From Science magazine
Skywatchers Await the Fleeting Shadow of Venus
On 8 June, Venus will cross directly in front of the sun for the
first time in 122 years
Venus usually shines like a brilliant beacon in the morning or evening
sky. But on 8 June, our sister planet will assume a darker guise: a
circular blot, slowly crossing the sun's face in a dramatic "transit."
No one alive has seen this mini-eclipse, which last occurred in 1882.
Astronomers of that era launched lavish excursions to capture the event
with newly invented cameras. Today, some research is planned, but the
emphasis is on getting students and the public to view the transit and
appreciate the workings of our clocklike solar system.
The transit's sporadic timing arises from the alignment of Venus's
orbit, which tilts 3.4 degrees relative to the plane of Earth's path
around the sun. As a result, Venus rarely crosses the line between Earth
and the sun. When it does, a second transit usually (but not always)
happens 8 years later. Those who miss the show in June had better catch
the next one in 2012--the last chance for 105 years.
English astronomer Jeremiah Horrocks was the first to predict and
observe a transit, in 1639. After that, each pair of transits grew in
cultural impact. The 1874 and 1882 events were such phenomena that
composer John Philip Sousa wrote The Transit of Venus March, while
Harper's Magazine featured a cover illustration of Appalachian children
watching the sun through a smoked pane of glass.
Astronomers were catalyzed as well. "It was like a space race in the
19th century to make accurate measurements of the transits," says NASA
chief historian Steven Dick of the U.S. Naval Observatory in Washington,
D.C. Indeed, the U.S. Congress funded 8 expeditions in 1874 for a
princely $177,000, while Russia fielded a whopping 26 teams. Their goal
was the same: to measure the exact moments when the full circle of Venus
entered and exited the sun's disk. Once they gauged those times at many
places on Earth, astronomers could use surveying methods to calculate
the Earth-Venus distance. Then, the orbital laws of Johannes Kepler
would yield the long-sought "astronomical unit" (AU)--the distance
between Earth and the sun.
The answers were close to the true value of about 150 million
kilometers, but scientists were skeptical. The problem was a weird
distortion called the "black drop effect." As the name implies, the
silhouette of Venus looks more like a water drop than a circle when the
transit begins and ends. "The black drop effect makes it extraordinarily
difficult to determine when the planet's edge actually touches the inner
edge of the sun," says astronomer Edward DeLuca of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
By the 1890s, other methods for measuring the AU were deemed far more
accurate.
Surprisingly, scientists haven't yet nailed the cause of the black drop
effect. Thick clouds around Venus don't bend light severely enough to
stretch the silhouette, all agree. Instead, a 2001 analysis by
astronomer Bradley Schaefer, now at Louisiana State University in Baton
Rouge, deduced that smearing within Earth's atmosphere--which also makes
stars twinkle--blurs Venus's disk in the observed way.
However, satellite images of a transit by the planet Mercury in 1999
also revealed a black drop effect, according to a forthcoming study in
Icarus. The team, led by astronomer Glenn Schneider of the University of
Arizona, Tucson, concludes that the distortion came from a combination
of the spread of light within the satellite's camera and the noticeably
dimmer appearance of the sun's edge, an effect called "limb darkening."
The same satellite--NASA's Transition Region and Coronal Explorer--will
observe the Venus transit in June. "It will solve the black drop mystery
totally," DeLuca says.
Others on the ground also plan to watch. For example, astronomers
Wolfgang Schmidt of the Kiepenheuer Institute for Solar Physics in
Freiburg, Germany, and Timothy Brown of the National Center for
Atmospheric Research in Boulder, Colorado, will use a 0.8-meter solar
telescope on the Canary Islands to take detailed spectrographic data
during the transit. They will try to measure wind speeds in the upper
atmosphere of Venus by detecting Doppler shifts in the spectral lines of
carbon dioxide gas, illuminated by the bright light of the sun behind.
"This is an unprecedented experiment," Brown says. " No one knows how it
will work." Ultimately, astronomers might adopt a similar approach to
study the atmospheres of transiting planets in other solar systems, he
notes, although any such measurement would have to be extraordinarily
sensitive to faint changes in the pattern of a distant star's light.
Beyond the potential research, astronomers are counting on a surge of
public interest as the transit approaches. Both NASA and the European
Southern Observatory are sponsoring major public-viewing campaigns* and
live webcasts. Participating students will submit their own records of
transit times and then learn how to calculate an AU. Viewers in most of
Europe, Africa, and Asia will get to watch the 6-hour transit from start
to finish, while those in the eastern half of the U.S. must settle for a
shorter taste at sunrise. Other Americans will miss out--but a sunset
view of the next transit awaits in 2012.
Even grizzled scientists are eager for 8 June to arrive. "The romance
and history of Venus transits are wonderful," says Brown. "If nothing
else, this will be a great time."
--Robert Irion
<footnote>
*http://sunearth.gsfc.nasa.gov/sunearthday/2004/index_vthome.htm
and http://www.vt-2004.org
--------------
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
heavens.
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
Earth?
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
have.
"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
systems.
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
say.
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
long periods.
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:
www.vt-2004.org
The Exploratorium in San Francisco
offers a Webcast of the event and instructions on observing it safely: www.exploratorium.edu/webcasts/index.html
-----------
From the Globe and Mail, Toronto
http://www.globeandmail.com/servlet/ArticleNews/TPStory/LAC/20040515/VENUS15//?query=venus
Venus ascending
'For
most people, it'll be about as exciting as watching paint dry,' but astronomers
are in a tizzy over the chance to see the planet pass between the sun and us
next month. DAN FALK explains the transit fever
By DAN FALK
Saturday,
May. 15, 2004
Why would
anyone travel thousands of kilometres to see a small black dot move slowly
across a big yellow circle?
For
amateur astronomers around the world, the question is just the opposite: How
could anyone think of missing a phenomenon so exotic that no living person has
seen it? You might as well ask a birdwatcher why he would bother tracking down
the black paradise flycatcher or a stamp collector why he would want a specimen
with an upside-down airplane on it.
"It's
one of these rare things," says Ralph Chou, a professor of optometry at
the University of Waterloo and an avid amateur astronomer.
Driven by
"sheer curiosity," Prof. Chou will go to Egypt next month for the
chance to see the black dot travel across the yellow circle, an event otherwise
known as a transit of Venus. At that spot, a combination of geography and
climate should guarantee him a front-row view.
A transit
of Venus occurs when the planet passes directly between the Earth and the sun.
Such transits occur in pairs eight years apart -- but the pairs are separated
by either 105 or 122 years.
In other
words, if you are born at the wrong time, you won't see it, period. Transits of
Venus are so infrequent that they have been recorded as being seen by human
beings on only five previous occasions, beginning in 1639; the last one was in
1882.
But the
dry spell is nearly over: The next transit of Venus will take place on June 8.
"There's
nobody alive on the face of this Earth that saw the last transit of
Venus," Prof. Chou says. "And we are just fortunate to be alive at a
time when we're going to have the possibility of seeing two transits - the one
in June of this year and another one in 2012."
A transit
is not as spectacular as an eclipse; indeed, without a telescope and the proper
filter needed to observe it safely, you would never know it was happening.
"For most people, it'll be about as exciting as watching paint dry,"
Prof. Chou says.
But the
rarity itself is enough to give sky watchers a case of transit fever.
Some
enthusiasts will be flying to exotic locations; in Egypt, Prof. Chou will be
leading a tour on behalf of the Toronto Centre of the Royal Astronomical
Society of Canada, a nationwide association of astronomy clubs.
Others
will stick closer to home. "I'm going to try to observe it locally, here
in the city -- if the weather is suitable," says Geoff Gaherty, a
Toronto-based computer consultant. "But I'm prepared to drive at least a
few hours if it looks more promising somewhere else."
Observers
in Central and Eastern Canada can indeed see the transit -- but only a portion
of it: It will already be under way as the sun rises. Viewers in Europe,
eastern Africa and western Asia will be able to see the entire event, which
lasts just over six hours.
The first
transit of Venus known to have been seen by human eyes occurred in 1639, and
only two people glimpsed it: a young Englishman named Jeremiah Horrocks and his
friend, William Crabtree.
By the
time of the next transit, in 1761, the situation was quite different. Transits
could be predicted with great accuracy, and astronomers knew that data from a
transit could be used to calculate the distance to the sun (and, from that, the
distance to the planets).
It boils
down to a problem of triangulation. As seen from different locations on the
Earth's surface, Venus takes a slightly different path across the face of the
sun. If you compare the path as seen from location A with the path recorded at
location B, and you know the distance separating A and B, along with the
duration of the transit as seen from the two locations, then -- with a bit of
geometry -- you can work out both the distance to Venus and the distance to the
sun.
Thus,
accurate transit observations became a top scientific priority, and about 70
expeditions were mounted to locations around the world to record the 1761
event.
The first
person to see a transit of Venus from North America, and the only one to view
the 1761 transit from this continent, happened to be standing on what is now
Canadian soil -- a hill in St. John's. The observer was John Winthrop, a
professor of mathematics and natural philosophy at Harvard College (now Harvard
University). The transit was not visible from Massachusetts, so he persuaded
Harvard and the governor of the colony to support his expedition -- and he set
sail for Newfoundland.
The trip
was a great success. "The morning of the 6th of June was serene and
calm," he wrote in his log. "And at 4h 18m we had the high
satisfaction of seeing that most agreeable Sight, VENUS ON THE SUN, and of
shewing it in our telescopes to the Gentlemen of the place, who had assembled
very early on the hill to behold so curious a spectacle."
This
year's transit will again be visible to early risers in St. John's, where keen
amateur astronomers are hoping to experience the same thrill that captivated
Prof. Winthrop nearly 21⁄2 centuries ago. The transit is of
"enormous historic interest," says Fred Smith, a professor in the
faculty of science at Memorial University of Newfoundland. "It does allow
us to recreate a little bit of history."
In fact,
Prof. Smith believes that he has figured out precisely where Prof. Winthrop was
stationed. Based on the Harvard professor's log, which gives the latitude of
the observing site but not the longitude, he believes that Prof. Winthrop
observed the transit from Kenmount Hill on the west side of the city.
The
transit eight years later, in 1769, motivated one of the more famous of all sea
voyages: the expedition of James Cook to the South Pacific. But there were also
four observing stations in Canada that year, including a remarkable expedition
by two English astronomers, William Wales and John Dymond, to Fort Prince of
Wales on Hudson Bay, the site of present-day Churchill, Man.
Dr. Wales
and Dr. Diamond spent 13 months at the fort, observing the local flora, fauna
and geology, complaining of mosquitoes, and were paid £200 by the Royal Society
of London for their efforts.
Astronomers
would eventually combine the data from the 1761 and 1769 transits and work out
the Earth-sun distance to within a few per cent of the modern value of 149.6
million kilometres.
The next
transit, in 1874, was visible from Asia, and European and American teams
observed the event from Russia, Japan, China and Mauritius. Despite the advent
of photography, the results were disappointing; a number of factors, including
hazy weather and turbulence in the Earth's atmosphere, hampered the
observations.
More
important, astronomers were now developing other methods for determining the
Earth-sun distance. One method involved tracking the position of Mars against
the background stars and turned out to be more accurate (and easier to do) than
the transit method. (Today, astronomers have more sophisticated ways of determining
distances in the solar system, including radar, and the average distance
separating the Earth and sun is now known to within a few dozen metres.)
By the
time the next transit rolled around, on Dec. 6, 1882, the event was no longer
of great scientific importance -- though members of the public were certainly
excited about it. Newspapers of the day record a high level of interest,
although bad weather hampered the viewing in many locations. "Cold and
miserable, the amateur astronomers stuck to the roof of The Mail until the
transit was over," the Toronto newspaper reported; observers in Halifax
experienced "very dark and rainy" conditions and "made no
observations," a local paper said.
Still,
the 1882 event triggered a surge of interest in astronomy and science. The
Royal Society of Canada was founded the same year, historians note. "The
prospect of the transit, and the hope that even Canadians could make some
useful observations, was, I think, the beginning of professional astronomy in
Canada," says Peter Broughton, a retired Toronto teacher who has written
extensively on the history of Canadian astronomy.
These
days, of course, Venus is hardly a stranger. We now recognize it as our sister
planet, with a diameter just a bit smaller than Earth's and a mass four-fifths
of Earth's. We know it has a thick (albeit poisonous) atmosphere, and no moons.
By now, 26 space missions, including eight landers, have explored the
cloud-covered world.
The June
8 transit will tell astronomers little that is new about either Venus or the
sun -- although there is still scientific interest in the "black
drop" effect, a complex phenomenon in which the disc of Venus appears
stretched out into the shape of a raindrop as it first passes in front of the
sun and later as it appears to exit the solar disc.
Jay
Pasachoff, an astronomer at Williams College in Massachusetts, will observe the
transit from northern Greece, together with a group of students, in order to
study the black drop effect in more detail.
But he
admits that transits are now primarily of educational and historical value.
This spring's event "is a great time for public education, and making
people appreciate the value of science in general and astronomy in
particular," Prof. Pasachoff says.
On June
8, people will again look up at that black dot inching across the sun, and
ponder the nature of these distant spheres -- at once unfathomably remote and
yet strangely familiar. Many will muse on the rarity of the event, just as
astronomer William Harkness of the U.S. Naval Observatory did the last time
around.
"What
will be the state of science when the next transit season arrives God only
knows," he told an audience in 1882. He could hardly imagine that far-off
day when "the 21st century of our era has dawned upon the Earth, and the
June flowers are blooming in 2004."
Dan Falk is a Toronto science writer,
broadcaster and amateur astronomer. He plans to view the June 8 transit from
somewhere in southern Europe.
Our sister planet
Venus is
named for the Roman goddess of love and beauty. It is known as the morning star
as well as the evening star, depending on its position in the sky.
Orbit: 108,200,000 kilometres from the sun, about two-thirds the size of
Earth's orbit.
Diameter: 12,100 kilometres, about 95 per cent size of Earth.
Mass: 4.87 x 10{+2}{+4} kilograms, about 80 per cent of the mass of
Earth.
Rotation: in an opposite direction to that of Earth.
Length of year: 225 Earth days (but its day is 243 Earth days
because it rotates so slowly).
Surface: craters and volcanoes.
Atmosphere: mainly carbon dioxide, with some nitrogen and virtually no water
vapour; several layers of clouds many kilometres thick composed of sulphuric
acid.
Atmospheric pressure: At the surface, 92 times that of the Earth's at
sea level.
Surface temperature: about 475 degrees Celsius.
Times to watch
For all
locations except the High Arctic, the transit will already be under way as the
sun rises on June 8. Canada's Far West is outside the zone of visibility. To
view the sun safely, you can use No. 14 welder's glass. All times are local.
Location Sunrise Transit ends
Iqaluit
2:22 a.m. 7:23 a.m.
St.
John's 5:04 a.m. 8:55 a.m.
Halifax
5:30 a.m. 8:26 a.m.
Quebec
4:51 a.m. 7:25 a.m.
Montreal
5:06 a.m. 7:25 a.m.
Toronto
5:35 a.m. 7:25 a.m.
Winnipeg
5:19 a.m. 6:24 a.m.
Regina
4:43 a.m. 5:23 a.m.
For more
locations, see the website: http://www.planetarium.montreal.qc.ca/Information/Actualite
Venus2004/tableau_a.html
Source: Montreal Planetarium