different stations, or the difference in the durations of the transit, can be without difficulty translated into difference of direction, and the distances of Venus and the sun can be deduced.[1]
Immense trouble was taken by Governments, Academies, and private persons in arranging for the observation of the transits of 1761 and 1769. For the former observing parties were sent as far as to Tobolsk, St. Helena, the Cape of Good Hope, and India, while observations were also made by astronomers at Greenwich, Paris, Vienna, Upsala, and elsewhere in Europe. The next transit was observed on an even larger scale, the stations selected ranging from Siberia to California, from the Varanger Fjord to Otaheiti (where no less famous a person than Captain Cook was placed), and from Hudson's Bay to Madras.
The expeditions organised on this occasion by the American Philosophical Society may be regarded as the first of the contributions made by America to the science which has since owed so much to her; while the Empress Catherine bore witness to the newly acquired civilisation of her country by arranging a number of observing stations on Russian soil.
The results were far more in accordance with Lacaille's anticipations than with Halley's. A variety of causes prevented the moments of contact between the discs of Venus and the sun from being observed with the precision that had been hoped. By selecting different sets of observations, and by making different allowances for the various probable sources of error, a number of discordant results were obtained by various calculators. The values of the parallax (chapter viii., § 161) of the sun deduced from the earlier of the two transits ranged between about 8" and 10"; while those obtained in 1769, though much more consistent, still varied between about 8" and 9", corresponding to a variation of about 10,000,000 miles in the distance of the sun.
The whole set of observations were subsequently very elaborately discussed in 1822–4 and again in 1835 by Johann Franz Encke (1791–1865), who deduced a parallax of 8"⋅571, corresponding to a distance of 95,370,000 miles,
- ↑ For a more detailed discussion of the transit of Venus, see Airy's Popular Astronomy and Newcomb's Popular Astronomy.
