Early human ancestor chewed bark

The first Australopithecus sediba fossil was discovered in South Africa

Continue reading the main story

An early relative of humans chewed on bark and leaves, according to fossil evidence.

Analysis of food trapped in the teeth of the two-million-year-old "southern ape" suggests it existed on a unique diet of forest fruits and other woodland plants.
The study, in Nature, gives an insight into the evolution of what could have been a direct human ancestor.
Other early African contemporaries had a diet suggesting a grassland habitat.
The first fossils of Australopithecus sediba, discovered in South Africa in 2008, were hailed as a remarkable discovery.
Teeth from two individuals were analysed in the latest research, focussing on patterns of dental wear, carbon isotope data and plant fragments from dental tartar.
The evidence suggests the ape-like creature ate leaves, fruit, bark, wood and other forest vegetation.
Dr Amanda Henry of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, led the research.
"We've for the first time been able to put together three quite different methods for reconstructing diet and gotten one cohesive picture of the diet of this ancient species and that picture is really quite different from what we've seen in other hominins (human ancestors)," she said.
"That's exciting, we're seeing a lot more variation among these species than we'd expected."
Human milestone Human ancestors from around this time period were probably exploring a wide variety of habitats.
Each species was finding its ecological niche a few hundred thousand years before the evolution of Homo erectus, which spread out of Africa into many different habitats around the world, heralding a milestone in human evolution.
Dr Henry said Australopithecus sediba walked on two legs but probably also spent time foraging in the trees.
"It was still quite primitive; it had a very small brain; it was quite short and it had fairly long arms but it was definitely related to us," she said.

Bark and woody tissues were found in the teeth

Dr Louise Humphrey of the palaeontology department at London's Natural History Museum said there was debate about the position of Australopithecus sediba in the human lineage.
"The question is, is this a great great granddad or grandma or is it a cousin?
"They were eating bark and woody substances, which is quite a unique dietary mechanism; it hasn't been reported for any other human relative before."
The animal may have eaten fruit and young leaves when food was plentiful, but turned to less nutritious food like bark when times were hard.
However, syrup beneath the bark may have provided a sugary treat.
Dr Henry said: "A lot of people have turned their nose at the idea of eating bark but I always think that what they're eating is probably not the coarse outer bark but potentially the softer inner bark where the sap is.
"And so if you think of maple syrup - it's the sap of maple trees - then it could have been quite a tasty substance."

Brian Greene: Exploring the unknown universe

These are the sorts of questions that Brian Greene, a professor of physics and mathematics at Columbia University, has spent his lifetime trying to answer.
In between, he has also found time to write several best-selling books on the subject. His latest, The Hidden Reality, explores the possibility that our universe is not the only universe.
He tells BBC Future how pushing the boundaries of scientific exploration will shape our future.
If you would like to comment on this video or anything else you have seen on Future, head over to our Facebook page or message us on Twitter.

Laser treatment for Earth-bound asteroids

In the cinematic world, our planet’s destruction by the impact of another cosmic body is one of the few science-fiction tropes to bridge the blockbuster (Meteor, Armageddon, Deep Impact) and the arthouse (Last Night, Melancholia). Whatever else that means, it seems to imply that we’re all thinking about it.
But how do you think about it? The impact of a 10-km (6-mile) asteroid would be apocalyptic, but the chances of it happening in the next few generations are all but negligible. The smaller the asteroid, though, the bigger the danger, as witnessed by the recent near miss of the 7-metre (23-feet) 2012 KT42, the sixth closest encounter of any known asteroid. But, Bruce Willis apart, can we even begin to think about averting such an event with today’s technologies?
This is one of those areas in which hard science can degenerate into idle speculation and fantasy. That’s why a paper, as yet unpublished, by two aerospace engineers at the University of Strathclyde in Glasgow, UK, is worth attending to. No one would claim that the plan sketched by Massimiliano Vasile and Christie Maddock to deflect Earth-threatening asteroids with solar-driven lasers is a blueprint for the survival of mankind, or that governments should be rushing to implement the idea. Rather, it’s the sort of ballpark calculation that lets us contemplate the magnitude of the task.
Deflection tactics
But there’s another reason to take note, which the authors don’t mention. Recent announcements of plans to mine asteroids for precious elements and minerals – in particular the launch of Planetary Resources, backed by Larry Page and Eric Schmidt of Google and commercial spaceflight entrepreneur Peter Diamandis – has got people talking about whether some of these cosmic goldmines might be nudged closer to Earth for easier access. Any technology that could alter the course of asteroids might therefore excite more interest from private speculators than from governments wanting to prevent doomsday.
The basic idea behind this approach isn’t new. In fact it goes back to 1994, when planetary scientist Jay Melosh, a specialist on meteorite impacts, and his colleagues proposed that asteroids on a collision course with our planet might be deflected by the type of nuclear blasts favoured in Armageddon.
Another possibility makes use of a huge reflector floating near an asteroid, which could focus sunlight onto the surface and burn off a jet of icy material. According to Newton’s third law of motion, this would gradually change the asteroid’s course: the momentum of the material flung out in one direction would be compensated by a change in the asteroid’s own direction of motion. Some researchers expanded the idea by proposing the use of a whole fleet of mirror-bearing spacecraft around the asteroid. But Vasile and Maddock pointed out that if the reflectors are going to be close enough to the asteroid to achieve strong solar heating, there’s a risk that the mirrors will be covered with the debris coming off the surface.
Laser blast
That’s why the duo now envisage using laser beams instead to heat the surface: they remain tightly focused over large distances, and so can be stationed further away. Lasers too have been considered before for this purpose. But they eat up a lot of energy, and previous proposals have imagined running them from a nuclear power source on a single spacecraft. In contrast, Vasile and Maddock propose using a swarm of craft – which are easier to build – equipped with modest-sized, electrically powered lasers driven by photovoltaic cells, powered in turn by light-collecting mirrors perhaps a few metres in size.
Sounds nice in principle. But can a realistically sized fleet of spacecraft induce enough deflection to head off a potentially hazardous Earth-bound asteroid? The two engineers formulate an answer to this question by looking at an asteroid named 99942 Apophis, which is known to have a trajectory that crosses the Earth’s orbit and has a very small chance of hitting in 2036 or 2037. It’s shaped like a potato, reaching almost 200m (655 feet) along the long axis, and would wreak serious havoc if it hit us.
There are lots of asteroid-deflecting parameters one could vary: the laser power (and consequent solar-cell requirements), the size, number and position of the spacecraft, how they are powered to hold their position, and so on. The craft would have to be actively held in place, not least to counteract the slight push that they will receive from the stuff being blasted off the asteroid. One of the key aims is to find a compromise distance that achieves enough heating without too much clogging of the collector mirrors. The effort any system needs to put in also depends on how much deflection is needed to avoid catastrophe, and how much warning you have: how long you can spend nudging the asteroid, in effect.
All this means that it’s not possible, or indeed meaningful, to say exactly what would be needed in terms of design or cost to get this idea to work. But Vasile and Maddock do manage to establish that if, say, we discovered in ten years time that Apophis really was going to strike, it should be possible to implement a strategy like this based more or less on the technologies already to hand, without any fear of bankrupting the economy. That’s surely a little reassuring, even if it wouldn’t make much of a movie.
If you would like to comment on this story or anything else you have seen on Future, head over to our Facebook page or message us on Twitter.

The Case of the Missing Carbon

Alone in a sealed jar, a mouse would die from exhaled CO2. But as scientist Joseph Priestley observed in 1771, adding a mint plant allows the mouse to thrive. In this proof of photosynthesis, the mint absorbed CO2, retained carbon for growth, and released oxygen. Two centuries later humans tried—and failed—to survive in a sealed environment in Arizona's Biosphere 2.

Photograph by Peter Essick

By Tim Appenzeller

Republished from the pages of National Geographic magazine

It's there on a monitor: the forest is breathing. Late summer sunlight filters through a canopy of green as Steven Wofsy unlocks a shed in a Massachusetts woodland and enters a room stuffed with equipment and tangled with wires and hoses.
The machinery monitors the vital functions of a small section of Harvard Forest in the center of the state. Bright red numbers dance on a gauge, flickering up and down several times a second. The reading reveals the carbon dioxide concentration just above the treetops near the shed, where instruments on a hundred-foot (30-meter) tower of steel lattice sniff the air. The numbers are running surprisingly low for the beginning of the 21st century: around 360 parts per million, ten less than the global average. That's the trees' doing. Basking in the sunshine, they inhale carbon dioxide and turn it into leaves and wood.
In nourishing itself, this patch of pine, oak, and maple is also undoing a tiny bit of a great global change driven by humanity. Start the car, turn on a light, adjust the thermostat, or do just about anything, and you add carbon dioxide to the atmosphere. If you're an average resident of the United States, your contribution adds up to more than 5.5 tons (5 metric tons) of carbon a year.
The coal, oil, and natural gas that drive the industrial world's economy all contain carbon inhaled by plants hundreds of millions of years ago—carbon that now is returning to the atmosphere through smokestacks and exhaust pipes, joining emissions from forest burned to clear land in poorer countries. Carbon dioxide is foremost in an array of gases from human activity that increase the atmosphere's ability to trap heat. (Methane from cattle, rice fields, and landfills, and the chlorofluorocarbons in some refrigerators and air conditioners are others.) Few scientists doubt that this greenhouse warming of the atmosphere is already taking hold. Melting glaciers, earlier springs, and a steady rise in global average temperature are just some of its harbingers.
By rights it should be worse. Each year humanity dumps roughly 8.8 billion tons (8 metric tons) of carbon into the atmosphere, 6.5 billion tons (5.9 metric tons) from fossil fuels and 1.5 billion (1.4 metric) from deforestation. But less than half that total, 3.2 billion tons (2.9 metric tons), remains in the atmosphere to warm the planet. Where is the missing carbon? "It's a really major mystery, if you think about it," says Wofsy, an atmospheric scientist at Harvard University. His research site in the Harvard Forest is apparently not the only place where nature is breathing deep and helping save us from ourselves. Forests, grasslands, and the waters of the oceans must be acting as carbon sinks. They steal back roughly half of the carbon dioxide we emit, slowing its buildup in the atmosphere and delaying the effects on climate.
Who can complain? No one, for now. But the problem is that scientists can't be sure that this blessing will last, or whether, as the globe continues to warm, it might even change to a curse if forests and other ecosystems change from carbon sinks to sources, releasing more carbon into the atmosphere than they absorb. The doubts have sent researchers into forests and rangelands, out to the tundra and to sea, to track down and understand the missing carbon.

Governments make 'pitiful' progress on oceans

By Richard Black Environment correspondent, BBC News, Rio de Janeiro

Scientists say the failures of governments to pursue 20-year-old ocean conservation goals are plain to see

Continue reading the main story

Related Stories

• Rio hears call for sustainability
• Australia creates marine reserve
• How fishing has expanded since 1950 Watch

Little has been done to protect marine life since the 1992 Rio Earth Summit, conservation scientists conclude.

On pledges to protect key habitat and restrict the size of fishing fleets, they say progress has been "pitiful".
Their analysis is carried in the journal Science and is being discussed during final preparations for the Rio+20 summit, which opens next week.
Conservationists were delighted by Australia's move to set up the world's largest network of marine reserves.
But globally, the picture is bleak, they say.
"Our analysis shows that almost every commitment made by governments to protect the oceans has not been achieved," said Jonathan Baillie, director of conservation at the Zoological Society of London (ZSL).
"If these international processes are to be taken seriously, governments must be held accountable and any future commitments must come with clear plans for implementation and a process to evaluate success or failure."
Progess reviewed

In Rio de Janeiro, leaders will be peering intently at how oceans play into global sustainability

The researchers assessed the various pledges made at the landmark 1992 Earth Summit and 10 years later at the Johannesburg World Summit on Sustainable Development.
Governments vowed to establish an ecologically sound network of marine reserves by 2012, eliminate subsidies that contribute to illegal fishing, protect critical habitat, look after the needs of local fishermen and restore depleted stocks to healthy levels by 2015.
Subsidies have not been eliminated, and illegal fishing is still a major issue in some parts of the world.
Little over 1% of the seas are protected. Two years ago, governments agreed to raise that to 10% by 2020, but the new analysis shows that at the current rate of progress, the world is off course to meeting that target.
The pledge to restore stocks to healthy levels by 2015 has also seen slow progress. European ministers meeting earlier this week voted to give themselves until 2020 to achieve the target in EU waters.
The latest exception to this rather gloomy picture emerged earlier this week, when the Australian government announced it was creating a network of marine reserves around its shores that will cover 3.1 million sq km of water, including the ecologically rich Coral Sea off the Queensland coast.
Continue reading the main story

Rio summit jargon buster

Use the dropdown for easy-to-understand explanations of key terms:

Johannesburg 2002

Johannesburg 2002

See World Summit on Sustainable Development

Coming just before ministers arrive in Rio, conservationists hope it will inspire other countries to commit to strong safeguards.

Commitment concern Oceans are one of the major themes of Rio+20.
The "package" of outcomes that government negotiators are discussing includes agreeing to establish marine reserves in international waters, agreeing on equitable use of the oceans' genetic resources, and western help for poor countries on technology.
A decision to phase out harmful subsidies is also possible.
But over the various rounds of preparatory talks that began six months ago, there has been little indication that all governments are keen on these moves.
The US is against pledging to share ocean genetic resources equitably; developing countries say that without such a pledge, they will not agree to protected areas on the high seas.
"We're worried that some countries are starting to back off commitments that they made 10 years ago in Johannesburg," said Sue Lieberman, director of international policy at the Pew Environment Group.
"But there is still time for countries to move beyond what they agreed to in 2002, especially on the high seas," she told BBC News.

Low-Cost Solar Brightens Lives in the Developing World

A women's group meets in Mityana, Uganda to discuss deployment of low-cost solar lights in a program spearheaded by Solar Sister, a nonprofit that encourages renewable energy and local entrepreneurship. Below, a child studies by the light of a solar lamp from U.S.-based d.Light Design.

Photograph courtesy James Akena, Solar Sister

A decade ago, Katherine Lucey oversaw a heavily subsidized $1,500 solar-light installation in the rural district of Mpigi in central Uganda. The 60-watt rooftop solar panel system could power three lights in the four-room, off-the-grid house.
The family's father wanted the lights in his office, bedroom, and main room. But his wife successfully argued instead for  a light in the room where she cooked dinner, a light outside for security, and a light for her chicken coop. After all, chickens lay more eggs when they have more light.
Lucey recalls being struck by how something as simple as light could profoundly change a family's life. Indeed, after solar-powered lighting was installed, the family prospered by selling more eggs and, over time, they bought a cow, a goat, and a pig. The woman even started a school and women's literacy club.
"It was such a simple, fundamental intervention," said Lucey, who now runs a solar lamp nonprofit called Solar Sister.
Today, solar lights are making similar differences in millions of lives in the developing world-at a fraction of what they cost when Lucey did the installation at Mpigi.
(Related: "Sunlight In the Dark")
Thanks to technological advances, simple solar lamps go for as little as $10 to $20 each, and ones that have multiple brightness settings and an outlet to charge a cell phone don't cost much more. For daytime use, there are even cheaper options, such as a $2 to $3 solar bottle bulb made of a plastic bottle of purified water and bleach, sealed into the roof. The water helps disperse sunlight into a room, while the chlorine keeps mold from growing.
In wealthy countries, where access to cheap fossil-fueled electricity from the grid is nearly universal, solar electricity is still seen as an expensive energy option. That's particularly true when considering that a rooftop photovoltaic installation of sufficient size to power an electronics- and appliance-packed home costs tens of thousands of dollars. But the calculus is much different when bringing electricity for the first time to homes and communities that have none, with an aim of providing basic needs such as lighting and cell-phone charging. Development organizations are finding that solar energy is one of the most cost-effective options for providing not only power, but also a better livelihood.

Beyond Candles and Kerosene
Private companies and nonprofits are tapping into an enormous global need. An estimated 1.6 billion people, or more than one-fifth of the world's population, don't have access to a public electricity grid and instead rely on other means of lighting such as kerosene and candles. Nearly 600 million of the energy-poor live in Africa.
(Related: "The Solvable Problem of Energy Poverty")
Buying kerosene fuel can strain already tight household budgets, often meaning little or no light for key evening and nighttime stretches when children could be studying and parents could be working indoors.
Kerosene also produces toxic smoke and soot, which damages lungs and causes other serious health problems. Kerosene lamps, especially makeshift ones, also are dangerous-tens of thousands of children and adults in the developing world die or are seriously burned in kerosene accidents each year.
It's unclear how fast the solar lamp market is growing, but India's 2011 census alone estimated 1.1 million homes with solar lighting devices.
Examples run the gamut:

• The Scandinavian furnishings company Ikea has partnered with UNICEF and Save the Children to provide solar-powered lamps to tens of thousands of children in rural schools in India and Pakistan.
• The International Organization for Migration provided thousands of solar lamp systems to people displaced by the 2010 earthquake in Haiti.
• Nonprofit EMACE Sri Lanka has distributed 172 solar lamps to fishermen in Sri Lanka for night fishing.
• A Philippines organization called Isang Litrong Liwanag (A Liter of Light) has helped install more than 30,000 solar bottle bulbs in mostly poor areas of the Philippines. On a sunny day, a one-liter bottle sealed into the roof of a shack refracts light at a strength roughly equivalent to that of a 55-watt light bulb. The bottle bulb was designed by students at the Massachusetts Institute of Technology, but the concept of a solar glass bottle reportedly was pioneered in Brazil by Alfredo Moser, a São Paulo mechanic who was seeking to light up his workshop.

(Related: "Solar Brings Light to Quake-Darkened Haiti" and "Linkin Park's Bid to 'Power the World'")
"A solar bulb works at daytime only given that the sun is the source of light but is also useful at night if the moon is bright or if there is a source of light (nearby) for example a street lamp post," said Katrina Cardenas, A Liter of Light's internal affairs director.
U.S.-based d.light Design was one of the pioneers in distributing rugged solar lamps and lanterns, and now distributes its products in 40 countries, focusing particularly on Sub-Saharan Africa and India.
In just five years, the company has distributed more than 1.4 million lanterns, ranging in price from about $10 for a student lamp to about $45 for a rugged, handheld lantern with four light settings and cell-phone charger. A partnership with the Shell* Foundation is aimed at implementing market awareness programs and supporting local entrepreneurship.
Donn Tice, chief executive officer, said that while d.light is a for-profit company, it has a social mission to help people replace kerosene lanterns, cheap flashlights and other throwaway items with safer, cleaner, more permanent lanterns.
"These products really have to be rugged to survive and do their job in a developing-world environment," Tice said in a call from the company's Delhi office.
Atul Mittal, d.light's India marketing director, said a family can recover their cost within three to four months, based on buying a kerosene lantern for $2.50 and spending $3 a month on fuel.
Surveys conducted in India showed students increased their study time and received higher grades.
Toyola Energy in Ghana is known for selling energy-efficient cookstoves but is using the same network to sell solar lamps. In 2011, the company sold 5,453 solar lanterns and small home solar systems, mainly in northern Ghana but also some in nearby Togo and Nigeria, said co-founder Suraj Wahab. He expects those numbers to double this year.
(Related: "Protecting Health and the Planet With Clean Cookstoves")
A lantern with three light settings, a small plug-in solar panel, and a cell-phone charger goes for $30.
Wahab said Toyola Energy has found that the "very basic" lantern can change the lives of rural dwellers. "They have aspirations to live the good life like everybody else and are willing to pay for it," he said by email.
In fact, he said, some customers have paid for their systems by setting up businesses to recharge villagers' cell-phone batteries. In rural villages without public electricity, people have to scramble to find places and ways to recharge their cell phones.

From Light to Economic Development
The family in Mpigi, Uganda, who used solar lights to power their egg-selling business inspired Katherine Lucey to form a U.S.-based nonprofit two years ago called Solar Sister, which trains women as entrepreneurs who sell solar light products in Africa. ExxonMobil Foundation's Women's Economic Opportunity Initiative provided a key grant to launch a full-scale enterprise.
(Related: "Nigeria's Solar Projects Yield Both Failure and Success")
Solar Sister is targeting women, because they typically are the ones who manage household utilities. The nonprofit now has 143 entrepreneurs who have sold more than 3,500 solar lighting products in Uganda, Rwanda, and South Sudan, benefiting an estimated 17,600 lives. According to the industry rule of thumb, one solar lamp system will benefit an average of five people in a household.
Lucey is candid about the challenges, which include a heavy investment in training in part to bridge the technology know-how gap.
About 50 percent of the women interested in becoming entrepreneurs get through the first phase of the training, and only about 40 percent of those wind up doing well, she said. The others participate at a lesser level, perhaps selling a few lamps a month to supplement other incomes.
Solar Sister provides entrepreneurs with a "Business in a Bag," which includes a vetted supply of inventory, marketing materials, a ledger sheet, a bag and a T-shirt.
The organization also provides a variation of micro-credit. Products are provided up-front and entrepreneurs repay in 30 days, after they get cash from sales. "This allows them to start up a business without access to capital," Lucey said. "If they are unable to sell the product, they can return it for full credit."
A successful entrepreneur typically would sell 10 products per month at an average retail price of $45 (student lamps, lamp/phone chargers, a home system or two) and earn a 10 percent commission, or $45. That's $540 in annual income, at least in line with per capita income figures in Uganda.
Women participate at the level that suits their needs, Lucey said. "We have many who sell less, and are happy just to make $10 a month to make ends meet or pay school fees for the fifth child in the family, and a few who sell much, much more."
(Related: "Fighting Poverty Can Save Energy, Nicaragua Project Shows")
She gave the example of a woman named Grace whose husband earns $250 a year as a counselor for families of AIDS patients. Grace has been able to triple her family's income by becoming a Solar Sister entrepreneur. The family needs the money: Grace and her husband support 10 children, including six from relatives who died of AIDS. The additional income has enabled all of the children to attend school.
Lucey said market saturation isn't a problem at this point. For example, Solar Sister has a team of 15 entrepreneurs in Mityana, Uganda, which "looks like a typical rural community with dirt roads, mud-walled, thatched-roofed houses, and chickens wandering about." But that rural town has a population of 350,000 people, and is growing rapidly.
While the worst-case scenario for this type of enterprise is that an entrepreneur disappears with a small amount of inventory, the worst case Solar Sister has encountered came when a woman wanted to supplement her income from selling fish at a roadside market.
"It did not work out too well for her, the product was not a natural fit for her customers to say the least, and the lamps came back to us a little smelly," Lucey said. She said that prompted Solar Sister to institute a "must be returned in good condition" policy.
(Related Quiz: "What You Don't Know About Solar Power")
*Shell is sponsor of National Geographic's Great Energy Challenge initiative. National Geographic maintains autonomy over content.

Transit of Venus 2012—Sun Show Will Be Last for a Century

How to see rare planetary lineup that may unlock puzzles of alien worlds.

Venus glides across the sun's face during its previous transit, seen from Flagler Beach Pier in Florida.

Photograph by Jim Tiller, Daytona Beach News-Journal/AP

Early this week sky-watchers around the world will be able to witness a transit of Venus—a celestial event that won't be seen again for more than a century.
Visible either Tuesday or Wednesday, depending on where you live, the transit will offer astronomers a chance to refine our understanding of Venus as well as to tweak models for searching for planets around other stars. (Pictures: See what the Venus transit will look like.)
Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than our planet, can undergo this unusual alignment.
With its relatively tight orbit, Mercury circles the sun fast enough that we see the innermost planet transit every 13 to 14 years. But transits of Venus are exceedingly rare, due to that world's tilted orbit: After the 2012 Venus transit, we won't see another until 2117.
During the upcoming transit, Venus will look like a black dot gliding across the face of the sun over the course of about six hours.
"Venus's diameter will appear only about a 30th the diameter of the sun, so it will be ... like a pea in front of a watermelon," said Jay Pasachoff, an astronomer at Williams College in Massachusetts. (Read a Q&A with Pasachoff about Venus transits.)
"The effect won't be visually impressive, but that black dot against the sun is a remarkable thing to see."
Watch a live broadcast of the 2012 transit of Venus.
The entire transit of Venus will be visible from Hawaii, Alaska, New Zealand, Japan, the Philippines, most of Australia, and parts of eastern Asia. Countries in the Western Hemisphere will see the transit on Tuesday, while those in the Eastern Hemisphere will see it on Wednesday.
(See a visibility map for the transit of Venus.)
Viewers in North America will see Venus start to cross the sun in the late afternoon on Tuesday, but the sun will set with the planet still in transit.
Observers in Europe, Africa, and western Australia, meanwhile, will see the sun rise Wednesday morning with Venus already on its face.
Venus on the Edge
The 2012 transit of Venus will be visible even to the naked eye—although astronomers caution that people should never look directly at the sun without proper protection.
To watch safely, observers should always use special "eclipse glasses" or telescopes equipped with solar filters.
Perhaps the safest way to watch the transit of Venus is to make a pinhole camera, said Pasachoff, who is also a National Geographic Society Committee for Research and Exploration grantee. (National Geographic News is a division of the Society.)
To do so, cut a hole about a quarter-inch (0.6-centimeter) wide in a piece of cardboard paper, and use the hole to project an image of the sun onto a flat surface, such as a wall or sidewalk.
During the transit, the leading edge of Venus's silhouette will first touch the upper left side of the sun's disk.
In less than half an hour, Venus's opposite edge will touch the same point of the solar limb. At this stage, the planet's circular shape will appear to be distorted into a teardrop for a few minutes—something astronomers call the black drop effect.
"It's believed that this is an effect of blurring by Earth's atmosphere, combined with the apparent slight darkening of the sun's visible surface near its edges," said Ben Burress, a staff astronomer at the Chabot Space and Science Center in Oakland, California.
Depending on local sky quality, the altitude of the observer, and the size of the telescope, observers may also glimpse Venus's atmosphere during the transit, Burress said.
(Related: "Venus Spinning Slower Than Thought—Scientists Stumped.")
"A bright rim around the edge of Venus against the background of space may be visible just as Venus is entering or leaving the solar disk," he said. "This is caused by sunlight refracting"—or bending—"in the dense upper atmosphere of Venus."
Measuring the Solar System
Transits of Venus are so rare because the planet's orbit is tilted just over three degrees from the plane of the solar system. This means that most of the time Venus passes above or below the sun's disk, as seen from Earth.
On average, we see four transits of Venus within 243 years. The events happen in pairs spaced eight years apart, and they alternate whether Venus crosses the top or the bottom of the solar disk, Williams' Pasachoff said. This year, for instance, the planet will transit the top of the sun.
Astronomers first used telescopes to observe a transit of Venus in 1639.
But it wasn't until 1769 that dozens of scientists scattered across the globe to make detailed measurements of the event, including the famous voyage of British lieutenant James Cook, who had astronomers collecting transit data from the island of Tahiti during his South Pacific expedition.
(Related: "Journals of Captain Cook Go Online.")
Observations from different locations on Earth allowed scientists to not only triangulate the true size of the sun but also to more accurately determine the distance between the sun and Earth.
"Prior to that, the estimates of those scales were mostly educated guesswork," Chabot's Burress said.
Based on the 18th-century transit, astronomers calculated that the sun is 95 million miles (153 million kilometers) away—only slightly off from the true Earth-sun distance of 93 million miles (150 million kilometers).
"Since we already knew the relative spacing between the orbits of all the planets, once we determined the Earth-sun distance, in one fell swoop we were able to calculate the distances to all the other planets."
Venus Transit a Key to Planetary Puzzles
Today, 21st-century astronomers hope to use the 2012 transit of Venus to collect data on the planet's atmosphere and compare their findings to measurements from the European Space Agency's Venus Express orbiter.
The orbiter has returned information on intriguing weather patterns in Venus's dense atmosphere, but at close range the craft can see only one region at a time.
(See "Venus Craft Reveals Lightning, Supports Watery Past.")
The transit, meanwhile, will allow astronomers to get a broader picture of Venuvian weather in the planet's upper atmosphere and see how different regions interact.
In addition, scientists using the NASA-ESA Hubble Space Telescope will use the transit to help improve techniques for finding and characterizing planets around other stars, aka exoplanets.
With its sensitive instrumentation designed to peer deep into the cosmos, Hubble can't look directly at the sun. Instead astronomers will have the orbiting observatory aimed at the moon to watch for the slight drop in reflected sunlight during the transit.
The hope is that Hubble's activity will be a good parallel to observations currently being carried out by NASA's Kepler spacecraft, which looks for dips in starlight caused by planets transiting their host stars, as seen from Earth.
A prolific planet hunter, Kepler has already racked up 61 confirmed planets and more than 2,300 planetary candidates. (Related: "NASA Finds Smallest Earthlike Planet Outside Solar System.")
However, "since the stars are so far away that no details can be seen, those exoplanet transits are visible only in the total light of the star," Williams's Pasachoff said.
Scientists therefore have to make a number of estimates when analyzing Kepler data to tease out a planet's size and atmospheric properties.
Watching how the sun's light changes during the Venus transit can show astronomers whether their calculations capture the known properties of a nearby planet, helping them to refine their models for studying more distant worlds.
"Understanding the details of a transit in our own system can be the key to unlocking the transits of exoplanets in faraway solar systems."