The Mysterious Zodiacal Light

By Joe Rao
SPACE.com Skywatching Columnist
posted: 24 October 2008
12:16 am ET

During the next few weeks on some clear moonless early morning, if you are fortunate to be far from any haze and bright lights, keep a close watch on the eastern horizon about two hours before sunrise. If you're lucky you might catch a glimpse of a ghostly column of light extending upward into the sky.

Many have been fooled into thinking that it's beginning of morning twilight and indeed the Persian astronomer, mathematician and poet Omar Khayyam (1050? -1123?) referred to this ghostly glow as the "false dawn" in his poem, The Rubaiyat.

That faint ghostly glow was once thought to be solely an atmospheric phenomenon: perhaps reflected sunlight shining on the highest layers of Earth's atmosphere. We know now that while it is indeed reflected sunlight, it is being reflected not off our atmosphere, but rather off of a non-uniform distribution of interplanetary material; debris left over from the formation of our solar system.

These countless millions, if not billions of particles – ranging in size from meter-sized mini-asteroids to micron-sized dust grains -- seem densest around the immediate vicinity of the sun, but extend outward, beyond the orbit of Mars and are spread out along the plane of the ecliptic (the path the sun follows throughout the year). Hence the reason for the name Zodiacal Light is because it is seen projected against the zodiacal constellations.

Before the break of dawn

The best time to see the Zodiacal Light is when the ecliptic appears most nearly vertical to the horizon. For those in the Northern Hemisphere, the best morning views in the eastern sky will come during the next few weeks without the interference of bright moonlight. Conversely, for those who live in the Southern Hemisphere, the best views now are in the western evening sky right after sunset.

Those who live in the tropics or at the equator are luckiest of all since the Zodiacal Light is always very conspicuous from these regions. This is probably because from these locations the ecliptic is always favorably oriented allowing views of the Zodiacal Light both in the western evening sky and eastern morning sky all year long.

For northerners at this particular time of the year, it is just before morning twilight begins (about 90 minutes before sunrise), that the Zodiacal Light should appear at its brightest and most conspicuous.

A ghostly slanted pyramid

The Zodiacal Light is something you have to see to believe.

My best views of it came from southeast Arizona back in 2001. I had chosen that part of the country for a view of a pristinely dark and starry sky to serve as the backdrop for an upcoming display of Leonid meteors. Imagine my surprise when I stepped outside on my first early morning watch, and was surprised to see what looked like the glow of a town or small city just beyond the horizon to my east. It took a while for me to realize that what I was seeing was not urban light pollution, but the Zodiacal Light!

To a discerning eye, its diffuse shape somewhat resembles a tilted cone, wedge or slanted pyramid. At the base of the cone, the light may extend some 20 to 30-degrees along the horizon. At its best, it can approach or even equal the Milky Way in brightness, but more often than not it is so faint that even a small amount of atmospheric haze can obscure it. On exceptionally clear nights, the tapering cone might be seen to stretch more than halfway to the zenith.

In fact, should you be blessed with such conditions – absolutely no artificial lighting, smoke or haze – you should also try to see the Zodiacal Band, which runs along the entire ecliptic and usually averages about 5 to 10-degrees in apparent width.

Counterglow

There is yet another ghostly glow in the sky which few have seen or heard about.

This is the Gegenschein, derived from a German word meaning "counterglow," since it is always opposite or "counter" in position to the sun in the sky. The Gegenschein is visible only with the unaided eye, as it is far too large and diffuse to be viewed with a telescope or even binoculars.

The exact nature of the Gegenschein is still somewhat of a mystery, though most think that – like the Zodiacal Light – it might be some sort of tail formed by minute particles from our atmosphere that are streaming out into space away from the Earth in the opposite direction from the sun.

• Online Sky Maps and More
• Sky Calendar & Moon Phases
• Astrophotography 101

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

Derailed particle collider feted with lavish gala

By SHARON GAUDIN, New York Times

While an orchestra played and dignitaries from around the world gathered, the Large Hadron Collider this week was formally inaugurated amid much fanfare, despite that the fact particle smasher has been broken down for the past month.
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Swiss President Pascal Couchepin and French Prime Minister Francois Fillon were flanked by international science ministers at Tuesday's gala, which was held on the Franco-Swiss border at the world's largest particle collider. Speeches and exhibitions were followed by a performance from the Orchestre de la Suisse Romande.

"Today is a day for CERN to thank its member states for their continued support for basic science, and for providing the stable framework that makes science of this kind possible," said CERN Director General Robert Aymar. "It is also a day for CERN and the global particle physics community to take a sense of pride in the achievement of bringing this unique facility from dream to reality, a process that has taken over two decades of careful planning, prototyping and construction, culminating with the successful circulation of the machine's first protons in front of a global audience on September 10."

But the pageantry and talk of dreams turning into reality come while the collider sits idle, sidelined last month by a faulty electrical connection. Scientists at CERN, the group that oversees the collider, initially predicted that the problem would only keep the collider offline for two months, but the estimate quickly was extended so the downtime now is expected to stretch throughout the whole winter.

The organization reported in September that it will need to investigate the issue further and do repairs. The repairs are unlikely to be completed before the project enters its "winter maintenance" period. That means particle beams won't be shot through the 17-mile long vacuum-sealed tube again until spring.

The wiring trouble surfaced less than two weeks after scientists conducted their first experiment. The collider shot two separate particle beams - one at a time - in different directions around the tube at speeds of more than 99% the speed of light. The initial experiment came 20 years after development of the collider began.

When the collider eventually smashes the beams together, it will create showers of new particles that should re-create conditions in the universe just moments after its conception.? Scientists predicted that the Large Hadron Collider will run particle-collision experiments for the next 10 to 15 years.

With the Big Bang theory, scientists largely believe that more than 13 billion years ago an amazingly dense object the size of a coin expanded into the universe that we know now -- with planets, stars, black holes and life.

A main goal of the collider experiments is to find the elusive Higgs particle, which is believed to be responsible for giving other particles their mass. Though its existence hasn't been proven, it's believed that Higgs particles are what give electrons their weight, for instance.

Scientists are also hoping that the particle collider will give them information about so-called dark energy and dark matter.

At the Tuesday inauguration, the collider's project leader, Lyndon Evans, said, "The LHC is the largest and most sophisticated scientific instrument ever built. There have been many challenges along the way, which have all been overcome one after the other. We are now looking forward to the start of the experimental program. The adventure of building the [collider] will end and a new adventure of discovery will begin."

Hatred on the Campaign trail

Study: ‘Hippie’ Apes Also Hunt Other Apes For Meat

From RedOrbit.com

Researchers have discovered the first evidence of wild bonobos hunting and eating the young of other primate species.

Once considered by experts as the only chimpanzee species to resist hunting and killing other apes for food, bonobos don’t appear to embrace a "make-love-not-war" philosophy, researchers reported in the October 14th issue of Current Biology.

Experts had previously believed that unlike their chimpanzee counterparts, bonobos – also known as “hippie” apes – would restrict what meat they do eat to forest antelopes, squirrels, and rodents.

Researchers made the discovery that these free-loving primates also hunt and kill other primates while they were studying a bonobo population living in LuiKotale, Salonga National Park, in the Democratic Republic of Congo. They had been observing the bonobos there for the last five years, which is what made the new observations possible.

"These findings are particularly relevant for the discussion about male dominance and bonding, aggression and hunting—a domain that was thought to separate chimpanzees and bonobos," said Gottfried Hohmann of the Max-Planck-Institute for Evolutionary Anthropology.

"In chimpanzees, male-dominance is associated with physical violence, hunting, and meat consumption. By inference, the lack of male dominance and physical violence is often used to explain the relative absence of hunting and meat eating in bonobos. Our observations suggest that, in contrast to previous assumptions, these behaviors may persist in societies with different social relations."

Bonobos live only in the lowland forest south of the river Congo, and, along with chimpanzees, they are humans' closest relatives. Bonobos are perhaps best known for their promiscuity: sexual acts both within and between the sexes are a common means of greeting, resolving conflicts, or reconciling after conflicts.

"This has implications for models on early humans that people have proposed how humans have evolved," said Hohmann.

Researchers living in LuiKotale, Salonga National Park observed the bonobos silently stalk through the woods on the ground, trying to get underneath a group of chimps before clambering up a tree in a sudden attack.

The bonobo hunts were successful on fewer than half the excursions and in some cases shared the meat. Hohmann said this presents evidence they were willing to share to encourage group hunting.

'Brain fingerprinting' could be breakthrough in law enforcement

By KOMO Staff

Watch the story

SEATTLE -- Science is becoming a more important part of catching a killer or terrorist and keeping the innocent out of jail.

A Seattle neuroscientist is leading the way with technology based on a simple fact: your brain can't lie.

An odd looking headband, flashing words on a computer screen, and a couple clicks of a mouse could be the secret to putting a murderer behind bars.

"It's a game changer in the field of global security," said Dr. Larry Farwell, Chairman of Brain Fingerprinting Labs who developed "brain fingerprinting" - a lie detector test for the 21st century.

While polygraph tests rely on emotional responses, brain fingerprinting records how your brain reacts to words and images related to a crime -- ones only the killer would recognize.

"If the person was there, they get 'ah ha!' response in the brain waves," he said. "The brain says 'Ah ha!' "

And that "ah ha" moment can't be covered up.

"This is an involuntary response that happens very quickly; it's not something you can control," he said.

Dr. Farwell has worked with the CIA, FBI and law enforcement agencies around the country. His cases include an innocent Iowa man finally freed after 23 years, and a serial killer in Missouri who eventually confessed.

It's technology Farwell says is fool proof. And unlike polygraphs, these can be admitted in court.

There's still resistance from some law enforcement agencies.

"It took some time, it always takes time," Farwell said. "It took time for fingerprints, for DNA and now for brain fingerprinting."

But Dr. Farwell says his guarantee will help more people accept it.

"I can't beat it, and I invented it."

He has even offered $100,000 to anyone who can fool the system, and so far nobody has.

Researchers discover baldness gene: 1 in 7 men at risk

From eurekalert.org

Scientists at McGill, King's College and GSK solve mystery of male pattern baldness

Researchers at McGill University, King's College London and GlaxoSmithKline Inc. have identified two genetic variants in caucasians that together produce an astounding sevenfold increase the risk of male pattern baldness. Their results will be published Oct. 12 in the journal Nature Genetics.

About a third of all men are affected by male pattern baldness by age 45. The condition's social and economic impact is considerable: expenditures for hair transplantation in the United States alone exceeded $115 million (U.S.) in 2007, while global revenues for medical therapy for male-pattern baldness recently surpassed $405 million. Male pattern baldness is the most common form of baldness, where hair is lost in a well-defined pattern beginning above both temples, and results in a distinctive M-shaped hairline. Estimates suggest more than 80 per cent of cases are hereditary.

This study was conducted by Dr. Vincent Mooser of GlaxoSmithKline, Dr. Brent Richards of McGill University's Faculty of Medicine and the affiliated Jewish General Hospital (and formerly of King's College), and Dr. Tim Spector of King's College. Along with colleagues in Iceland, Switzerland and the Netherlands, the researchers conducted a genome-wide association study of 1,125 caucasian men who had been assessed for male pattern baldness. They found two previously unknown genetic variants on chromosome 20 that substantially increased the risk of male pattern baldness. They then confirmed these findings in an additional 1,650 caucasian men.

"I would presume male pattern baldness is caused by the same genetic variation in non-caucasians," said Richards, an assistant professor in genetic epidemiology, "but we haven't studied those populations, so we can't say for certain."

Though the researchers consider their discovery to be a scientific breakthrough, they caution that it does not mean a treatment or cure for male pattern baldness is imminent.

"We've only identified a cause," Richards said. "Treating male pattern baldness will require more research. But, of course, the first step in finding a way to treat most conditions it is to first identify the cause."

"Early prediction before hair loss starts may lead to some interesting therapies that are more effective than treating late stage hair loss," added Spector, of King's College and director of the TwinsUK cohort study.

Researchers have long been aware of a genetic variant on the X chromosome that was linked to male pattern baldness, Richards said.

"That's where the idea that baldness is inherited from the mother's side of the family comes from," he explained. "However it's been long recognized that that there must be several genes causing male pattern baldness. Until now, no one could identify those other genes. If you have both the risk variants we discovered on chromosome 20 and the unrelated known variant on the X chromosome, your risk of becoming bald increases sevenfold."

"What's startling is that one in seven men have both of those risk variants. That's 14 per cent of the total population!"

Cosmic strings might emit cosmic sparks, answer cosmological questions

From PhysOrg.com -- For astronomers, understanding what happened in the early moments of the universe could answer many questions in physics and astronomy. One possible player in the early universe is cosmic strings, which arise naturally in particle physics models. However, cosmic strings are quite strange hypothetical entities: they’re thinner than a proton, but can be as long as the universe. Cosmic strings might have formed as imperfections when the early universe was undergoing drastic phase changes.
“If cosmic strings were found to exist, it would tell us that the universe was very hot (trillion trillion degrees) in the first fraction of a nanosecond,” physicist Tanmay Vachaspati told PhysOrg.com. “It would tell us that the fundamental theory must admit string solutions. Further studies of the properties of the strings could tell us if string theory may be correct. So the discovery of cosmic strings would be truly remarkable for a wide cross-section of physicists and astronomers.”

Cosmic strings are also superconducting, and can be viewed as elastic, current-carrying wires that permeate the cosmos as closed loops and infinitely long curves. The strings oscillate under their own tension, giving off very strong electromagnetic radiation.

Possibly, a recent observation of a radio burst, or spark, could have been caused by such a superconducting cosmic string. That’s the idea being suggested by Vachaspati, from the Institute for Advanced Study in Princeton, New Jersey, and Case Western Reserve University in Cleveland, Ohio.

Vachaspati has developed a prediction that cosmic strings could produce potentially observable radio sparks, and recently published his study in Physical Review Letters. Previous attempts to observe cosmic strings have focused primarily on high-energy emission such as gamma rays, rather than lower-energy radio waves.

The recent radio spark was observed by Dunc Lorimer and colleagues, and reported in 2007. No host galaxy has been identified for the spark, which lasted only a millisecond, and had a high central frequency of 1.4 GHz. Vachaspati found that the radio spark’s properties, such as its duration, fluence, spectrum, and event rate, match well with a superconducting cosmic string that carries a current of about 100,000 GeV.

“Lorimer et al’s result is the first radio burst to be detected at cosmological distances,” Vachaspati said. “Their observation triggered the idea that radio bursts may be a good way to search for strings.”

Such a spark could come from a point called a “cusp” on an idealized, one-dimensional cosmic string. For a brief instant, a cusp reaches the speed of light, and this localized region emits a very strong electromagnetic radiation. Vachaspati found that an observer located at a large distance and slightly off the beam direction could see this radiation as a spark similar to the one observed by Lorimer’s group.

Whether this particular radio spark was caused by cosmic strings or something else, Vachaspati explains that the important thing is how his predictions could influence particle physics. For example, the existence or absence of cosmic strings could be used to constrain various fundamental models.

He explained that the superconducting cosmic string model may be tested in a variety of ways, such as looking for signatures of decaying particle emission, looking for unusual “fanlike” radiation patterns from kinks on the strings, and finding more radio sparks located outside of galaxies.

More information: Vachaspati, Tanmay. “Cosmic Sparks from Superconducting Strings.” Physical Review Letters, 101, 141301 (2008).

Moscow calls for anti-US alliance

From The Sydney Morning Herald

Divide and rule … the Russian President, Dmitry Medvedev, centre, and France's Nicolas Sarkozy leave their meeting in Evian.
Photo: AFP

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Adrian Blomfield in Moscow
October 10, 2008

THE President of Russia has called on Europe's leaders to create a new world order that would minimise the role of the United States.

Confident that a row with Europe prompted by Russia's invasion of Georgia in August was over, Dmitry Medvedev arrived in the French spa town of Evian on Wednesday determined to woo his fellow leaders into creating an anti-US front.

Gone was the kind of wartime rhetoric that saw Mr Medvedev lash out at the West and describe his Georgian counterpart, Mikheil Saakashvili, as a "lunatic". Instead Mr Medvedev spoke of a Russia that was "absolutely not interested in confrontation", and outlined plans for a new security pact to ban the use of force in Europe.

Yet there was little doubt that Mr Medvedev was playing the divide-and-rule tactics of Vladimir Putin, his predecessor and now Prime Minister, by seeking to pit the US against its European allies.

In a speech delivered to European leaders at a conference hosted by the French President, Nicolas Sarkozy, to discuss the international financial crisis, Mr Medvedev sought to show that the US was at the root of all the world's problems. He blamed Washington's "economic egotism" for the world's financial woes and then accused the Bush Administration of taking Europe to the brink of a new cold war by pursuing a deliberately divisive foreign policy.

He also maintained that the US was once again trying to return to a policy of containing Russia.

"After toppling the Taliban regime in Afghanistan, the United States started a series of unilateral actions," Mr Medvedev said.

"As a result, a trend appeared in international relations towards creating dividing lines. This was in fact the revival of a policy popular in the past and known as containment."

While he called for a cooling of the noxious rhetoric that had blighted East-West relations over the past two years, Mr Medvedev clearly laid the blame for the deterioration on the US, which he said was again viewing Russia through the prism of the Cold War. "Sovietology, like paranoia, is a very dangerous disease, and it is a pity that part of the US Administration still suffers from it," he said.

In order to end the "unipolar" model in which the world depended on the US, he proposed creating new financial systems to challenge the dominance of the International Monetary Fund and the World Trade Organisation, both of which had fallen under Washington's spell.

Attacking the enlargement of NATO, which he said had advanced provocatively towards Russia, he proposed a new European security treaty.

The new European pact would include "a clear affirmation of the inadmissibility of the use of force - or the threat of force - in international relations" and would be built on the principle of the territorial integrity of independent nations.

While Russia has insisted it was not intending to supplant NATO, Mr Medvedev made it clear that the US-dominated alliance was partly responsible for the war in the Caucasus by its failure to rein in Georgian "aggression".

The Russian President won praise from Mr Sarkozy after he announced that all Russian troops had been withdrawn from buffer zones around Georgia's rebel enclaves of South Ossetia and Abkhazia before today's deadline.

Describing his guest as a man who had "kept his word", Mr Sarkozy immediately declared that talks on an EU-Russia partnership deal, suspended as punishment for Russia's military operation in Georgia, could resume.

Telegraph, London; Guardian News & Media

Huge Planet Defies Explanation

By SPACE.com Staff

Astronomers have sighted a very dense planet-sized object that orbits its parent star in just four days and six hours.

The object, COROT-exo-3b, fits into the category of a failed star known as a brown dwarf, but the team that made the discovery has not ruled out the possibility that it is a planet. Brown dwarfs are failed stars. They burn lithium but are not massive enough to generate the thermonuclear fusion of hydrogen and helium that powers real stars. Planets do none of that.

"It has puzzled us; we're not sure where to draw the boundary between planets and brown dwarfs," said Hans Deeg, an astronomer at the Instituto de Astrofisica de Canarias (IAC) in the Canary Islands, Spain.

The object has a mass 20 times greater than that of Jupiter, but is roughly the same size. It falls outside the range of planets and stars discovered to date, with the largest planets having 12-Jupiter-mass and the smallest stars 70-Jupiter-mass.

If astronomers confirm the object as a planet, it would weigh in as the most massive and densest planet found so far. A full study will be detailed in the journal Astronomy and Astrophysics.

"COROT-exo-3b might turn out to be a rare object found by sheer luck", said Francois Bouchy, an astronomer at the Institut d'Astrophysique in Paris. "But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our sun. We're now beginning to think that the more massive the star, the more massive the planet."

Ground-based telescopes around the world helped pinpoint the object, including observatories in France, Chile, Germany, Hawaii, Israel and Spain's Canary Islands.

The hunt for exoplanets has intensified over recent years, with astronomers usually finding such objects indirectly by observing their gravitational influence on parent stars. Another team showcased what might be the first direct image of an exoplanet around a sun-like star in September.

Prosecutors Expected To Spare Wall St. Firms

By Carrie Johnson

Washington Post Staff Writer

Justice Department officials yesterday vowed to unravel the complex financial deals that helped prompt a market crisis in an effort that will generally seek criminal charges against individual brokers and bankers, rather than companies themselves, according to interviews with lawyers involved in the cases.

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Mindful of the fallout from the last wave of business fraud cases six years ago, authorities are leaning against seeking indictments of major banks and insurers that may have inflated the value of their mortgage-related investments. Instead, prosecutors will look for such garden-variety crimes as false statements and insider trading by executives who tried to disguise financial problems or pad their wallets.

Exhibit A is Bear Stearns, the investment bank that collapsed in March and was bought by J.P. Morgan Chase during a cash squeeze that ultimately gripped Wall Street. Two former fund managers there are fighting criminal charges for allegedly misleading investors about the financial health of their unit. The company will avoid indictment, according to two sources familiar with the case who spoke on condition of anonymity because the process is not final. Robert Nardoza, a spokesman for the U.S. Attorney's office in the Eastern District of New York, declined to comment yesterday.

That pattern is likely to persist even as fallout from the liquidity crisis intensifies. Last week, the FBI announced 26 investigations underway at companies including Fannie Mae, Freddie Mac, Lehman Brothers, American International Group, Countrywide Financial and IndyMac.

At an American Bar Association conference yesterday, Deputy Attorney General Mark R. Filip vowed that prosecutors would press ahead to decode the obscure financial products at the heart of the market's troubles. He said there would be "no unwillingness to take the facts and the law where they lead."

Yet the tenor is markedly different from the last wave of financial scandals, which began with the indictment of accounting firm Arthur Andersen six years ago. The firm swiftly collapsed, costing tens of thousands of jobs. More recently, corporate executives and civil liberties advocates pressed for legislation that would bar strong-arm prosecution tactics. In August, Filip issued guidance that reminds prosecutors to consider the rights of corporate employees.

Among other factors, the guidelines require government lawyers to take into account the health of a business when they make decisions about whether to file criminal charges. Given the current landscape, with Lehman in bankruptcy proceedings, Fannie Mae and Freddie Mac under federal control, and AIG surviving only after an $85 billion infusion from the Treasury, lawyers with experience in such cases predict few major criminal prosecutions of businesses.

"It would be a very rare company that would ever be prosecuted," said Joshua Hochberg, former chief of the Justice Department's fraud section. "These are all negotiated settlements. . . . A criminal conviction brings mandatory debarment and effectively puts a corporation out of business."

Prosecutors for months have been sifting through documents in an effort to separate bad business decisions from possible criminal conduct.

The initiative has been complicated in part by gaps in regulation of mortgage-backed securities. Credit-default swaps, a kind of insurance against defaults on housing-related investments, are not considered a security under the laws that govern the Securities and Exchange Commission, according to Columbia University law professor John Coffee Jr. That means companies that sell the swaps do not have an affirmative duty to advise investors about their risks. Rather, brokers would be subject to law enforcement action only if they made a misleading statement about their risks or value, perhaps in a bid to win greater fees.

"It's always fraudulent when you have a material misrepresentation, deliberately made, with the intent to deceive and for personal gain," said Gil M. Soffer, who oversees corporate fraud prosecutions at the Justice Department.

Such allegations go to the heart of a criminal case filed last month against two former Credit Suisse employees and may underpin an investigation of individuals who had worked at Lehman and who were involved in the same market, known as auction-rate securities. The Lehman probe was reported yesterday by the Wall Street Journal.

Government officials with experience investigating corporate fraud say some of the patterns they are detecting -- lying to investors, shifting debt off corporate balance sheets -- are familiar.

"The more things change, the more things stay the same," said Benton Campbell, U.S. for the Eastern District of New York and a former member of the government's Enron Task Force.

Scientists trace AIDS virus origin to 100 years ago

From Cnn.com

NEW YORK (AP) -- The AIDS virus has been circulating among people for about 100 years, decades longer than scientists had thought, a new study suggests.

Genetic analysis pushes the estimated origin of HIV back to between 1884 and 1924, with a more focused estimate at 1908.

Previously, scientists had estimated the origin at around 1930. AIDS wasn't recognized formally until 1981 when it got the attention of public health officials in the United States.

The new result is "not a monumental shift, but it means the virus was circulating under our radar even longer than we knew," says Michael Worobey of the University of Arizona, an author of the new work.

The results appear in Thursday's issue of the journal Nature. Researchers note that the newly calculated dates fall during the rise of cities in Africa, and they suggest urban development may have promoted HIV's initial establishment and early spread.

Scientists say HIV descended from a chimpanzee virus that jumped to humans in Africa, probably when people butchered chimps. Many individuals were probably infected that way, but so few other people caught the virus that it failed to get a lasting foothold, researchers say.

But the growth of African cities may have changed that by putting lots of people close together and promoting prostitution, Worobey suggested. "Cities are kind of ideal for a virus like HIV," providing more chances for infected people to pass the virus to others, he said.

Perhaps a person infected with the AIDS virus in a rural area went to what is now Kinshasa, Congo, "and now you've got the spark arriving in the tinderbox," Worobey said.

Key to the new work was the discovery of an HIV sample that had been taken from a woman in Kinshasa in 1960. It was only the second such sample to be found from before 1976; the other was from 1959, also from Kinshasa.

Researchers took advantage of the fact that HIV mutates rapidly. So two strains from a common ancestor quickly become less and less alike in their genetic material over time. That allows scientists to "run the clock backward" by calculating how long it would take for various strains to become as different as they are observed to be. That would indicate when they both sprang from their most recent common ancestor.

The new work used genetic data from the two old HIV samples plus more than 100 modern samples to create a family tree going back to these samples' last common ancestor. Researchers got various answers under various approaches for when that ancestor virus appeared, but the 1884-to-1924 bracket is probably the most reliable, Worobey said.

The new work is "clearly an improvement" over the previous estimate of around 1930, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. His institute helped pay for the work.

Fauci described the advance as "a fine-tuning."

Experts say it's no surprise that HIV circulated in humans for about 70 years before being recognized. An infection usually takes years to produce obvious symptoms, a lag that can mask the role of the virus, and it would have infected relatively few Africans early in its spread, they said.