Sky & Telescope - October 2022 - 38

Pioneering Science
from those we know today. Even though
he was on the right track, it seems that,
somehow, he just didn't get the arithmetic
right. Unfortunately, like other astronomers
of his epoch, he never used any of
these data to speculate on the moons'
compositions. Not properly of astronomical
interest, perhaps.
The Ice Men Cometh
So, who did finally make careful (and
correct) measurements of both the
brightnesses and densities of the Galilean
moons, and consider what the data meant?
Edward Charles Pickering, director of the
Harvard College Observatory. In a paper
published in 1907, Pickering gives tables of diameters, densities,
and albedos for the moons - and his numbers are very
close to modern values.
First, Pickering had to figure out the intrinsic brightness of
each moon. In the 1870s he had developed a technique that
measured by how much he needed to dim Jupiter's light with
filters to match a moons' observed brightness. Using Engelmann's
diameters, Pickering came up with albedos for the
moons. His results were only about half the modern accepted
values, but nevertheless he calculated that Europa is about
twice as reflective as Earth's Moon. (Today we know that
Europa's albedo is more than five times that of the Moon.)
TIBER TELESCOPE This 9-inch (25-cm) Merz refractor at the Collegio
Romano Observatory, located on the roof of the church of St. Ignatius in
Rome, was used by the Italian astronomer Fr. Angelo Secchi to observe
Jupiter's moons. After his death in 1878, the Italian government confiscated
the telescope and moved it to their observatory on Montemario,
Rome. It was lost in a fire in 1958.
PIONEEERING OBSERVER Angelo Secchi was
a Jesuit priest and director of the Collegio Romano
Observatory from 1850 until his death in 1878. He
made the first measurements connecting solar activity
to terrestrial magnetic storms, but his studies
into the physical and chemical natures of stars and
planets were his greatest achievement.
And what's more interesting is what
Pickering did not observe. In the 1879
Annals of the Astronomical Observatory of
Harvard College, he wrote, " It has been
thought by many astronomers that the
light of the satellites of Jupiter was variable.
This view is not sustained by the
present measurements . . . During the past
two or three years, I have frequently had
occasion to compare the satellites, and in no case have I been
able to perceive any marked change in their relative brightness
beyond that due to the proximity of Jupiter. "
That " proximity of Jupiter " seemingly is what had led
other astronomers astray. When Europa is close to Jupiter and
you look at it through a telescope, your eye will adjust to the
brightness of the planet. This makes Europa appear dimmer
than when it's farther from Jupiter's bright disk.
Pickering's explanation for this effect had actually been
proposed 250 years earlier, shortly after Galileo first discovered
the planet's moons. Fr. Christoph Scheiner, S. J. (famous
for his dispute with Galileo over sunspots) and his student
Johann Georg Locher, wrote in 1614 about this. " Anyone
who can see, has seen a lesser light obscured by a greater; a
weaker, by a stronger, " they wrote. It appears that 19th-century
astronomers didn't know about Scheiner and Locher's
analysis, demonstrating that when we lose touch with past
scientists, we lose knowledge.
Pickering took the novel step of actually interpreting the
data in his table. " It will be noted that the densities . . . are
extremely small for solid bodies, " he wrote. " The density
of our Moon, which is of about their size is 3.44 [times the
density of water], and that of the Earth much larger . . . " And
then, with density and albedo measurements in hand, he proceeded
to suggest what the moons might be made of: " Their
density and brightness are what we should expect if they were
composed of . . . "
Wait for it!
" . . . loose heaps of white sand. "
Sand? Why didn't Pickering think of ice?
For one thing, even though Secchi had identified hydrogen
in stars, most astronomers hadn't yet accepted that the most
abundant element in the universe was hydrogen. Pickering
wouldn't have expected ice to be a common substance in
space. But he was also convinced that the moons couldn't be
solid because, as he explains, unless they're made of something
that can change shape, like sand, " . . . it is impossible to
understand how rigid bodies could assume the varying ellipticities
exhibited by these bodies . . . the variations of whose
38 OCTOBER 2022 * SKY & TELESCOPE
VATICAN OBSERVATORY (2)
Sky & Telescope - October 2022

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