Earth's Reflection Varies on the Moon
The sunlit crescent portion of the Moon is blocked by a filter. This allows researchers to measure earthshine on the Moon, as seen in the lower left of this image, created by the Big Bear Solar Observatory lunar on Dec. 12, 1999.
Credit: BBSO/NJIT/CALTECH

The moon will be full Thursday, which means we'll see it in all its illuminated glory. But when the moon is just a sliver, we sometimes see our own reflection shining back at us from the moon's shrouded side, in a phenomenon called "earthshine."

Now scientists say the difference in light reflection from the Earth?s land masses vs. the oceans can be seen on the moon. By tracking changes in earthshine as Earth rotates, scientists measured brightness variations that correspond to the brilliant, mirror-like reflections from oceans compared to the dimmer reflections from land.

Earthshine was first proposed by Leonardo da Vinci, who suggested that sunlight could bounce off our planet and be reflected back to us by the moon. This light is only visible when there is little sunlight reflecting directly off the moon, which would otherwise drown out the much dimmer earthshine. Thus, Earth's reflection is only visible to the naked eye on the darker portion of thin crescent moons, and not full moons.

The phenomenon can sometimes be seen by the naked eye as a ghostly glow, and is easily visible with a telescope. It is best seen once a month when the crescent moon hangs just above the western horizon right after sunset.

Full moons occur when the sun, Earth and moon are lined up. The moon will look pretty much full tonight and Thursday night. But it is never truly full. In fact, because the moon's orbital plane around Earth is slightly different than Earth's plane in relation to the sun, the three objects rarely line up perfectly, and when they do, the moon falls into shadow and an eclipse occurs.

Sally Langford, a physics graduate student at the University of Melbourne, used the observatory at Mount Macedon in Victoria, Australia, to measure changes in reflected earthshine as our planet rotates. She observed the moon for about three days each month while it was rising and setting.

In the evening, when the moon was a waxing crescent, the reflected earthshine originated from the Indian Ocean and the coast of Africa. In the morning, when the moon was a waning crescent, the reflected light came only from the Pacific Ocean.

"When we observe earthshine from the moon in the early evening, we see the bright reflection from the Indian Ocean, then as the Earth rotates the continent of Africa blocks this reflection, and the moon becomes darker," Langford said.

The discovery, detailed in this week's issue of the journal Astrobiology, could help scientists learn more about distant planets around other suns, she said.

"In the future, astronomers hope to find planets like the Earth around other stars. However these planets will be too small to allow an image to be made of their surface. We can use earthshine, together with our knowledge of the Earth's surface to help interpret the physical makeup of new planets."

Changes in the apparent brightness of an exoplanet's reflection could signal that continents and oceans are rotating through.

"If we find Earth-sized planets and watch their brightness as they rotate, we will be able to assess properties like the existence of land and oceans," she said.