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



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








From The New York Times

May 18, 2004


Venus Returns for Its Shining Hour



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



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:


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:


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:


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:


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:




From the Globe and Mail, Toronto


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




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:




Source: Montreal Planetarium