Turner studies planet through earthshine

First explained by Leonardo Da Vinci as the pale glow reflected from the Earth onto the dark side of the moon, the phenomenon is known as earthshine. In a paper published this week in the scientific journal Astrobiology, Turner and his Australian colleagues demonstrated that changes in the brightness of earthshine correspond to the presence of oceans and continents on our planet.

The study, Turner explained, aimed to understand how Earth-like planets with seas and landmasses would appear from great distances. “We wanted to see what the light coming from the Earth would look like. We were looking for the signal of coastline boundaries,” Turner said.

“When we look at planets in other solar systems we can just watch the planets as they rotate — they will appear as a point of light that may get dimmer and brighter if they have oceans and continents,” University of Melbourne researcher and co-author Sally Langford said in an e-mail. Langford has also worked as a visiting student research collaborator at Princeton.

She also noted that the study was the first to measure short-term variations in earthshine. Other studies, she explained, have averaged out the intensity of the Earth’s reflection on the moon.

Turner explained that the intensity of earthshine changes because water and land reflect light differently. For example, the Earth illuminates the moon more when sunlight glints off the water to form an intensely bright area.

“If you were on the moon, you would see that bright spot on the Earth. When the land comes into view, the Earth dims away,” he said.

Langford said the researchers observed earthshine weakening as the sun rose over Africa, blocking the bright reflection from the Indian Ocean. The earthshine, however, was constant as the sun gleamed over the landless Pacific.

Measuring this variation in earthshine required extra coordination. Turner explained that he had to set up and test the equipment during the initial observations, as well as interpret results and write up the report, even when he was not physically with the Australian researchers.

“I would say [the challenges to international collaboration] were not terribly great,” he said. “E-mail is pretty efficient, and we could do video conferencing.” Turner noted that he and his colleagues used Skype, software that allows computer users to contact phones at a low cost or other computers for free.

Turner was in Australia when the study began, he explained. The researchers had to find a dark spot where light pollution could not contaminate their data, Turner said. He explained that as faint earthshine typically brightens and dims by only around 10 percent, a location devoid of excess light would best facilitate capturing these small variations.

Mount Arapiles was the first observation spot they found. “It was fun,” Turner said. “We were on a small dirt road on a station wagon with equipment, running everything on a car battery, with kangaroos hopping around. It was scenic, not like our usual observatory trips.”