Planetshine lets you see the dark side of the moon. But climate scientists say it could also tell us more about our future.
Planetshine is a pretty straightforward phenomenon. But if you look up the definition, you may get caught in a sea of scientific jargon with words like “albedo” and “diffuse reflection.” Simply put, planetshine occurs when sunlight reflecting off a planet reflects onto and lights up the dark side of one of its moons. Some climate scientists say that albedo could be a measure of our climate impact, too.
What Is Planetshine?
The example most of us can identify with is Earthshine, which is when Earth reflects its light onto its waxing or waning Moon. There, between the horns of the thin, bright crescent Moon, you will see Earthshine—the ghostly image of the full Moon. It’s sometimes called “the old moon in the new moon’s arms” when it’s waning or “the new moon in the old moon’s arms” when it’s waxing.
The best time to see Earthshine is to gaze up at the Moon on an evening or two before or after the next new moon. Shift your eyes slightly to the left of the glowing crescent and you should see the ashen glow of the dark side of the moon. Scientists say Earthshine is most intense, and thus easier to see, in April and May. That means you’ll want to head outside around sunset a day or two before or after the new moon.
No doubt, Earth’s inhabitants have marveled at this site for hundreds of thousands of years. But it took an artist to actually figure out what it could be.
Planetshine is the Earth’s albedo, a fancy word that means the measure of how much light hits a surface is reflected without being absorbed. In other words, albedo is reflectivity. An object that appears white reflects most of the light that shines upon it and is said to have a “high albedo,” whereas an object that looks dark absorbs most of the light, and is said to have “low albedo.”
Earthshine is actually a double reflection of sunlight—the reflection of sunlight onto Earth and the reflection of that sunlight from Earth onto the Moon. When the light from the Moon reflects back toward the Earth, it creates Earthshine.
Each time light reflects off the Earth or Moon, it becomes dimmer because the light is absorbed by the surface from which it is reflecting. The fact that Earthshine (or planetshine) is reflected twice means that it is dimmer than moonlight, which is reflected just once. Furthermore, the Moon’s albedo is less than the Earth’s, which makes Earthshine even dimmer, appearing more ghostlike than the brilliant glow of the Moon.
As the Moon gets fuller, the light reflected on it begins to outshine Earthshine, which is why Earthshine is only visible during crescent phases of the moon.
Only about 10 percent of the Earth’s shine reflects off the ocean, as Da Vinci had suggested. But most—about 50 percent—comes from the clouds. About 10 to 25 percent comes from the land. The only things that have more reflexivity than clouds are snow and ice. But those areas are usually covered in clouds. Because cloud cover changes season-to-season, the Earth’s albedo changes seasonally as well.
The reason Earthshine peaks in the spring is because the Northern Hemisphere is tilted toward the sun. Winter snow and ice, which reflect sunlight the best, are still on the ground in these higher latitudes during those months. In the winter, the Arctic receives much less sunlight and reflects less light.
Who Discovered Planetshine?
When you hear the name Leonardo Da Vinci, most people think of the Mona Lisa. It turns out there was much more to the renowned painter of the Italian Renaissance. He is also credited for solving the ancient riddle of Earthshine.
In 1510, Da Vinci published a collection of scientific writings, one of which was titled “Of the Moon: No Solid Body is Lighter than Air.” In it, he posed the theory that the Moon had an atmosphere with oceans and was a fine reflector of light because it contained so much water. The Moon’s glow, he suggested, was due to sunlight reflecting off Earth’s oceans and hitting the moon.
Back then, there was a lot of speculation about the heavens. In Da Vinci’s day, most people believed that the Earth was the center of the universe. (Copernicus wouldn’t publish his sun-centered theory until 1543, nearly a quarter century after Da Vinci died.) Centuries later, Da Vinci’s theory of oceans on the moon would prove not to be the case. But he understood the basic premise of Earthshine enough to go down in history as the person who explained the Moon’s ghostly glow.
Planetshine and Climate Change
In 1998, Project Earthshine, a NASA-supported ground-based Earthshine observing project, was launched at Big Bear Solar Observatory, where researchers began studying how much sunlight our planet reflects and how Earth’s cloud cover varies over time. Preliminary data shows a 6.5 percent drop in cloud cover from 1985 to 1997 and an increase from 1997 to 2003.
Researchers are now working to find out how fluctuations in the Earth’s albedo might affect climate change and global warming.
At this point, it still remains a mystery. All clouds contribute to an increase in albedo. But some clouds have a net warming effect because they trap more heat than they reflect and others have a net cooling effect because their increased albedo reflects more radiation than absorbs heat.
Planets aren’t the only celestial beings that shine. Ringshine is another phenomenon involving sunlight reflecting off a planet. It occurs when the rings of a planet reflect the light from the sun onto the planet itself or its moons.
One cool example of ringshine can be seen in images taken during the Cassini-Huygens mission, a collaboration between NASA, the European Space Agency, and the Italian Space Agency, which involved sending a probe into space to study Saturn and its systems.