SkyandTelescope.com July 2014 55J. Kelly Beatty
Senior contributing editor Kelly Beatty gets a big grin every time
he spots earthshine. Follow him (@NightSkyGuy) on Twitter.
The fi rst useful measurements of earthshine’s varying
intensity came from André-Louis Danjon (1890–1967),
well known to skygazers for his gauge of umbral darkness
during total lunar eclipses. He concluded that the albedo
(refl ectivity) of a “full Earth” could vary from a low of 32%
in July to a peak of 52% in October.
Danjon’s pioneering work has been continued and
expanded thanks to Project Earthshine, coordinated by
Philip Goode of Big Bear Solar Observatory in southern
California and Enric Pallé of Instituto de Astrofísica de
Canarias on the island of Tenerife. Since 1999 their small
team has used dedicated telescopes to monitor the bright-
ness of earthshine and then to compare those results with
satellite imagery of Earth’s daylit hemisphere.
You might think that the advent of weather satellites
would make measurements of Earth’s cloudiness routine.
But reliably accurate satellite readings have only been
possible in the past two decades. Initially, Goode and
Pallé couldn’t reconcile their Project Earthshine observa-
tions with the weather imagery, but better analysis and
techniques (such as using robotic telescopes) have fi nally
brought convergence.
These refl ectivity data have become increasingly
important in the ongoing debate over global climate
change. For example, the Project Earthshine work shows
that Earth’s albedo made a distinct jump (by roughly
0.5%) from late 1998 through mid-2000, and then
afterward it largely leveled off. By comparison, the Sun’s
output varies only by about 0.1% over an 11-year-long solar
cycle. So if Earth has become 0.5% more refl ective, then
the amount of sunlight reaching the ground has been
reduced by about 2 watts per square meter.
This change can’t be explained by an increase in
greenhouse gases — in fact, this increasing albedo trend,
if sustained over time, would tend to make Earth cooler,
not warmer. Goode says that more observations, covering
the period since 2007, should be published soon.
Meanwhile, exoplanet specialists are excited about how
earthshine can be used to “detect” our planet’s abun-
dant plant life. Leafy plants exhibit a sharp increase in
refl ectivity at wavelengths longer than about 700 nm. So
when sunlight refl ects off a particularly verdant continent
(South America, for example), the spectrum of earthshine
mirrored in the Moon gains a near-infrared enhancement
that’s very distinctive. Researchers think this “red edge”
might become a crucial spectral fi ngerprint in the search
for habitable worlds.
So the next time you’re greeted with a Moon grinning
at you like a celestial Cheshire Cat, smile back with a
knowing nod — and enjoy the wonder of it. ✦Earthshine is obvious in this view of a two-day-old waxing cres-
cent Moon. Features most visible in the lunar night, such as the
rayed crater Tycho, are the same ones seen during a full Moon.
Jerry Lodriguss combined eight diff erent exposures to capture
both the Moon’s sunlit crescent and its dimly lit dark side.In the early 1500s, Leonardo da Vinci deduced that the ghostly glow seen on the night side of the Moon is due to refl ected light shining
on it from Earth. The backwards writing in his drawing is not a mistake; Leonardo wrote all his personal notes in mirror writing.CODEX LEICESTER