British naval officer James Cook prepares to…
1772 CE
British naval officer James Cook prepares to observe the solar eclipse of the planet Venus, having arrived sailing from Cape Horn to Tahiti on the ship HM Bark Endeavour on April 13, 1769, and commissioned the building of a small fort and observatory at what is now known as Point Venus.
The astronomer appointed to the task is Charles Green, assistant to the recently appointed Astronomer Royal, Nevil Maskelyne.
The primary purpose of the observation is to obtain measurements that can be used to calculate more accurately the distance of Venus from the Sun.
If this can be achieved, then the distances of the other planets can be worked out, based on their orbits.
The transit takes place on June 3.
Disappointingly, the separate measurements of Green, Cook and Solander vary by more than the anticipated margin of error.
Their instrumentation is adequate by the standards of the time, but the resolution still cannot eliminate the errors.
When their results are later compared to those of the other observations of the same event made elsewhere for the exercise, the net result is not as conclusive or accurate as had been hoped.
The difficulties are today thought to relate to the Black drop effect, an optical phenomenon that precludes accurate measurement—particularly with the instruments used by Cook, Green and Solander.
Once the observations are completed, Cook opens the sealed orders for the second part of his voyage: to search the south Pacific for signs of the postulated rich southern continent of Terra Australis, acting on additional instructions from the Admiralty.
The Royal Society, and especially Alexander Dalrymple, believe that it must exist and that Britain's best chance of discovering it and claiming its fabled riches before any other rival European power manages to do so would be by using Cook's Transit of Venus mission (on an inconspicuous small ship such as the Endeavour) as a cover.
Cook, however, has his own personal doubts on the continent's existence.