They occur at intervals of eight years, a small section of individual life; but alternating with intervals of more than a century, during which whole generations pass away, thrones crumble, and dynasties change. (Thomas Milner)
1631: Before the 17th century nobody had ever seen a transit of Venus, because nobody had ever thought to look. The German astronomer Johannes Kepler was the first man to work out the exact shape of planetary orbits (in 1627), and hence the first to predict that Venus would cross the Sun on 6th December 1631. Unfortunately it was night-time in Europe, nobody elsewhere knew anything about it and Kepler died in 1630. So nobody saw anything. 1639: A 20-year old Liverpudlian astronomer called JeremiahHorrocks made a careful study of Kepler's tables and worked out, with only a month to go, that another transit of Venus was due on 24th November 1639. He set up a projecting telescope in his house in MuchHoole, near Preston. It was a cloudy, wintry Sunday and Jeremiah had to attend church twice during the day. However at 3:15pm the clouds finally broke and he was allowed half an hour of uninterrupted viewing before sunset. Horrocks was the first man ever to see this astronomical phenomenon, and used his observations to calculate that the Sun was far bigger and far further away than any scientist had ever imagined. Alas he died the following winter, a young genius extinguished. You can read his notes from the day here, read a full biography here and visit his local church here.
1761: Astronomers planned to use accurate measurements of the 1761 transit from different points on the globe to calculate the exact distance from the Earth to the Sun. English and French observers sailed on long perilous voyages to chosen viewing points across the world, not helped by the two countries being at war at the time. However it proved very difficult to determine the exact time that Venus crossed the edge of the Sun (due to the 'Black drop' effect) so the final calculations turned out to be less than conclusive. 1769: The Royal Society sent Captain James Cook on an antipodean voyage to observe the 1769 transit from the newly-discovered island of Tahiti. At least that was the cover story - the Endeavour was also on an secret mission to discover a 'Continent or Land of great extent' (which turned out to be the eastern coast of Australia). Cook built a fort in Matavai Bay to protect his expedition from the natives, establishing an observatory in preparation for the transit. He successfully (if inaccurately) recorded his observations, but it was two eventful years before he could return this information to London.
1874: Science had moved on over the previous century, with the invention of photography to record events with greater accuracy and the telegraph to send data across the world faster than any ship. Astronomers journeyed to far-flung locations to record the 1874 transit, including Hawaii, New Zealand and Vladivostok. When all the results were compiled the distance to the Sun was calculated to be 92,885,000 miles - not bad, but still not as accurate as scientists would have liked. 1882: This December transit was widely observed across Europe and North America but by now serious astronomers had given up on transit observations, realising they could never yield really useful results. Over a mere five occurrences these transits of Venus had shifted from events of astronomical importance to a sideshow with mere curiosity value. But that didn't stop Professor Robert Stawell Ball, the Patrick Moore of his day, from peering through the snow at an observatory in Dublin and looking forward wistfully to the sixth...
Transit of Venus: the history bit
