Moon's Dark Side Gives Climate Hints

If he were alive today, Leonardo da Vinci could spend most of his time saying "I told you so." Four centuries ago he postulated that the light that makes the dark part of the crescent moon visible to us some of the time comes from the Earth, not the moon.

And now, all these years later, scientists have found a way to use Leonardo's discovery to help us understand the Earth’s incredibly complex climate system. They have figured out how to use "Earthshine," the sunlight reflected from the Earth to the moon and back again, to shed a little light on how the planet’s climate is changing.

When you think about it, Leonardo's finding was quite extraordinary. In those days, no one knew the moon didn’t generate its own light, and didn’t even have an atmosphere that could scatter light into areas that would otherwise be dark.

He figured out that when the Earth reflects enough light, we can see the entire moon, not just the crescent.

Measuring the Light

Early last century the French astronomer Andre-Louis Danjon made the first measurements of the intensity of earthshine, but no one really knew what to do with them. Then in 1991, Steven E. Koonin, professor of theoretical physics at the California Institute of Technology, co-authored a paper in which he speculated that if earthshine could be measured very precisely, it might tell much about the health of the Earth’s climate.

The amount of radiation that the Earth reflects back into space, called its "albedo," depends on a number of factors, including cloud cover and seasonal changes in the amount of snow and ice. When the sun rises on a region covered with winter snow, for example, more solar rays are reflected back into space.

If you were somewhere in space, in the path of that radiation, the Earth would appear much brighter because the planet’s albedo would be very high.

Koonin theorized that it should be possible to monitor earthshine for a long period of time, thus providing a constant record of changes in the Earth’s climate. If the Earth is getting warmer because of the greenhouse effect, as many experts believe, then that should reduce the amount of snow and ice and produce a gradual decrease in the planet’s albedo.

… A Surprisingly Simple Experiment …

So Cal Tech teamed up with the New Jersey Institute of Technology and in 1994 a surprisingly simple experiment was launched. In these days of ultra-sophisticated gadgets costing millions and millions of dollars, the two institutions began taking measurements with a 6-inch telescope at the Big Bear Solar Observatory in the mountains northeast of Los Angeles.

The small telescope was equipped with a charge-coupled device, a very precise version of the electronic gizmo in video cameras that captures the light to produce an image.

"We measure the intensity of the dark part of the moon," Koonin says, which is no small accomplishment because the adjacent crescent is about 10,000 times brighter.

Measurements were taken on 70 nights in 1994-95, and the scientists recently completed a two-year observation during which measurements were taken on about 200 nights.

…And Surprising Results

Right out of the barrel, the experiment produced some surprising results.

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