Captain Cook and Mister Green
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On April 11, 1769, Endeavour arrived at Tahiti, followed by a generally friendly
introduction between the crew and the Tahitians. After almost eight months
at sea, the men of Endeavour enjoyed the luxury of fresh food and the company
of women. To maintain discipline in the face of Tahiti's manifold pleasures,
Cook kept his crew busy building a fortification on shore, which he christened “Fort
Venus.” A cluster of tents intended to house the scientists and their
instruments was surrounded by a wooden palisade. The fort was completed well
before the time of the transit.
Fort Venus, the transit viewing site for the Endeavour crew at Tahiti. (Courtesy of the British Library)
A major setback occurred on the morning of May 2, when the expedition's
heavy brass quadrant was stolen from a tent inside the fort. “It was
a matter of astonishment to us all,” Cook wrote, “how it could
be taken away, as a Centinal stood the whole night within 5 yards of the door
of the Tent where it was put.” The quadrant was essential for measuring
the latitude and longitude of the fort and calibrating the expedition's
clocks by comparing them to local solar time. Green and Cook had not brought
a spare for the expensive piece of equipment; the quadrant had to be recovered.
Green and Banks, accompanied by a Tahitian interpreter and an English midshipman,
set off to track down the quadrant. “The weather was excessive hot,” Banks
reported in his diary. “Sometimes we walked, sometimes we ran when we
imagined…that the chase was just before us.” After traveling for
seven miles in the tropical heat, exhausted and apprehensive, the Englishmen
were finally met by a party of Tahitians bearing pieces of the quadrant, which
they had dismantled. A quick examination of the pieces by Green revealed that
nothing essential was missing. After the long hot return to Fort Venus, the
quadrant was repaired by Herman Spöring, one of Banks' assistants
who had been a London watchmaker before joining the expedition.
June 3, the day of the transit, was cloudless and intensely hot. (A thermometer
placed in the sun read 119 degrees Fahrenheit.) Cook and Green staunchly remained
in the midday sun, carefully measuring the duration of the transit. Cook's
and Green's measurements, however, differed significantly. Cook timed
the transit at 6 hours, 6 minutes, and 20 seconds, but Green's measurement
was 17 seconds longer. The problem, they realized, was that the blurring effect
of Venus' atmosphere made it difficult to estimate the exact time when
Venus first touched the Sun's disk.
Nevertheless, having done their best, they packed their instruments and continued
westward.
Departure from Tahiti did not end Cook and Green's observational labors.
They spent half a year meticulously mapping the coast of New Zealand. At the
end of the survey, Cook proudly noted that “the situation of few parts
of the world are better determined that these Islands are, being settled by
some hundreds of Observations of the Sun and Moon and one of the transit of
Mercury made by Mr. Green.” After nearly succumbing to disaster when
she ran aground on the Great Barrier Reef, Endeavour sailed onward to the Dutch
outpost of Batavia (now the city of Jakarta, Indonesia). Cook took the opportunity
to forward a glowing report to the British Admiralty. “I was very much
assisted by Mr.
Green,” he wrote in appreciation, “who let slip no one opportunity
for making of Observations for settling the Longitude during the whole Course
of the Voyage.” Ironically, immediately after Cook penned his optimistic
report, the Endeavour's crew was stricken by a deadly outbreak of dysentery,
graphically known in the eighteenth century as the “bloody flux.” Cook
escaped the illness, but Green was seriously afflicted; he died on January
26, 1771.
According to a later newspaper report, he “was directed by the surgeon
to keep himself warm, but in a fit of phrensy got up in the night and put his
legs out of the portholes, which was the occasion of his death.” Green
had not yet organized his astronomical notes when he died, so Cook spent much
of the return to England making sense of the disorder. When Endeavour finally
landed in England in July 1771, after an absence of nearly three years, only
56 of the original 94 men were still alive. But Green and his transit observations
left their mark on humanity's ongoing quest for understanding its place
in the universe.
As results trickled in from expeditions all over the globe — Norway,
Hudson's Bay, Baja California, and dozens of other sites — astronomers
began using them to calculate the distance from Earth to the Sun. More than
600 scientific papers were published concerning the 1769 transit. Using different
subsets of the available data and different algorithms for computing the distance,
astronomers computed values for the Earth-Sun distance ranging from 92.8 million
miles to 96.9 million miles.
Today we have much better methods for measuring distances within the solar
system (we can find the distance to Venus by bouncing radar signals off its
surface), and the average Earth-Sun distance is known to be 92.96 million miles.
The next transits of Venus, which will occur on June 8, 2004, and June 5, 2012,
will be interesting curiosities, but astronomers will not use them to measure
the size of the solar system. Despite the fact that the techniques of Cook
and Green have been rendered obsolete, their courage and resolution have not
been forgotten.
Barbara Ryden is an assistant professor of astronomy at Ohio State University
and a frequent contributor to StarDate. Her article about four generations
of Cassini astronomers appeared in the January/February 1994 issue.
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