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.
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