Earth's largest satellite, the Moon, has been drafted to act as a giant mirror, working in concert with other satellites to help study Earth's climate and measure changes in global cloud cover and temperatures.

The technique, based on an idea first described by Leonardo da Vinci, measures sunlight that bounces off our planet and is reflected back by the Moon. This "Earthshine," as researchers call it, is sometimes visible to the naked eye as an eerie dim glow on the otherwise dark portion of a crescent Moon. Any kid with a telescope can spot it easily.

As Earth's weather and climate change, increasing and decreasing cloud cover alters the amount of sunlight reflected by the planet, a number scientists call Earth's albedo.

For an average albedo, the study arrived at 0.297, give or take 0.005. This is in line with estimates made by conventional means using satellite data and, the researchers say, proves that the relatively inexpensive and simple concept works.

"As far as scientific enterprises go, this is cheap," said Philip R. Goode, a New Jersey Institute of Technology physics professor and coauthor of the study. "It's the kind of science you could have imagined doing when you were a kid."

Goode and his colleagues hope to soon refine the technique and use it to study possible climate changes caused by increases and decreases in solar activity. Meanwhile, early results indicate a potentially promising research tool that might one day also help measure potential long-term global warming, say the scientists whose early work will be published in the May 1 issue of the journal Geophysical Research Letters.

Other experts caution that using Earthshine to measure changes in Earth's albedo is fraught with challenges. Goode and his colleagues say they have taken these challenges into account.

Bruce Barkstrom, a researcher at NASA's Langley Research Center who studies terrestrial radiation as it relates to the ever-changing cloud cover, cautioned that any measurement of Earth's albedo is a complex undertaking. Even the best satellite data run through the most powerful computers does not yet provide a complete picture of how the albedo changes over time.

Earthshine data have their own limitations. Barkstrom said the Moon tends to reflect light that comes mostly from Earth's equatorial region, making it difficult to pin down the exact meaning of Earthshine data. Ground-based detection is also made difficult by changing amounts of water vapor in the atmosphere.

"What you end up with are things that tend to bias the measurement and that are going to be very difficult to correct," Barkstrom said.

Hugh Christian, who runs the Global Hydrology and Climate Center at NASA's Marshall Space Flight Center, said the new study is legitimate science and quality work. "But it's a very difficult measurement," Christian said. "It would probably be very premature to say you could monitor global change this way, but it's certainly an intriguing approach and has merit."

He added that it remains to be determined how well it can work.

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Old idea, new use

Using Earthshine as a research tool is not a new idea. While da Vinci long ago explained the phenomenon, systematic measurement of Earthshine dates back at least to 1926, when French astronomer André-Louis Danjon began a series of more than 200 measurements during a five-year period. Other researchers made sporadic measurements in the decades that followed.

But most of this information is of little use to climate research, because it did not take into account global cloud cover, which is one of the most significant factors controlling how much sunlight the planet reflects back into space. Satellite observations now make that possible, and such a melding of data was first suggested about 20 years ago.

But no one has done it on a useful scale until now. Steven E. Koonin, a Caltech researcher who coauthored the current paper, proposed combining Earthshine measurements with satellite data 10 years ago. Now Koonin, Goode and their colleagues use a 6-inch (152-millimeter) refractor telescope at the Big Bear Solar Observatory in California. This telescope is otherwise devoted to observing the Sun, and is therefore available at night when a dark sky makes Earthshine easily visible in a telescope.

The new albedo measurements are based on 70 nights of observations during 1994-95 and another 200 that began in 1998 and are ongoing. The data are averaged over long periods of time to account for changes caused by local weather, seasonal snow cover and other factors.

The researchers say the data provide hints that Earth's albedo has decreased 2.5 percent during the past five years, as the Sun's magnetic activity has climbed from solar minimum to maximum. If accurate, this finding supports a hypothesis that the Sun's magnetic field plays an indirect role in Earth's climate.

Previous studies have shown that the energy released by the Sun varies by about one-tenth of a percent during the 11-year solar cycle. A 1999 study reported in the journal Science found evidence that increased solar radiation during the peak of the cycle produces more ozone in Earth's upper atmosphere. In turn, the increased ozone warms the stratosphere, a region from about 6 to 30 miles (10 to 50 kilometers) up in the atmosphere, by trapping heat energy. Upper-level wind patterns are then altered.

A trickle-down effect, according to the 1999 study, changes wind patterns around the globe, fueling stronger westerly winds in North America, for example. Such changes in wind patterns would alter cloud development, which relies on how large air masses move over land and sea.

Goode said his team would like to develop a more accurate, dedicated system of telescopes to watch possible changes in Earth's albedo during the decline of the solar cycle in coming months and years. Solar activity is expected to begin a decline within the next year or so, a slide that will last four or five years.

Working with a telescope designer in Taiwan and other partners in Europe, the researchers hope to set up a half-dozen relatively small, robotic telescopes around the world to make more accurate measurements. The telescopes would not be a lot more sophisticated than those used by backyard astronomers, Goode said, and the total cost would be around $1 million.

The Earthshine study was funded by NASA and the Western Center for Global Environmental Change.