Transit of Venus (detail)
from James Cook and Charles
Green's
"Observations Made ...
at King George's Island
in the South Sea"
in
Philosophical Transactions
of the Royal Society.
Vol. 61, 1771.
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The main mission of Captain Cook’s
first voyage was to extend knowledge in the field of astronomy.
The astronomer Edmond Halley had explained in 1716 that if the transit
of Venus were to be observed from a number of different vantage
points on the earth, then the distance to Venus, and hence the distance
to the sun, could be accurately calculated. The value of the mean
distance between the earth and the sun needed to be more precise,
because that unit (now called the astronomical unit, or the AU)
is used to describe the scale of the solar system. As part of an
international effort, the Royal Society and the British Admiralty
sent James Cook and the Endeavour to Tahiti for the express
purpose of observing Venus as it passed across the sun in 1769.
In addition, Cook was to explore the Pacific Ocean and search for
the great southern continent that was supposed to lie about the
South Pole.
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On June 3, Cook reported that the weather at Tahiti was excellent
for viewing the transit of Venus. The report in the Philosophical
Transactions includes an engraved illustration of his and
astronomer Charles Green’s observations. It depicts
"an atmosphere or dusky cloud round the body of
the planet,"
reported generally among the observers worldwide, which caused
a disappointing lack of the precision that had been hoped for
by all.
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