Moon Reveals New Way to Find Oceans, Land on Other Earths

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An Australian doctoral researcher using a backyard telescope has made a potentially big discovery: Earth’s oceans and continents shine differently on the dark side of the moon.

Now, Sally Langford, a doctoral candidate in physics at the University of Melbourne, is suggesting the “earthshine” of planets around other stars could provide long-distance windows into their surface features.

Langford and her colleagues, from Melbourne as well as Princeton University, have shown for the first time that the difference in reflection of light from the Earth’s land masses and oceans can be seen on the dark side of the moon, a phenomenon known as earthshine. Their paper appears in this week’s edition of the international journal Astrobiology.

This is the first study in the world to use the reflection of the Earth to measure the effect of continents and oceans on the apparent brightness of a planet. Other studies have used a color spectrum and infrared sensors to identify vegetation, or for climate monitoring.

The researchers peered at the dark side of the crescent moon using a 20 cm (8 inch) telescope, on the bigger side of what most amateur astronomers use in their yards.

For three years, Langford took images of the Moon to measure the earth’s brightness as it rotated. Observations of the Moon were made from Mount Macedon in Victoria, for around three days each month when the Moon was rising or setting. The study was conducted so that in the evening, when the Moon was a waxing crescent, the reflected earthshine originated from Indian Ocean and Africa’s east coast. In the morning, when the Moon was a waning crescent, it originated 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.

Langford said the variation revealed the difference between the intense mirror-like reflections of the ocean compared to the dimmer land.

“In the future, astronomers hope to find planets like the Earth around other stars,” Langford said. “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.” 

LEAD IMAGE CAPTION: Earthshine on a crescent moon. Credit: Edward W. Szczepanski, Houston Astronomical Society (click on the photo to visit Szczepanski’s page)

Source: University of Melbourne. The paper is available here.