would occasionally pass between the Earth and the Sun when they would appear as a black dot against the bright surface of the Sun. He predicted that transits of both Mercury and Venus would occur in 1631.

The transit of Mercury was seen by Pierre Gassendi in Paris on 7 November 1631, but the transit of Venus in the following month was missed because it occurred during the European night. The first actual observation of a Transit of Venus was made by Jeremiah Horrocks in 1639, an outstandingly brilliant young mathematician from Lancashire.

The suggestion that a Transit of Venus, observed from different parts of the world, could be used to measure the actual distance of the Earth from the Sun, was first made by James Gregory and Edmund Halley, (Astronomer Royal 1720-1742). Such transits, where Venus moves across the face of the Sun as seen from the Earth, occur twice in every approximately 113 years, with the two transits separated by 8 years. Transits of Venus occurred in 1761 and 1769, 1874 and 1882, and will next occur in 2004 and 2012 AD.

The realisation that the transits of Venus could solve, what many saw to be `the last great problem in astronomy' provoked enormous interest in the 18th century, and even countries that technically were at war (Great Britain and France) collaborated in this great international scientific experiment. Expeditions were dispatched to distant lands to observe the transits over as large a geographical area as possible. Captain Cook was sent on his first voyage to the Pacific by the Royal Society to observe the Transit from the island of Tahiti. Other astronomers travelled to Africa and throughout Europe to time the exact instant of transit. King George III built himself an observatory at Kew specifically for the purpose and Charles Mason travelled to Ireland to view the transit from Donegal.

Click on image to enlarge.

11. Jeremiah Horrocks (1619-1641) An artist's impression of Horrocks observing the Transit of Venus in 1639. Horrocks was the first to realise that the Moon's orbit was elliptical and made a number of other discoveries before his early death at the age of 22. (Courtesy of the National Museums Liverpool - The Walker)
Mappemonde 12. Mappemonde showing the areas of the globe where the 1769 transit would be visible. To improve the accuracy of the measurements of the transit, it was necessary to make observations simultaneously from sites widely separated in latitude. The optimum regions to observe the transit were the South Pacific and Northern Europe. The Royal Society decided to send observers to the recently discovered Society Islands (Tahiti) and Northern Norway. As local weather conditions could prevent observation at the crucial times, it was important to disperse observers over a reasonably large area. Partly for this reason, observations were also made by the Royal Society from the townland of Cavan in County Donegal.( Courtesy of the Melbourne Observatory)
Captain James Cook 13. Captain James Cook (1728-1779), by John Webber in 1776. Cook, the explorer, was sent by the Royal Society to observe the Transit of Venus of 1769 from Tahiti. In the course of his three voyages to the Pacific basin he charted many unknown lands including the East coast of Australia, New Zealand, various Pacific islands and the West coast of North America. ( Courtesy of the National Portrait Gallery)
Fort Venus, Tahiti 14. Fort Venus, Tahiti. The Royal Society commissioned Charles Green, an astronomer from the Royal Greenwich Observatory, to travel with Captain Cook to the South Seas. In this contemporary drawing we see the observatory set up by Cook on the principal island of the archipelago, Tahiti, where the Transit observations were made.
Venus Point, Tahiti 15. Monument that marks the spot on Venus Point, Tahiti where the observations of the Transit of Venus were conducted in 1769. (photo by C.J. Butler)
island of Morea 16. Morea a neighbouring island, where backup observations were also carried out by Cooks' party. (photo by C.J. Butler)
Drawings of the Transit 17. Drawings of the Transit of Venus from Tahiti by Charles Green. They show the characteristic distortion of the image of Venus when near to the limb of the Sun. This made the timing of the exact moment of contact difficult and compromised the results of the observations.
botanical specimens 18 Botanical Specimen The benefits to science and humanity of Cook's voyages were not restricted to astronomy.On Captain Cook's first voyage, he was joined by Joseph Banks the naturalist. Through the prodigious efforts of Banks, the expedition made many botanical discoveries that were new to science, including 1300 new species. The artists on board recorded many of these specimens which were later engraved for printing. This is one of over 900 engravings made of botanical specimens which regrettably were never published for financial reasons. The metal plates were subsequently lost and only came to light during the second world war when they were rediscovered in London. (Courtesy of James Trimble)
Charles Mason 19. Charles Mason's Journal of Observations Charles Mason (1730-1786) was an experienced astronomer who had been assistant at the Royal Greenwich Observatory. With his partner Jeremiah Dixon, he had successfully observed the Transit of Venus in 1761 from the Cape of Good Hope and, between 1764 and 1768, had surveyed the boundary line between Maryland and Pennsylvania which was subsequently known as the Mason-Dixon Line. Here we show the first page of his diary of observations in Donegal leading up to and including the Transit of Venus in 1769. (Courtesy of the Royal Astronomical Society)
Mason's observations 20. Observations of Transit from Donegal Mason was chosen by the Royal Society to observe the 1769 transit from Donegal. His observations were regarded by Maskelyne, the Astronomer Royal, as an important contribution to the project because Dixon and others in Lapland were clouded out. Mason's careful preparatory work contains some of the earliest accurate observations for the determination of position carried out in Ireland. (Philosophical Transactions of the Royal Society)
The King's Observatory at Richmond 21. The King's Observatory at Richmond, Kew (1769) This building was designed by Sir William Chambers for King George III specifically for the royal family to observe the Transit of Venus in 1769. It incorporates several features that derive from earlier observatories in Sweden and Denmark. It was influential in the design of the two Irish observatories at Dunsink and Armagh in 1785 and 1789, respectively. (photo by C.J. Butler)
King George III 22. King George III (c 1761) by Allan Ramsey King George III was interested in many aspects of science and particularly astronomy. His collection of scientific and mathematical apparatus was extensive with much of it now preserved in the Science Museum, South Kensington. Some items from his astronomical collection, including the telescopes and clocks used to observe the 1769 Transit of Venus were presented to Armagh Observatory in 1840 by Queen Victoria, after the Kew Observatory ceased to function. Some of them are on display here. (Courtesy of the National Portrait Gallery)
Short Telescope 23. Telescope (1768) by Thomas Short (1710-1768) used by George III to observe the Transit of Venus in 1769. It has several alternative configurations: Cassegrain, Gregorian or Newtonian.
Sisson Quadrant 24. Quadrant by J. Sisson This instrument is also part of the Kew Collection now housed at Armagh Observatory. It was probably used to measure the altitude of Venus as it crossed the Sun in 1769.
Shelton Regulator 25. Regulator by John Shelton With this clock, Dr Demainbray, the Kings Astronomer timed the transit of 1769. It was one of several made by Shelton for the Royal Society's expeditions to observe the transit. One was taken by Captain Cook to Tahiti, another by Dixon to Lapland and a third by Mason to Donegal.
Records of the Transit of Venus observed from Kew 26. Records of the Transit of Venus observed from Kew by Dr Demainbray and members of the Royal family. They are remarkable for their agreement, with only one second difference between all of the observers, whereas at Greenwich, a similar set of observations yielded a spread of times ranging over one minute. It must have been difficult to disagree with the King. ( Courtesy of Kings College, London)

Last Revised: 2009 November 18th