Did you ever wonder what happened to Mason and Dixon [Line] before they became famous? It's a bit like Oscar Levant's quip "I'm so old I knew Doris Day before she was a virgin". [that's a misquote].
I'm reading Andrea Wulf's 2012 book Chasing Venus The Race to Measure the Heavens which cashed in on the most recent transit of Venus 5/6 June 2012. I picked if up remaindered for €2.50 in June 2017 and almost didn't. There are so many books which set out to explain something at the interface between science and history and they are not all of equal quality. If it's remaindered it's probably for good reason, I reasoned. I was wrong, I got stuck in and read the first 100 pages without coming up for air, It was favorably reviewed in The Guardian by Nicholas Lezard who expresses his ennui - yawn! - about transits of Venus when our generation had the chance to see two. Because he wasn't properly primed about how exciting the transits of 1761 and 1769 had been and how they had given science and the public understanding of science a huge boost. And it turns out that I secured a copy of one of the few 5 year old books that is not available on Amazon for £0.01.
Johannes "Ellipse Orbit" Kepler had calculated the orbit of Venus from numerous observations of its position against the fixed stars at specific points of Earth-time. For those of us who are not astronomers with 20/20 hindsight, it seems a colossal feat of the imagination and trigonometry to be able to do this. In 1627, Kepler predicted that Venus would traverse the face of the Sun in 1631 and have a near-miss in 1639. He died in 1630 without being able to verify his prediction. Two British observers Jeremiah Horrocks in Preston and William Crabtree in Manchester were able to refine Kepler's calculations to be convinced that there would be another transit in 1639. Lucky with the weather, they were, on 4th December 1639, able to project the image of the planet against the Sun's disc through a telescope and onto a sheet of paper. Winter in N England has a very short day, the weather is usually crappy and it was only possible for Horrocks to make an estimate of a) the relative size of Venus and the Sun and a guesstimate of how long the transit was. His calculations of the Earth-Sun distance were out by about 50% which is pretty good given the constraints and lack of any precedent.
In 1716 Edmond "Comet" Halley wrote a paper predicting the 1761 and 1769 transits were a) going to happen and b) would be really useful for measuring the scale of the solar system. That's rather a long lag time between this call to arms and the events at point and indeed Halley died 20 years before the first one. The measurements are only really useful if they are carried out from widely separate parts of the world and getting 'foreign' was a mighty logistical problem in the days before steam-ships let alone bookings with Air France. The astronomical collective came within an ace of literally and figuratively missing the boat and it fell to Joseph-Nicolas DeLisle to serve as the catalyst to get some Effectives in place. The British Royal Society and the French Académie des sciences led the charge and were, remarkably, in contact despite the fact that their political masters were 1756-1763 engaged in the Seven Years War = Guerre de Sept Ans = [in USA] the French & Indian War which could qualify as the First World War. Thus in addition to the hazards of weather in the day, the risks of storms on the way there was the possibility of being killed or captured in a naval action.
Charles Mason, aged 31 in 1761, was the assistant to the Astronomer Royal in Greenwich. He was contracted for £200 to sail Sumatra to make southern observations of the 1761 transit. This was abut 4x his meagre salary for being the gopher at Greenwich. He took with him an even younger surveyor and amateur astronomer called Jeremiah Dixon who was a neighbour of a Fellow of the Royal Society. Dixon also accepted £200 to sail to the far side of the world for an ephemeral chance at making astronomical history. It was as important to know where your were making the observations from as it was to measure the length of time of the transit; so they made a good team. But 4 days after leaving Plymouth their ship was attacked by a larger French frigate and badly mauled. Putting back to Plymouth for major repairs Mason and Dixon seem to have temporarily lost the bottle for adventure and sent a series of truculent letters to their employers at the Royal Society refusing to go to Sumatra. Eventually they were shown their contract and told that they didn't have any choice in the matter. And there was no time to lose; indeed they were now too late to make Sumatra in time for the 6 hour window of opportunity. Accordingly they broke their contract again by electing to stop-off in Cape Town to make observations from there - despite the fact that the first part of the transit would, from South Africa be invisible because the Sun would not have risen. Fortune favored the bold (and stroppy) and Mason and Dixon obtained the best set of data from the Southern hemisphere! Their success outshone their previous tantrums and they won a surveying contract in 1763 which makes their names far more recognisable today than any of the other astronomers involved in the events of 1761. That contract was to survey and mark the boundary between Pennsylvania and Maryland which is now immortalised as the Mason-Dixon Line - sing Dixie!
Here Andrea Wulf and CBS sets out the transit stall. Here's the maths for how timing the transit allows an estimate of 1AU the distance from Earth to Sun. Matt Parker explains (with ExCel!) why the transits come in pairs separated by 8 years every 100+ years.