1769 Transit of Venus observed from Tahiti

On June 3, 1769, British navigator Captain James Cook, British naturalist Joseph Banks, British astronomer Charles Green and Swedish naturalist Daniel Solander recorded the transit of Venus on the island of Tahiti during Cook's first voyage around the world. During a transit, Venus appears as a small black disc travelling across the Sun. This unusual astronomical phenomenon takes place in a pattern that repeats itself every 243 years. It includes two transits that are eight years apart, separated by breaks of 121.5 and 105.5 years. These men, along with a crew of scientists, were commissioned by the Royal Society of London for the primary purpose of viewing the transit of Venus. Not only would their findings help expand scientific knowledge, it would help with navigation by accurately calculating the observer's longitude. At this time, longitude was difficult to determine and not always precise. A "secret" mission that followed the transit included the exploration of the South Pacific to find the legendary Terra Australis Incognita or "unknown land of the South."

In 1663, Scottish mathematician James Gregory came up with the idea of using Venus or Mercury transits to determine the astronomical unit by measuring the apparent solar parallax between different points on the surface of the Earth. In a 1716 issue of the Philosophical Transactions of the Royal Society, Edmund Halley illustrated Gregory's theory more fully and explained further how it could establish the distance between the Earth and the Sun. In his report, Halley suggested places that a full transit should be viewed due to a "cone of visibility." Places he recommended for observing the phenomenon included the Hudson Bay, Norway and the Molucca Islands. The next transits would occur in 1761 and 1769. Halley died in 1742, almost twenty years before the transit.

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