Preparations For The 1769 Transit

Both the British and the French redoubled their efforts in the quest to obtain the desired benefit from the 1769 transit. The middle of the event was at about 22:20 UT (i.e., London time), so it was clear that stations in the Pacific were required if the entire six-hour transit was to be followed.

Simplistically, one could imagine that locations further east (for instance in the Caribbean) might be able to see the ingress, those further west (in India) the egress, and only at longitudes for which local midday is near 22:20 UT would the complete transit be observable. From that perspective Tahiti was an excellent prospect. Hawaii, which Cook was to map on a later voyage and name the Sandwich Islands, meeting his death there in 1779, would have been at a good longitude, although a more southern latitude was desired.

But that discussion is too simple. The transit was on June 3, only 19 days before the summer solstice, and so the Northern Hemisphere was tilted toward the Sun. In consequence the event might be seen throughout the Arctic, almost independent of longitude. Lapland is often called the Land of the Midnight Sun for a good reason, and it was realized that the transit could be observed from, say, the Russian town of Murmansk, and right across northern Siberia to the Pacific and thence Canada. At that time of the year the Sun is above the horizon for most of the day at such latitudes.

This meant the observational baseline could be stretched far to the north, and an international effort was organized, the British taking responsibility for extrapolating that baseline as far south as possible. Apart from Murmansk, and Hudson's Bay, ingress-to-egress timings were made in Norway. In fact it was our old friend Jeremiah Dixon who went to Norway, this time unaccompanied by Charles Mason. The idea that Cook was sent to Tahiti because the transit could not be observed from anywhere near the longitude of Britain is therefore incorrect: paradoxically, he sailed to Cape Horn and then westwards for reasons of latitude. That is, Cook's party went to the Pacific in order to observe the transit from as far south as possible.

One other important location from which the transit in 1769 was observed deserves special mention, a French expedition. It was the Abbé Jean Chappe d'Autoroche who had watched the

1761 transit from Tobolsk, in Siberia, an observation mentioned earlier. This time he wanted to sail to the Solomon Islands. These islands lie in the western Pacific, northeast of Australia, and were the site of ferocious fighting during the Second World War. Back in the eighteenth century the Solomons were under Spanish control, but despite intending to take two Spanish naval officers along the court of Spain refused him leave, suspecting him of wanting to spy out the territory on behalf of France. Thus Chappe sailed across the Atlantic, through the Caribbean, and landed in Mexico at Veracruz. From there his party traveled overland through Mexico City at great personal danger from banditos, the localViceroy then providing them with an escort of soldiers as they pushed on to the Pacific coast through Guadalajara. From San Blas they sailed, with some difficulty, northwest towards Cape San Lucas, the tip of Baja California, and observed the transit from San José del Cabo. This has been the source of much confusion, "San José, California" being a totally different place, deep in Silicon Valley.

Chappe got to this lesser-known San José a fortnight before the transit and fixed its latitude by observing the culmination of stars, its longitude using the moons ofJupiter, and then the transit. A complete success, except that a contagious disease—a strain of typhoid it seems—was already sweeping San José when his party arrived. Ignoring the danger, Chappe insisted on remaining not only for the transit, but also thereafter for a lunar eclipse on June 18. Timing of that eclipse was required to secure the site's longitude, an essential parameter if the transit project was to succeed. By then Chappe had himself succumbed to the illness. He died six weeks later, as did one of the Spanish officers, but the remnants of the party ensured that the invaluable timings were returned to Europe.

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