The Venus Hunters
For nearly four centuries, astronomers have chased the planet to
the ends of the Earth
Chart: How to Catch a Planet
By RICHARD MONASTERSKY
Early next month, billions of people
around the world will have a chance to see something that nobody alive has ever
witnessed. On June 8, Venus will slowly march across the face of the Sun
-- creating a small black period that will punctuate the fiery solar disk.
The celestial event, called a
transit of Venus, last took place in 1882, when it created such a stir that
spectators jammed Wall Street, and global powers opened their coffers in fits
of patriotic frenzy to see which nation could best observe the phenomenon from
remote spots on Earth.
Astronomers risked their lives, and
occasionally lost them, because they hoped to use the transit to measure the
distance between the Earth and the Sun. That value, named the astronomical
unit, once provided the fundamental means to map the positions of heavenly
bodies and determine the size of the universe. The measurement was so central
to those tasks that the British Astronomer Royal in the mid-19th century called
it "the noblest problem in astronomy."
"At one time it was the most
important thing in astronomy," says Jay M. Pasachoff, a professor of astronomy
at Williams College. Nations spent the equivalent of millions of dollars
mounting transit expeditions that prefigured the Apollo missions and the
robotic rovers now driving across the surface of Mars.
"In the 19th century it was
really analogous to the space race," says Steven J. Dick, chief historian
at NASA. "Any country that had a scientific reputation sent out [transit]
expeditions. It was a race to see who could come up with the best technique and
final answer."
Today astronomers have more accurate
ways of determining the astronomical unit, but many researchers will
nonetheless take up their telescopes next month to observe the transit of
Venus. They will gather, in small groups and at major conferences in England
and Iran, to pay homage to intellectual forebears who struggled so heroically
to document this rare occurrence. The tales of those exploits recall some of
the biggest names in science and provide snapshots of key moments in the past
four centuries when astronomy intersected with wars, empires, and exploration.
Britain, for example, sent Captain
James Cook off to the South Pacific to observe a transit in 1769, in a mission
that eventually took on far more importance as a means of expanding the British
Empire. Charles Mason and Jeremiah Dixon set off to measure the 1761 transit
and so impressed their superiors that they later were assigned to survey the
border between Maryland and Pennsylvania, a project that resulted in the
Mason-Dixon line. And the 1882 transit made such an impression on the young
John Philip Sousa that he wrote a march and later a novel about the event.
Chance of a Lifetime
Although Venus has been passing
between Earth and the Sun since the dawn of the planets, history has no record
of anybody witnessing the event until 1639. Even then, only two people had that
honor -- and they almost missed it.
Since then, Venus has transited
across the Sun only four times, in 1761, 1769, 1874 and 1882. The planetary
show happens so infrequently because Venus's orbit tilts slightly relative to
Earth's. Roughly every 115 years the orbits line up, and Venus appears for six
hours as a tiny dot migrating across the Sun's face.
If people can't catch the June 8
transit this year, they might have a chance -- depending on where they
live -- to see the next occurrence, on June 6, 2012. Miss that one, too,
and it will take a breakthrough in medical science for them to have another
such opportunity. The next pair of transits will not occur until 2117 and 2125.
The astronomer Johannes Kepler was
the first person to recognize the potential for such displays when he wrote in
1627 that Venus would appear against the backdrop of the Sun late in the year
1631. He also realized that meticulous observers at different spots on Earth
could use the transit indirectly to calculate the distance to the Sun. An
observer in London, say, would see Venus from a different angle than would
somebody in Cape Town, South Africa. Knowing the distance between the two
cities, astronomers could use the angle between the observers to calculate the
distance to Venus, and from that, the distance between Earth and the Sun.
Kepler died the year before the transit, but he would have missed it anyway,
because it happened during the night in Europe and it was visible only on the
other side of the Earth.
Although Kepler made tremendous
leaps in understanding the motions of the planets, he was not immune to
mistakes. He predicted the next transit for the middle of the 18th century, not
recognizing that another would occur only eight years later. That 1639 transit
would have passed unnoticed by all had it not been for Jeremiah Horrocks, an
insightful young man whose accomplishments might have rivaled Isaac Newton's if
his life had not ended so early.
Despite the important role he played
in astronomy, Horrocks is a historical black hole. Scholars have no records
detailing when he was born, what kind of occupation he held, or even what
caused his death at the age of 22 or 23.
Born near Lancashire, Horrocks
entered the University of Cambridge at age 13 as a sizar -- a member of
one of the original work-study programs. In return for his tuition, he acted as
a servant to a senior fellow. At Cambridge he took an interest in astronomy and
started to read the classics in the field, while at the same time perfecting
his techniques for observing the sky.
Later a friend named William
Crabtree suggested that Horrocks read the work of Kepler, whose ideas were not
well received in England at the time. Horrocks recalculated some of Kepler's
tables and realized in 1639 that Venus would transit the Sun in just a few
weeks' time. By letter, he urged Crabtree to try to observe it as well.
Horrocks set up a telescope in a
darkened room and projected the image of the Sun onto a screen. (Looking at the
Sun directly, with or without a telescope, can quickly cause blindness.) Not
sure of his own calculations, he started watching two days before he
anticipated the event, according to Wilbur Applebaum, a Horrocks scholar and an
emeritus professor of history at the Illinois Institute of Technology.
On the fateful Sunday, Horrocks was
called away on some unknown but urgent business, but returned in time to see
the spot of Venus against the Sun's disk just before sunset. Using the limited
transit data he collected, he succeeded in measuring the apparent size of
Venus.
Horrocks spent the next year
perfecting his calculations and writing several drafts of his work. In December
1640, he sent a letter to Crabtree, telling him that the manuscript was almost
complete. The two men, who had never met in person, arranged to get together in
January. But the day before their scheduled meeting, Horrocks died.
His work on the transit of Venus did
not appear until 1662, but Isaac Newton and Robert Hooke learned about the work
before its publication. Mr. Applebaum suggests that Horrocks's ideas, in fact,
formed the basis for Newton's theory about the motion of the Moon. Newton
acknowledged his predecessor by thanking "our countryman Horrox" in
his Principia.
Immortal Fame
From that quiet beginning in the
17th century, the study of transits turned into a frenzy the next time an
opportunity arrived, thanks to the work of Edmond Halley, whose name graces the
famed comet. Halley, who was Britain's Astronomer Royal, conceived of a much
simpler way to use the transits of Venus to determine planetary distances.
Instead of using angles, Halley
suggested that observers in different locales time the duration of the transit.
An observer in Europe would see Venus pass across a different part of the Sun than
would an observer in Asia; hence the transit would last longer in one spot than
another. By determining the time to the nearest second, they could determine
how far away Venus was from Earth, says J. Donald Fernie, an emeritus professor
of astronomy at the University of Toronto.
Halley knew that he would not live
to see the next transit -- he died in 1742 -- and he pleaded with
"those curious astronomers who (when I am dead) will have an opportunity
of observing these things." He promised "immortal fame and
glory" to astronomers whose observations succeeded in improving the
measure of the planet's orbits.
His successors took those words to
heart. Despite the hazards of 18th-century travel and the various wars raging
among European powers, more than 270 teams went off around the world to record
the pair of transits later in that century.
The Royal Society of London sent
Charles Mason and Jeremiah Dixon to Sumatra to witness the 1761 event. Only
hours into their trip, their ship came under fire from a French frigate in the
English Channel. With 11 dead and dozens injured, the British ship returned to
port. The rattled scientists sent word to the Royal Society thanking it for the
opportunity but graciously declining a second attempt. The society bullied Mason
and Dixon into setting off again, however, by threatening their careers and
their social standing. Now pressed for time, the two never made it to Sumatra
but stopped in South Africa and successfully observed the transit from there.
Mason and Dixon's trials pale in
comparison with the sufferings of French astronomers of the time, whose
exploits have become legendary in the annals of science. One of them, Guillaume
Joseph Hyacinthe Jean Baptiste Le Gentil de la Galaisiere embarked on an
ill-fated journey that stretched almost as long as his name.
Le Gentil, as he has come to be
known, planned to observe the June 6, 1761, transit from Pondicherry, a
French-controlled city on the east coast of India. He left France in March 1760
and arrived at the island of Mauritius, in the southern Indian Ocean, that
July. Although he had plenty of time to reach Pondicherry, ill winds and
British forces conspired to keep him away. Had he stayed put on Mauritius, he
could have observed the transit there. But by bad luck, Le Gentil was at sea
trying to reach Pondicherry during the transit and could make no meaningful
observations.
Determined not to miss the next one,
he decided to remain in the Indian Ocean for several years, studying Madagascar
and nearby islands. In 1766 he sailed to Manila to prepare for observing the
1769 transit from there. He tried to circumvent any political problems by
requesting letters of recommendation from the Spanish royal court to give to
the Spanish governor of Manila. After 14 months the letters arrived, but the
governor declared that they must be forgeries because he could not conceive of
a response from Europe arriving so quickly.
Le Gentil worried that he might land
in prison or suffer a worse fate if he stayed in Manila, so he decided to head back
to India. In March 1768, he finally reached Pondicherry, built an observatory,
and prepared for the morning of June 4, 1769, when all his years of effort
would reach a climax.
The whole month of May brought
beautiful weather, and the night of June 3 was clear enough for the astronomer
to see a moon of Jupiter. But at 2 a.m., when he awoke to check the conditions,
"I saw with the greatest astonishment that the sky was covered everywhere.
... From that moment on, I felt doomed, I threw myself on my bed, without being
able to close my eyes," he said in his published account, translated by
the late Helen Sawyer Hogg.
The blanket of clouds blocked out
the Sun until after the transit ended -- and then the sky cleared for the
rest of the day. Le Gentil had missed his last chance, while conditions in
Manila that day had remained perfectly clear.
"That is the fate which often
awaits astronomers," he wrote. "I had gone more than ten thousand
leagues; it seemed that I had crossed such a great expanse of seas, exiling
myself from my native lands, only to be the spectator of a fatal cloud which
came to place itself before the Sun at the precise moment of my observation, to
carry off from me the fruits of my pains and my fatigues."
Le Gentil's luck turned no better when
he tried to head home. He made several attempts, only to be blocked by a
hurricane on one voyage and by a petulant French captain, who refused him
passage, on another.
When the wayward astronomer finally
returned to France, after an absence of 11 years, he found his estate in a
shambles and his spot occupied in the Academy of Sciences -- an outrage
considering that the academy had sent him on his trip in the first place. He
eventually regained his position, married, had a daughter, and lived until the
age of 67.
Difficult Measures
However long and trying, Le Gentil's
odyssey was a vacation in contrast to the fate of his colleague Jean-Baptiste
Chappe D'Auteroche, who in 1760 slogged overland to the Ural Mountains to
observe the transit from there. After that mission proved a success, Chappe
requested a warmer climate for the next transit, and the Academy of Sciences
sent him to Baja California for the 1769 event. His crew sailed to the east
coast of Mexico and crossed the country overland, fighting heat, rain, insects,
and rough terrain.
Chappe's engineer, a man known to
history only as Pauly, wrote that "high mountains, dreadful precipices,
dry deserts offered to us every day some new dangers. We came near dying a
thousand times." They finally reached the Pacific and went by boat to the
southern tip of Baja. When they arrived, though, Pauly said, "some savage
people who came to us informed us that a most dreadful epidemic was laying
waste the country."
With only 13 days left before the
1769 transit, Chappe waved off the warning to move north. Instead his team
quickly built an observatory and observed the transit. Days later, the disease
starting claiming the Europeans. "A burning soil such as there is in that
country was our bed," wrote Pauly. "The few medicines we had brought
from France were useless for want of knowing how to use them. In that miserable
situation we were bound, to quench our burning thirst, to drink stagnant water
full of copper."
Chappe died on August 1, and most of
the rest of the team also perished. Pauly survived and carried the observations
back to the Academy of Sciences.
Astronomers back in Paris reaped the
benefits of such extravagant and ill-fated efforts. Using the hard-won data
from Chappe and others, the scientists calculated the astronomical unit to be a
little more than 90 million miles, close to the actual value of nearly 93
million miles. But the 18th-century observers had run into an unforeseen
difficulty in timing the transits. Nobody, it seemed, could agree on how to tell
when Venus began intersecting the disk of the Sun.
The problem was that the planet did
not cleanly meet up with the edge of the sun. Instead, as the two bodies
neared, the black circle of Venus would seem to ooze toward the edge of the
Sun, like oil spreading across a table. That made it difficult for two
observers, even right next to each other, to agree on when Venus actually
entered and exited the solar disk.
Captain Cook, observing the 1769
transit from Tahiti, described the problem in his journal. Despite perfectly
clear skies, "we very distinctly saw an Atmosphere or dusky shade round
the body of the Planet which very much disturbed the times of the
Contacts," he wrote, "... and we differ'd from one another in
Observeing the times of the Contacts much more than could be expected."
Scientists once attributed the
problem, called the black-drop effect, to the atmosphere of Venus. But new
research points toward the Sun itself, and the way it grows darker toward its
edges, which can cause strange optical tricks.
The Patriotism Card
More than a century after Cook's
voyage, the United States banked on the new technique of photography to get
around the dreaded black-drop effect. To secure the necessary support, the
superintendent of the U.S. Naval Observatory played the patriotism card by
saying that observing the 1874 transit would "afford our countrymen a
peculiarly favorable opportunity to exercise their inventive ingenuity in the
introduction of improved modes of observation." He helped his case by
noting that Britain, Russia, and Germany had provided significant money for
their astronomers, says Mr. Dick, the NASA historian.
Congress appropriated a total of
$177,000 (the equivalent of more than $2-million today) to send teams of
observers to Vladivostok, Russia; Nagasaki, Japan; Beijing; the Kerguélen
Islands, in the Indian Ocean; two places in Tasmania; New Zealand; and the
Chatham Islands, near New Zealand.
In 1882 Congress appropriated
$85,000 to finance another eight expeditions, several of which could stay
closer to home because the transit was visible from the United States.
Virtually every telescope in the country, says Mr. Dick, was turned toward the
Sun on December 6 to witness the transit.
For John Philip Sousa, the 1882
event provided inspiration for a "Transit of Venus March," which was
performed early the next year, according to an exhibit titled "Chasing
Venus: Observing the Transits of Venus, 1631-2004," on display at the
Smithsonian Institution's National Museum of American History through April
2005. Even decades later, the planetary encounter continued to move Sousa, who
wrote a comic novel in 1920 called The
Transit of Venus, about a group of men taking a holiday away from women to
watch the transit.
For scientists, though, the transit
work was serious business. William Harkness, a Naval Observatory astronomer,
used the data collected by the observing teams to calculate a value of
92,797,000 miles, plus or minus 59,700 miles, for the distance between the
Earth and Sun. His finding represented a significant improvement over the past
century's work, coming within 0.17 percent of the actual value. But astronomers
had hoped for even better results. Just a year after Harkness made his
calculation, his colleague Simon Newcomb used measurements of the speed of
light to calculate the astronomical unit with more than twice the accuracy of
Harkness's work. That technique and others eventually eclipsed the transit
method for measuring the size of the solar system.
So when Venus appears in front of
the Sun next month, astronomers will use the event for other purposes. Mr.
Pasachoff, of Williams College, will travel to Greece to study the black-drop
effect and to collect data on Venus's atmosphere. He and others will also take
advantage of the transit to refine their techniques for studying so-called
exoplanets around far-off stars.
Astronomers in the past decade have
found more than 100 exoplanets, but it is difficult to measure their sizes and
to determine whether they have atmospheres. By watching how the Sun's light
changes when Venus transits, researchers can learn how to study transits of
exoplanets passing in front of their own parent stars.
While astronomy professors test out
new techniques on June 8, high-school students around the world will be re-creating
history, trying to repeat the work of Harkness and his predecessors who
measured the astronomical unit. "We're very excited about this opportunity
to bring an exceedingly rare astronomical event before millions of
students," says Mr. Pasachoff, who is president of an education panel of
the International Astronomical Union. He hopes that the transit will spark
students' interest in science and inspire the next generation of astronomy
professors.
More than a century ago, Harkness
also recognized the power of Venus's appearance before the Sun. He viewed the
event as the chance of a lifetime -- one that marked major milestones in
scientific achievement whenever it came.
With an eye toward history, he wrote
in defense of his efforts that "when the last transit season occurred the
intellectual world was awakening from the slumber of ages, and that wondrous
scientific activity which has led to our present advanced knowledge was just
beginning."
Another opportunity would not come
until "the June flowers are blooming in 2004," he wrote. "What
will be the state of science when the next transit season arrives God only
knows. Not even our children's children will live to take part in the astronomy
of that day. As for ourselves, we have to do with the present."
------------------------------------------------------------------------
HOW TO CATCH A PLANET
If you like your eyes, don't look at
the Sun to catch Venus's transit across the solar disk on June 8. That's a
quick way to go blind. To watch the big event, try setting up one of the simple
projection systems (described on the Web sites below), or take an easier
approach and tune in to a Webcast. In fact, residents of the Western United
States will have to turn to their computers, because the transit will end
before sunrise in that part of the country. In the Eastern time zone, the
transit will end at about 7:25 a.m, providing an hour or more of viewing time
after sunrise. People in Europe and much of Asia and Africa will be able to
watch the entire six-hour transit, weather permitting. Below are links to Web
sites with information about transits past and present.
* The
Smithsonian Institution's National Museum of American History has an online
exhibit, "Chasing Venus: Observing the Transits of Venus, 1631-2004":
http://www.sil.si.edu/exhibitions/chasing-venus/intro.htm
* The
Smithsonian offers an MP3 file of the "Transit of Venus March," by
John Philip Sousa: http://www.sil.si.edu/Exhibitions/chasing-venus/mp3/Sousa-Transit_of_Venus_March.mp3
* The
European Southern Observatory will provide a Webcast of the transit and is
coordinating the efforts of amateur astronomers around the globe who will be
timing the transit: http://www.vt-2004.org
* NASA
has information about past transits and predictions for the timing of the June
8 event: http://sunearth.gsfc.nasa.gov/eclipse/transit/TV2004.html
* The
Exploratorium, in San Francisco, offers a Webcast of the event and instructions
on observing the transit safely: http://www.exploratorium.edu/webcasts/index.html
SOURCE: Chronicle reporting
------------------------------------------------------------------------
http://chronicle.com
Section: Research & Publishing
Volume 50, Issue 36, Page A16