Venus
A vaguely Valentine’s Day related post today 
Observing the transit of Venus, the time at which the planet passes between the sun and earth, is a constant feature of the early records of the Royal Society, who received correspondence from around the world recording the precise times at which it was observed. The purpose of this romantic exercise, similarly to the mapping of the transits of Mercury, was to make observations relating to the position of the sun and planet, using the parallax method. This was pioneered by Edmund Halley, who explains the theory in Philosophical Transactions (translated here for those not keen to read the original in Latin). Mapping transits also has practical applications in navigation as it provides a natural clock with the aid of which longitudes, still a thorny problem in the seventeenth and eighteenth century due to the dearth of reliable timekeepers, could be calculated according to the difference in time between locations on a reference meridian. Perhaps this explains the interest of the East India Company, who allowed Halley to visit St Helena, where he built an observatory, the remains of which can still be observed on the island.
Halley’s work promoted international collaboration between scientists as well as voyages of exploration: most notably Captain Cook’s voyage to Tahiti to observe the 1769 transit. Romantics will have to wait a while to emulate the early stargazers, however, as the next transit is predicted to be visible in 2012.
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