There is something about the Transit of Venus that touches the imagination in ways that are not all to do with astronomy. The last Transit occurred in 2012, and if nobody who watched it will ever see another, the sight of that small black dot, making its almost imperceptible progress across the disc of the sun, offered, across the span of 243 years, an oddly moving connection with the scientists and sailors who quartered the globe and, sometimes, risked their lives in 1769 to attempt the first comprehensive observations of the event.