54 July 2014 sky & telescopeOBSERVING
Exploring the Solar System
Illuminating Earthshine
The pale glow on the Moon can teach us about Earth’s changing climate.
After a long, snowy winter in New England, early
April brought the fi rst pleasant evening for stargazing. A
delicate waxing-crescent Moon hovered over the west-
ern horizon, and even a quick glance showed there was
more to see than just a delicate swoosh of lunar light. In
fact, it was easy to make out the entire lunar disk, even
the bloated lion’s share hidden in shadow, thanks to the
ghostly phenomenon known as earthshine.
The cause is, by now, well known: whenever the Moon
presents a thin crescent to us, in the days before or after
new Moon, sunlight refl ecting off our planet brightens
the lunar night like a giant fl ashlight. At these times, a
hypothetical lunar astronaut would see a brilliant, nearly
full Earth hanging in the sky. This mirrored sunlight isenough to make the Moon’s darkened, desolate landscape
plainly visible to us.
Leonardo da Vinci described the phenomenon at some
length 500 years ago. The great Renaissance master
correctly reasoned that the unlit portion of the Moon
is refl ecting light shining on it from Earth, though he
imagined the lunar surface to be covered with liquid seas.
But, remarkably, da Vinci’s keen celestial insight wasn’t
appreciated for nearly two centuries, when a collection of
his writings known as the Codex Leicester was rediscovered
at the end of the 17th century.
Earthshine can be startlingly obvious. One of the most
vivid descriptions of this “ashen light,” as it’s sometimes
called, comes from J. F. J. Schmidt, a German-born
astronomer who served as director of Athens Observa-
tory from 1859 to 1884. Schmidt was mesmerized by the
appearance of a two-day-old Moon in January 1867. Scan-
ning the glowing lunar night through a 6-inch refractor,
he could trace many large craters and maria, mountain
peaks, and the long rays of Tycho.
Notably, the lunar features seen when bathed in
Earth’s pale light are the same ones you see well during
times near full Moon — splashes of rays, for instance,
rather than jagged rims and mountain chains — because
in both cases the illumination is coming directly from
our line of sight (or nearly so). The crater Aristarchus and
the prominent rays emanating from Tycho, Copernicus,
and Kepler are easy to spot with optical aid.
You’ve probably encountered earthshine most often
when a thin lunar crescent hangs low in the evening
sky — romantically termed “the old moon in the new
moon’s arms” — or in the predawn sky a few days before
new Moon. But various observers have spotted it well
after the waning crescent has fattened to fi rst quarter and
beyond. In 1949, Audouin Dollfus reported seeing a sliver
of earthlight’s glow just 38 hours before full Moon. The
renowned French observer had some help, though — he
used a coronagraph at Pic du Midi Observatory to mask
out the bright lunar disk.
If you’ve noticed that earthshine can vary in intensity
from night to night, your eyes aren’t playing tricks on
you. Clouds cover about 60% of Earth’s surface on aver-
age, and the ashen glow can appear stronger or weaker
depending on the cloud cover across the hemisphere of
Earth that’s facing the Moon at that moment.The crescent Moon awash with earthshine (refl ected light
from Earth on its night side) is one of the most splendid
scenes in nature.
P.- M. H ED ÉN