Researchers from the University of Melbourne and 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.

Sally Langford from the University of Melbourne's School of Physics who conducted the study as part of her PhD, says that the brightness of the reflected earthshine varied as the Earth rotated, revealing 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. However these planets will be too small to allow an image to be made of their surface," she said.

"We can use earthshine, together with our knowledge of the Earth's surface to help interpret the physical make up of new planets."

Light from the Sun is reflected off of the Earth (See Point A) onto the Moon (See Point B). Some of this light is then reflected off of the Moon back towards the Earth (See Point C). Because of this we see part of the Moon illuminated by the Sun and the rest of the Moon dimly illuminated by this doubly reflected light (which we call earthshine). Each time light reflects off a surface (like a planet), it gets dimmer because some of the light is absorbed by the reflecting surface. This means that earthshine is dimmer than moonlight because earthshine is sunlight that has been reflected twice and moonlight is sunlight that has been reflected only once (off the surface of the moon). In addition, the reflectivity of the moon (its "albedo") is less than that of the Earth, which makes earthshine even dimmer.   Credit: Keith Cooley

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 three year study involved taking images of the Moon to measure the earth's brightness as it rotated, allowing Ms Langford to detect the difference in signal from land and water.

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," Ms Langford said.

"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."

The paper will be published this week, in the international journal Astrobiology.