|
The 2004 Transit of Venus was special to those
who witnessed this once in a lifetime event. History records that
in 1769 Captain James Cook was similarly enthralled by a Transit
of Venus.
by Dr Tony Phillips
Every 120 years or so a dark spot
glides across the Sun. Small, inky-black, almost perfectly circular,
it's no ordinary sunspot. Not everyone can see it, but some who
do get the strangest feeling, of standing, toes curled in the damp
sand, on the beach of a South Pacific isle....
Sea gulls fluttered
upward, screeching. City odours drifted in from Plymouth, across
the ship, shoving aside the salt air. Sails snapped taut. The wind
had changed and it was time to go.
On August 12th, 1768,
His Majesty's Bark Endeavour slipped out of harbour, Lt.
James Cook in command, bound for Tahiti. The island had been
"discovered" by Europeans only a year before in the South Pacific,
a part of Earth so poorly explored mapmaker's couldn't agree if
there was a giant continent there ... or not. Cook might as well
have been going to the Moon or Mars. He would have to steer across
thousands of miles of open ocean, with nothing like GPS or even
a good wristwatch to keep time for navigation, to find a speck of
land only 20 miles across. On the way, dangerous storms could (and
did) materialize without warning. Unknown life forms waited in the
ocean waters. Cook fully expected half the crew to perish.
It was worth the risk,
he figured, to observe a transit of Venus.
"At 2 pm got under
sail and put to sea having on board 94 persons," Cook noted in his
log. The ship's young naturalist Joseph Banks was more romantic:
"We took our leave of Europe for heaven alone knows how long, perhaps
for Ever," he wrote.
Their mission was to
reach Tahiti before June 1769, establish themselves among the islanders,
and construct an astronomical observatory. Cook and his crew would
observe Venus gliding across the face of the Sun, and by doing so
measure the size of the solar system. Or so hoped England's Royal
Academy, which sponsored the trip.
The size of the solar
system was one of the chief puzzles of 18th century science, much
as the nature of dark matter and dark energy are today. In Cook's
time astronomers knew that six planets orbited the sun (Uranus, Neptune, and Pluto hadn't been discovered yet),
and they knew the relative spacing of those planets. Jupiter, for
instance, is 5 times farther from the Sun than Earth. But how far
is that … in miles? The absolute distances were unknown.
Venus was the key.
Edmund Halley realized this in 1716. As seen from Earth, Venus occasionally
crosses the face of the Sun. It looks like a jet-black disk slowly
gliding among the Sun's true sunspots. By noting the start- and
stop-times of the transit from widely spaced locations on Earth,
Halley reasoned, astronomers could calculate the distance to Venus
using the principles of parallax. The scale of the rest of the solar
system would follow.
But there was a problem.
Transits of Venus are rare. They come in pairs, 8 years apart, separated
by approximately 120 years.
Halley himself would never live to see one. An international team
did try to time a Venus transit in 1761, but weather and other factors
spoiled most of their data. If Cook and others failed in 1769, every
astronomer on Earth would be dead before the next opportunity in
1874.
Cook's expedition is
often likened to a space mission. "The Endeavour was not only on
a voyage of discovery," writes Tony Horwitz in the Cook travelogue
Blue Latitudes, "it was also a laboratory for testing the
latest theories and technologies, much as spaceships are today."
In particular, the
crew of the Endeavour were to be guinea pigs in the Navy's fight
against "the scourge of the sea" - scurvy. The human body can store
only about 6 week's worth of vitamin C, and when it runs out seamen
experience lassitude, rotted gums, haemorrhaging. Some 18th century
ships lost half their crew to scurvy. Cook carried a variety of
experimental foods onboard, feeding his crew such things as sauerkraut
and malt wort. Anyone who refused the fare would be whipped. Indeed,
Cook flogged one in five of his crew, about average in those days,
according to Horwitz.
By the time Cook reached
Tahiti in 1769, he'd been sailing west for 8 months - about as long
as modern astronauts would spend en route to Mars. Five crewmen
were lost when the ship rounded stormy Cape Horn, and another despairing
marine threw himself overboard during the 10-week Pacific passage
that followed. The Endeavour was utterly vulnerable as it angled
toward Tahiti. There was no contact with "Mission Control," no satellite
weather images to warn of approaching storms, no help of any kind.
Cook navigated using hourglasses and knotted ropes to measure ship's
speed, and a sextant and almanac to estimate Endeavour's position
by the stars. It was tricky and dangerous.
Remarkably, they arrived
mostly intact on April 13th, 1769, almost two months before the
transit. "At this time we had but very few men upon the Sick list
… the Ships company had in general been very healthy owing in a
great measure to the Sour krout," wrote Cook.
Tahiti was as alien to Cook's men as Mars might seem to us today.
No spacesuit was required to survive. On the contrary, the island
was comfortable and well provisioned for human life; the islanders
were friendly and eager to deal with Cook's men. Banks deemed it
"the truest picture of an arcadia (idyllic and peaceful) … that
the imagination can form." Yet the flora, fauna, customs and habits
of Tahiti were shockingly different from those of England; Endeavour's
crew was absorbed, amazed.
Credit: National
Library of Australia
The
view from Point Venus, Tahiti, where Cook and his men
observed the transit of Venus. Oil on canvas, William
Hodges, 1744-1797.
|
No wonder Cook and
Banks had so little to say about the transit when it finally happened
on June 3rd, 1769. Venus' little black disk, which could only be
seen gliding across the blinding sun through special telescopes
brought from England, couldn't compete with Tahiti itself.
Banks' log entry on
the day of the transit consists of 622 words; fewer than 100 of
them concern Venus. Mostly he chronicled a breakfast-meeting with
Tarróa, the King of the Island, and Tarróa's sister Nuna, and later
in the day, a visit from "three handsome women." Of Venus, he says,
"I went to my Companions at the observatory carrying with me Tarróa,
Nuna and some of their chief attendants; to them we shewd the planet
upon the sun and made them understand that we came on purpose to
see it. After this they went back and myself with them." Period.
If the King or Banks himself was impressed, Banks never said so.
Cook was a little more
expansive: "This day prov'd as favourable to our purpose as we could
wish, not a Clowd was to be seen … and the Air was perfectly clear,
so that we had every advantage we could desire in Observing the
whole of the passage of the Planet Venus over the Suns disk: we
very distinctly saw an Atmosphere or dusky shade round the body
of the Planet which very much disturbed the times of the contacts
particularly the two internal ones."
The "dusky shade round
the body of the Planet" was a problem. Intense sunlight filtering
through Venus' atmosphere fuzzed the edge of the disk and decreased
the precision with which Cook could time the transit. For this reason,
his measurements disagreed with those of ship's astronomer Charles
Green, who observed the transit beside Cook, by as much as 42 seconds.
Cook
and Green also observed the "black drop effect." When Venus is near
the limb of the sun - the critical moment for transit timing - the
black of space beyond the Sun's limb seems to reach in and touch
the planet. You can recreate the black drop effect with your thumb
and index finger: Hold the two in front of one eye and narrow the
distance between them. Just before they touch, a shadowy bridge
will spring across the gap. According to John Westfall, writing
for Sky & Telescope magazine in June 2004, "this is simply the
result of how two fuzzy bright-to-dark gradients add together."
The black drop effect, like the fuzziness of Venus' atmosphere,
made it hard to say just when the transit began or ended.
This
was a problem for observers elsewhere, too, not only Cook in Tahiti.
In fact, when all was said and done, observations of Venus' 1769
transit from 76 points around the globe, including Cook's, were
not precise enough to set the scale of the solar system. Astronomers
didn't manage that until the 19th century when they used photography
to record the next pair of transits.
Cook wouldn't dwell
on these matters; there was a lot more exploring to do. Secret orders
from the Navy instructed him to leave the island when the transit
was done and "search between Tahiti and New Zealand for a Continent
or Land of great extent."
Credit: National
Library of Australia.
The Endeavour is beached in Australia following a collision
with the Great Barrier Reef. An engraving from John Hawkesworth's
An Account of the voyages….
|
For much of the next
year Endeavour and her crew scoured the South Pacific, searching
for a continent that some 18th century scientists claimed was necessary
to balance the great land masses of the northern hemisphere. At
one point they were out of sight of land for almost two months.
But the terra australis incognita, the unknown "south land,"
didn't exist, just as Cook suspected. Along the way Cook met the
fierce Maori of New Zealand and the Aborigines of Australia, explored
thousands of miles of Kiwi and Aussie coastline, and had a near-disastrous
collision with the Great Barrier Reef.
Later, during a 10-week
stopover in Jakarta for repairs, seven seamen died of malaria. The
port city was densely populated by people and diseases. Cook left
as quickly as possible, but the damage was done. Ultimately 38 of
the Endeavour's original company (and 8 who joined later) perished,
including astronomer Charles Green, most from diseases picked up
in Jakarta. "The ship's 40% casualty rate wasn't considered extraordinary
for the day," writes Horwitz. "In fact, Cook would later be hailed
for the exceptional concern he showed for the health of his crew."
On July 11, 1771, Cook
returned to England at Deal. The surviving crew of the Endeavour
had circumnavigated the globe, catalogued thousands of species of
plants, insects and animals, encountered new (to them) races of
people, and hunted for giant continents. It was an epic adventure.
In the end, the transit
was just a tiny slice of Cook's adventure, out-enchanted by Tahiti
and sabotaged by black drops. But because of the voyage Venus and
Cook are linked. In fact, it might be said that the best reason
to watch a transit of Venus is James Cook.
Indeed, you may already
have decided for yourself as to the wonders of a Transit of Venus.
On June 8th, 2004, Venus crossed the face of the Sun again. The
event was web cast, broadcast, and targeted by innumerable sidewalk
telescopes. In other words, you couldn't miss it. Think back in
your mind to the inky black disk.... Perhaps the memory of it can
carry you back to a different place and time: Tahiti, 1769, when
much of Earth was still a mystery and the eye at the telescope belonged
to a great explorer.
Can you feel the sand
between your toes?
|
