Sky & Telescope - October 2022 - 36

CROSS-SECTIONS OF JUPITER'S MOONS: NASA / JPL; JOVIAN MOON DIAGRAM: ETHBIBLIOTHEK
ZÜRICH, ALTE UND SELTENE DRUCKE / PUBLIC DOMAIN
Pioneering Science
Io
Europa
Ganymede
could calculate the densities of the Galilean moons.
The second key indicator is albedo. Observers can estimate
how bright a moon is by comparing it to a " nearby " star of
known magnitude. However, without knowing the size of the
satellite, it's not possible to tell if it's small but shiny (high
albedo), or big and dusky (low albedo). Both would reflect
the same amount of sunlight. Once we know the size of the
moon, we can then determine its albedo.
So now astronomers had the rudiments of the two indicators
in hand. In the 1860s, pioneers in astrophysics like Fr.
Angelo Secchi, S. J. identified the signatures of hydrogen and
oxygen gases in stars' spectra, which suggested that water ice
might be common in the universe. So, was some astronomer
in the 19th century able to put
all the pieces together and discover
the icy natures of Callisto,
Ganymede, and Europa? Nope!
And the story of how they missed
it is fascinating.
On Thin Ice
From the standpoint of the early
21st century, it's perhaps difficult
to believe that there was ever
a time when scientists simply
weren't interested in knowing
the compositions of the stars
and planets. German astronomer
Friedrich Wilhelm Bessel,
for example, wrote in 1848 that
astronomy " must lay down
the rules for determining the
motions of the heavenly bodies
as they appear to us from the
Earth " ; other information, such
as knowing the density, was, " not
properly of astronomical interest. "
That meant that even with
the correct data right in front
of him, Bessel (and his contemporaries)
might not even have
thought to ask about the nature
of Jupiter's moons. However,
36 OCTOBER 2022 * SKY & TELESCOPE
Callisto
ICE HERE AND THERE
These cutaway diagrams show
the differences in composition
among Jupiter's four largest
moons. Blue and white indicate
water in solid (ice) or liquid form,
while the metallic (iron, nickel)
cores are shown as gray and
rock is indicated with brown. Io is
the one moon lacking significant
water, and Callisto is thought not
to have a metallic core.
pioneers like Secchi and English astronomer William Huggins
had begun to speculate on the compositions of the stars.
Would some brave planetary scientist follow suit?
One way to gauge the state of knowledge of astronomers
of the time is to read the books they wrote for popular audiences.
Consider the 1873 edition of Reverend Thomas W.
Webb's Celestial Objects for Common Telescopes and Simon
Newcomb's widely read 1878 book, Popular Astronomy. What
they say about Jupiter's moons is surprising. According to
Newcomb, " The light of these satellites varies to an extent
which it is difficult to account for, except by supposing very
violent changes constantly going on on their surfaces. "
Newcomb was an accomplished astronomer at the U.S. Naval
Observatory and should have been
a reputable source, so where did
he get this idea?
Webb, for his part, goes on at
length, citing observers who noted
radical changes in the brightnesses
of the moons. For example,
he writes about Callisto, " as far
back as 1707 Maraldi noticed
that, though usually faintest, it
was sometimes brightest (a variation
which he ascribes to all the
satellites); in 1711 Bianchini and
another once saw it for more than
1h so feeble that it could hardly
be perceived; 1849, June 13 Lassell
made a similar observation with
far superior means . . . "
GALILEO'S MOONS This illustration
by the German astronomer Fr. Christoph
Scheiner, S. J. and his student
Johann Georg Locher shows Jupiter,
its shadow, and its four largest moons.
The diagram appeared in their book
Mathematical Disquisitions, only four
years after Galileo first discovered the
satellites with his crude telescope. In
the book they proposed that measuring
the time required for the moons to pass
through the planet's shadow, or across
its disk, could provide more detailed
information about the Jovian system.
Sky & Telescope - October 2022

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