Sky & Telescope - December 2024 - 78

BEGINNER'S SPACE by Peter Tyson
What Are the Galilean Moons?
THE GALILEAN MOONS are the four
largest moons of the planet Jupiter.
History. The name honors Galileo
Galilei, who discovered them in 1610
using the newly invented telescope. The
find was one of the most earthshaking
in the history of science. The four
moons were the first new solar system
worlds identified since antiquity, and
their unveiling revolutionized observational
astronomy by demonstrating
that optics could reveal far more in the
heavens than the naked eye could.
Most significantly, Galileo's revelation
provided unequivocal evidence
that worlds circled other bodies besides
Earth. That realization helped upend
the long-accepted geocentric system of
Claudius Ptolemy, which held that our
planet was the center of the universe
and all bodies revolved around it.
Galileo labeled the four moons I, II,
III, and IV, in order of their increasing
distance from Jupiter. But in time they
gained names drawn from classical
mythology: Io, Europa, Ganymede, and
Callisto. All were lovers of Zeus in the
Greek mythos. (Zeus's equivalent in the
Roman world was Jupiter.)
Science. The Galilean moons look
tiny beside Jupiter, but they are major
solar system bodies. Io, Ganymede, and
Callisto are all larger than our Moon,
and Europa is only slightly smaller. All
other Jovian moons - the current count
78 DECEMBER 2024 * SKY & TELESCOPE
is close to 100 - are orders of magnitude
smaller than the Galilean moons.
Each of the four big satellites boasts
its own superlative. Io is the most volcanically
active body in the solar system,
with more than 400 active volcanoes.
Europa is the smoothest, and Callisto is
the most heavily cratered. Ganymede,
for its part, is the largest moon in the
entire solar system - appropriate, given
that Jupiter is the largest planet. Notably,
the outer three Galilean moons
stand out from Io in likely having
oceans of liquid water beneath their icy
crusts (S&T: Apr. 2022, p. 14).
Observing. For a novice observer of
the night sky, one of the most memorable
moments is their first sight of Jupiter
through a telescope. Suddenly, where
the unaided eye saw just the planet, up
to four tiny dots appear arrayed around
it. It's like a mini solar system, with
those four worlds orbiting the king
of planets. If it weren't for Jupiter's
brightness overwhelming theirs, they'd
actually be visible to the naked eye. But
through even a small telescope, the Galilean
moons are not only easily visible
but resolve into disks that are clearly
not stars but worlds of their own.
Observers see the moons strung in
a rough line on one or both sides of the
planet, like beads on a necklace. That's
because the moons circle Jupiter in the
plane of its equator, and the Jovian axis
GALILEO'S MOONS From left to right, they
are Io, Europa, Ganymede, and Callisto. They
appear here scaled to the moons' relative sizes
and placed in order of distance from Jupiter.
is tipped only about 3° from its orbit, so
we view that miniature " solar system "
almost edge-on.
Of course, the number of moons you
can see at any time changes constantly
as they revolve around Jupiter. All four
may appear on one side of the planet,
for instance, or two on each side. Or
you may see fewer than four, because
one or more of the moons may then be
behind or in front of Jupiter.
To find out where the moons are on
any given night this month, turn to
page 51 to see our " Jupiter's Moons "
graphic and " Phenomena of Jupiter's
Moons. " This table lists key events
occurring between the moons and the
planet, including when a moon disappears
behind Jupiter or within its
shadow as well as when a moon or its
shadow passes over the planet's face.
For other nights beyond this month,
consult our interactive tool (https://
is.gd/GalileanMoons). It displays the
four satellites' positions for any date
and time between January 1900 and
December 2100. The tool is customizable,
so you can change the view
depending on whether your telescope
shows the sky north up, south up, or
mirror-reversed. Happy observing! ¢
IO: NASA / JPL / UNIVERSITY OF ARIZONA; EUROPA & CALLISTO: NASA / JPL / DLR;
GANYMEDE: NASA / JPL / TED STRYK / CC BY-NC-ND 3.0
http://https:// http://www.is.gd/GalileanMoons
Sky & Telescope - December 2024

Table of Contents for the Digital Edition of Sky & Telescope - December 2024
