From The New York Times
May 18, 2004
Venus Returns for Its Shining Hour
By WARREN E. LEARY
he world is about to witness a rare
spectacle that once launched expeditions to ideal vantage points around the
globe and inspired millions of people to venture outside and stare at the
On June 8, people in the right
places on Earth will be able to see Venus move across the face of the Sun in a
kind of minieclipse that is visible twice every century or so. The last such
occurrence, called a transit of Venus, was in 1882. It inspired an
international effort to use the event to answer one of the most pressing
scientific questions of the day: What is the exact distance between the Sun and
Although studies of the event failed
to provide an exact answer, they did narrow the range of estimates and
measurements, and ushered in an era of investing in science as a symbol of
national prestige. For the last event, the United States government mustered
eight expeditions to make observations around the world, partly because
Britain, France, Russia and other rivals did the same.
By bouncing radar signals off the
Sun and Venus and using spacecraft measurements, scientists in the 1960's
calculated that the average Sun-to-Earth distance is 92,955,859 miles, a
measure called the astronomical unit.
Scientists realized for centuries
that if they could find out that number, they could use the formulas of the
17th-century astronomer Johannes Kepler to calculate the size of the solar
system and the exact distances between the planets.
"This was the most important
question of its day in astronomy," said Dr. Jay M. Pasachoff, a professor
of astronomy at Williams College. "And using the transits of Venus to
calculate the astronomical unit was the best way to do it."
Although transits of Venus have
occurred for thousands of years, the first report of its subtle crossing of the
Sun was in 1639. The transits occur when the orbits of Venus, Earth and the Sun
put them into alignment along the same plane.
Since 1639, transits have occurred
in 1761, 1769, 1874 and 1882. If someone misses the one next month, there will
be another opportunity on June 6, 2012. After that, more than a century will
pass before the next transits, in 2117 and 2125. Because of its rarity, the
transit next month, best viewed from Europe and the Mideast, is generating
great scientific and public interest, said Dr. Steven J. Dick, chief historian
for the National Aeronautics and Space Administration. Dr. Dick has written
extensively on the 18th- and 19th-century transits.
No one alive today saw the last
transit, he said, and seeing the next two will be the only chance most people
"These are truly
once-in-a-lifetime events," Dr. Dick said. "Although the scientific
importance has diminished, I think there will be a lot of interest this time
among the public, based on e-mail I've seen from around the world."
Dr. David DeVorkin, curator of the
history of astronomy at the National Air and Space Museum, said the 1874 and
1882 transits were prominently featured in newspapers and magazines. A carnival
atmosphere pervaded Wall Street for the transit on Dec. 6, 1882, with people
crowding the area and staring up through smoked glass.
"It was a popular
diversion," Dr. DeVorkin said. "Something maybe everybody didn't try
to see, but everybody talked about it."
Scientific interest persists.
Instruments aboard at least three Sun-watching satellites, as well as ground
telescopes, will follow the event. Researchers will use Venus' trek to test
techniques and instruments that can be used to detect planets in other solar
More than 120 extrasolar planets
have been discovered orbiting other stars, most of them huge bodies found
because their gravity affected the motion of their stars.
Astronomers have recently detected a
small number of far planets by measuring the fluctuations that they cause in
light from the stars they circle. In 2007, NASA plans to launch the Kepler
spacecraft to monitor Sun-like stars in hope of detecting Earth-size planets
through small decreases in star brightness.
Although denied a direct view of the
transit because it occurs at night in the American West, astronomers with the
University of Arizona hope to get an indirect view. Dr. Glenn H. Schneider said
he and a colleague, Paul S. Smith, will try to use the Steward Observatory in
Tucson to measure about a half-hour of sunlight from the end of the transit as
it reflects off the Moon.
"We want to see if we can
detect the signature of Venus' atmosphere spectroscopically from sunlight
reflecting off the moon, as if it was a reading coming from a faraway
star," Dr. Schneider said.
The transits generally occur in a
predictable pattern of two occurring in an eight-year period, followed by one
105 1/2 years later and another eight years after that. After an
additional 121 1/2 years, the pattern repeats. The paired eight-year
sightings occur because a Venusian year equals 224.7 Earth days, making 13
Venusian years equal to eight Earth years.
That allows the planets to return to
about the same alignment with the Sun they had been in eight years earlier,
after which they go out of sync for more than a century.
On Tuesday, June 8, observers lucky
enough to view the entire transit will see Venus as a small black spot crossing
the southern hemisphere of the Sun from left to right. The planet, entering the
disc of the Sun at the 8 o'clock position, will take six hours to cross the
bright face before exiting at the 5 o'clock position.
Venus, appearing as a round black
dot with a diameter one thirty-second of the Sun's, is widely expected to cause
a one-tenth of 1 percent drop in sunlight that reaches Earth during the event.
Location is everything, particularly
when trying to witness celestial events. The entire transit will be visible in
Europe, most of Africa, the Mideast and most of Asia. The unlucky regions of
the globe where the event occurs at night, and is unviewable, include western
North America, including most of the United States west of the Rockies;
southern Chile and Argentina; Hawaii; and New Zealand.
Some regions will see just part of
the transit, because the Sun sets while it is in progress. Those areas include
Australia, Indonesia, Japan, the Philippines, Korea and Southeast Asia.
Likewise, the Sun rises with the
transit in progress over eastern North America, the Caribbean, western Africa
and most of South America, allowing observers a brief view before the event
ends. How much early risers see will depend on the weather and how high the Sun
rises above the horizons before Venus moves out of view.
In New York, sunrise will be at 5:25
a.m., and Venus is to begin exiting the solar disc at 7:06, when the Sun is 17
degrees above the horizon. The planet's final contact with the edge of the Sun
should occur at 7:26 a.m., when the Sun is 20 degrees high. Times are similar
for most cities in the Eastern time zone and one hour earlier in the Central
time zone. But moving West means that the Sun is lower on the horizon.
Modern interest in planetary
transits can be traced from Kepler. Based on his calculations of planetary
motion, he wrote in 1627 that Mercury would cross the face of the Sun in
November 1631 and that Venus would follow on Dec. 6 that year. Kepler suggested
that observers placed at widely different points on Earth could indirectly calculate
the distance to the Sun by using Venus.
Knowing the distance between
observers and the different angles from which they viewed the transit,
astronomers could calculate the distance to Venus and use that to compute the
Earth-to-Sun measurement, he reasoned.
Kepler died the year before the 1631
Venus transit, but he would not have seen it had he lived, because it occurred
at night in Europe. He would have also missed the next transit, in 1639,
because he made a miscalculation that failed to predict it.
Fortunately, a young English
astronomer, Jeremiah Horrocks, became interested in Kepler's work and, in
recalculating some of the German's tables, discovered that a transit would
occur on Nov. 24, 1639. Horrocks witnessed part of the transit from his home in
Much Hoole, Lancashire, and a friend whom he notified by letter, William
Crabtree, saw it from Manchester.
The next transits, in the 18th
century, drew much more attention, thanks to Edmond Halley, the British
astronomer best known for the comet that bears his name. Halley suggested using
the 1761 and 1769 transits to calculate the Sun-to-Earth distance by having
observers time the events from widely spaced latitudes and trace the planet's
path across the Sun's face as they saw it from their positions. By measuring
the angular shifts of the paths based on the timings, Halley reasoned, the
astronomical unit could be calculated.
Although Halley died in 1742, his
plan guided many observations made of the two transits from around the world.
But the results varied widely and were disappointing. Among those trying to
work on the problem in 1769 was the British explorer Capt. James Cook, who took
his ship, the Endeavour, on its first voyage to the South Pacific to observe
the transit from Tahiti.
Cook and others were frustrated in
their observations by the inability to time the exact moment when the edges of
the planet and the Sun appeared to touch. When Venus nears the edge of the disc
of the Sun, its black circle appears to ooze toward the edge of the sun without
showing a clear point of contact. Although the precise second of contact was
needed for calculations, this so-called "black drop" phenomenon
caused observers watching the same event to disagree by several seconds up to a
minute on when the outer edges touched.
Cook and other observers speculated
that the problem was the distortion of light through the Venusian atmosphere.
Earlier this year, using spacecraft
observations, Dr. Pasachoff and other scientists concluded that the black drop
effect was caused by a combination of images' blurring in small-aperture
telescopes and the natural dimming of sunlight near the Sun's visible edge.
In the 19th-century transits,
scientists tried to overcome that effect and other imperfections with better
telescopes and the introduction of photography. Still measuring and timing
transits never led to finding the precise Sun-to-Earth distance.
William Harkness of the United
States Naval Observatory refined results from the 1882 transits and in 1894
came up with an astronomical unit measure of 92,797,000 miles. But the work of
another Naval Observatory scientist, Simon Newcomb, was adopted as the world
standard at a 1896 meeting in Paris, Dr. Dick said. Newcomb, who gave little
credence to transit data, combined values from several sources including
speed-of-light star readings, to come up with a figure of about 92,872,000
miles. Both were close to the modern value of 92,955,859 miles, but precision
is critical in astronomical terms.
Nevertheless, Dr. Dick said, the transits
of Venus remain important because the desire to define the astronomical unit —
and to maintain or gain scientific prestige — led many nations to mount
competing expeditions. In 1874, Russian sent out 26 expeditions, Britain 12,
the United States 8, Germany and France 6 each, Italy 3 and the Netherlands 1.
"You could compare it with the
space race in the 20th century," he said.
May 18, 2004
How to Watch Without Harm
By WARREN J. LEARY
hen viewing solar events like
eclipses or the transit of Venus, precautions are needed. Never look directly
at the Sun. The direct gaze can lead to severe eye damage or blindness, experts
Sunglasses and clouds do not protect
the eyes, and viewing the Sun through unfiltered telescopes, binoculars or
cameras can result in instant and permanent damage. Telescopes and binoculars
should be equipped with special undamaged solar filters. Glasses with solar
lenses are available commercially, but even then do not stare at the Sun for
The transit of Venus can safely be
seen if viewed indirectly, using telescopes or pinhole boxes to focus the image
on a screen or paper opposite the Sun.
Information on viewing solar events
is at these Web sites:
HISTORY The Smithsonian Institution
Libraries online exhibition "Chasing Venus: Observing the Transits of
Venus, 1631-2004'' includes many historical facts and illustrations: www.sil.si.edu/exhibitions/chasing-venus/intro.htm
VIEWS The annual Sun-Earth Day for
2004 has selected the transit of Venus as this year's theme. Information on
viewing is online at this NASA site: sunearth.gsfc.nasa.gov/sunearthday/2004/vt_webcasts_2004_4.htm
EDUCATION This extensive site,
prepared by Chuck Bueter of the International Planetarium Society, has
information on safe viewing, interactive education and hands-on activities,
global-observation programs for students, historical endeavors and the role of
spacecraft and the search for extrasolar planets, among other items, at: www.transitofvenus.org
FUTURE TRANSITS This
site, prepared by Fred Espenak of the NASA Goddard Space Flight Center,
provides history and information on viewing the 2004 and 2012 transits: sunearth.gsfc.nasa.gov/eclipse/transit/venus0412.html
IN REAL TIME 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:
The Exploratorium in San Francisco
offers a Webcast of the event and instructions on observing it safely: www.exploratorium.edu/webcasts/index.html