Sky & Telescope - March 2023 - 24

GIBBOUS EARTH: NASA; MOON WITH EARTHSHINE: SEAN WALKER / S&T
Lunar Science
on that date, he wondered: " Might it have been these heavy
snow-laden clouds which had some effect on the intensity of
sunlight reflected toward the Moon, perhaps contributing to
the prominence of earthshine visible in the UK evening sky? "
The first weather satellite became operational in 1960
when NASA lofted Tiros 1 into orbit. However, the notion
that the Moon might allow us to monitor Earth's weather
dates back to a paper presented to the Paris Academy of Sciences
in 1833 by the director of the Paris Observatory at the
time, François Arago. He pointed out that the brightness of
earthshine should be enhanced by widespread cloudiness
on Earth and suggested: " When we have better photometric
instruments at our command, we may be able to read in the
Moon the record of the average clearness of our atmosphere. "
A quarter of a century later Arago's prophecy was echoed
by the German explorer and naturalist Alexander von Humboldt
in his multi-volume treatise Kosmos: " It is therefore not
impossible, notwithstanding the surprise which such a result
might excite on first view, that one day meteorologists will
derive valuable ideas as to the mean state of the diaphanity
[transparency] of our atmosphere in the hemispheres which
successively contribute to the production of the ashy light. "
Upon Reflection
The improved photometric instrument that Arago hoped for
was devised in the 1920s by his countryman André-Louis
Danjon. Ingenious yet elegantly simple, Danjon's design
employs a pair of right-angle prisms to produce a double
image. One prism is mounted centrally in front of the objective
lens of a small refracting telescope, the other off to the
side. By adjusting the radial orientation and distance between
the prisms, the observer can see a pair of adjacent lunar
images, with the dark earthlit limb of the first image just
touching the bright sunlit limb of the second.
To attenuate the brightness of the second image, a cat'seye
diaphragm is interposed in the light path of one of the
prisms. This clever device consists of a pair of overlapping
rectangular metal plates with opposing ends notched with
a V profile. Rack-and-pinion gears impart equal but opposite
motions to each plate. As the plates slide together, a square
aperture of constantly decreasing area is formed, allowing the
second image to be dimmed. The aperture of the diaphragm
is read on a calibrated scale. The juxtaposed double image
eliminates errors introduced by atmospheric extinction and
scattered light from the luminous veil of the twilight sky.
By reducing the brightness of the Moon's sunlit limb until
it appears equal to the brightness of the earthlit limb, the
observer can determine the intensity of incident sunlight
relative to the intensity of earthlight. Earth's planetary albedo
or overall reflectivity can be extrapolated from this ratio with
a high degree of precision.
Danjon used his portable earthshine telescope to make
over 200 measurements of earthshine intensity between 1926
and 1935. His observing program was resumed by Jean Dubois
at Bordeaux Observatory in 1940 and continued until 1960.
These early studies revealed that the brightness of earthshine
EXTREMES OF REFLECTIVITY This 1968 photograph of Earth
by the Apollo 8 astronauts shows the dramatic differences in the
brightness of clouds, land, and water. Nearly all of South America
was overcast except for the Andes Mountains along the Pacific
coast. A small portion of cloud-free western Africa is also visible
along the evening terminator.
24 MARCH 2023 * SKY & TELESCOPE
A DISTANT MIRROR When we view a thin, earthlit Moon, an observer
on the lunar surface would be presented with the inverse phase on
Earth, resembling the photograph at left. This inverse relationship partly
explains why earthshine fades as the Moon waxes, since the amount
of light reflecting off our planet is diminishing as Earth's phase wanes,
thereby reducing the amount of reflective surface area.
Sky & Telescope - March 2023

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