Sky & Telescope - March 2025 - 58

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MARCH 2025 OBSERVING
Pro-Am Conjunction by Diana Hannikainen
Shades
of Red
Help rebuild a database
logging how dark the Moon
appears during lunar eclipses.
W
hen a volcano blows its top - as
Krakatoa did in 1883 and Mount
Pinatubo did in 1991 - it spews a lot of
ash and particles into the atmosphere.
That particulate matter can have global
consequences on Earth's climate (not to
mention our view of the universe). One
neat way of estimating the particles'
volume is by recording how dark lunar
eclipses appear following an eruption.
It's in the shadow. Graduate
student Lucas Boissel (Université Paris
Cité, France) discovered his passion for
paleoclimatic studies while working on
his master's thesis, which explored the
impact of early 20th-century volcanic
eruptions on climate and society. He
specifically focused on measuring the
stratospheric aerosol optical depth. Volcanic
ash in the stratosphere, upon contact
with water molecules, transforms into
sulfate aerosols, little particles of sulfurbearing
compounds. These reflect a fraction
of the incoming sunlight and absorb
outgoing infrared radiation, affecting
Earth's energy balance. By using reports
of contrasts in shading during lunar
eclipses, scientists can estimate the concentration
of aerosols in the atmosphere
and their climatic effects.
During a total lunar eclipse, Earth
sits between the Sun and the Moon. Any
sunlight that reaches the lunar surface
must first pass through our atmosphere,
where it's refracted (bent) and directed
moonward. Thanks to simple geometry
(and a pinch of physics), we can
58 MARCH 2025 * SKY & TELESCOPE
pA measure of how well aerosols in Earth's atmosphere block and scatter light during a lunar
eclipse tells scientists how much particulate matter a volcano such as Mount Pinatubo (shown
here in 1991) has belched into the atmosphere.
determine how high up in the atmosphere
the rays were when they traveled
through en route to the Moon. Sunlight
that passes closer to the ground in the
troposphere (below 10 km, or 7 miles)
is refracted and absorbed the most.
During an eclipse, this light is bent so
strongly that it focuses on the Moon's
surface in the inner umbra, the darkest
part of Earth's shadow. The troposphere
is where weather " happens, " so cloud
conditions along the sunrise and sunset
lines largely determine the shading of
the inner umbra.
Higher up in the atmosphere, in
the stratosphere, refraction is less
pronounced. Sunlight passing through
this layer falls in the outer parts of the
umbra. Under normal circumstances,
that light is less diluted than the rays
traversing the troposphere, and so we
see lighter shading. But a particularly
potent volcanic eruption will spew ash
into the stratosphere, which translates
into a darker-than-usual lunar eclipse.
Some eclipses have even been so dark
they've been referred to as " invisible. "
Wanted: Lunar eclipse data. Boissel's
passion for his master's subject propelled
him into his current PhD research
- he's done a deep dive into lunar eclipse
data from the 17th and 18th centuries.
But now he'd like to extend that timeline
into the 19th and 20th centuries. He's
already perused multiple archives, such
as those of the Paris Observatory library
and the National Library of France, but
collecting the relevant data is challenging,
he says. Some readers may be
familiar with Richard Keen (University
of Colorado), who had established a
network of aficionados to collect data on
observations of lunar eclipse shadings.
But Keen passed away in 2020, leaving
much of his lifetime's work unpublished.
Now, Boissel hopes to revive that
network (which he's planning to name
LUNAE-Volc). If you're a professional
astronomer and have access to your
institute's archives or are an amateur
and have kept extensive notes of
past lunar eclipses, Boissel would be
delighted to hear from you. In addition,
he's looking for your observations
of upcoming eclipses - for this, he's
requesting brightness estimates using
the Danjon scale (ranging from 0 for
very dark to 4 for very bright eclipses),
as well as information on local weather
conditions. You can reach him at
l.boissel@hotmail.com. Your data may
help determine the impact of volcanism
on Earth's atmosphere and ultimately
refine climate models.
¢ Editor in Chief DIANA HANNIKAINEN
will henceforth view differences in shading
during lunar eclipses in a new way.

Sky & Telescope - March 2025

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