Galileo discovers the moons of Jupiter - January 7th, 1610

On this day in 1610, Italian astronomer Galileo Galilei made the first recorded sighting of the planet Jupiter’s moons.

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Episode Transcript

Speaker 1 (00:00):
This Day in History Class is a production of I
Heart Radio, Hello and Welcome to This Day in History Class,
a show that reveals a little bit more about history
day by day. I'm Gay Bluesier, and today we're looking
at a pivotal moment in astronomical history, the time when

(00:22):
humans learned that the Earth's moon wasn't the only one
out there. The day was January seven, six Italian astronomer
Galileo Galilei made the first recorded sighting of the planet

(00:43):
Jupiter's moons. He made the discovery using his newly improved
homemade telescope, which made objects appear twenty times larger. At first,
Galileo wasn't sure what he was looking at. As far
as anyone knew, their planets didn't have their own moons,
So when Galileo noticed three bright lights near Jupiter, he

(01:07):
just assumed they were distant stars. However, scientists don't let
assumptions stay assumptions for long if they can help it,
So Galileo continued observing the lights over the course of
several nights, and it's lucky for us that he did.
The discoveries he made that week ultimately changed how humans

(01:28):
view the universe. As well as our place within it.
The first thing Galileo noticed was that the three lights
near Jupiter seemed to move in the opposite direction of
the stars behind them. The lights also changed their positions
relative to one another, but always stayed close to Jupiter.

(01:49):
It was puzzling behavior for a group of stars, so
the astronomer kept watching. Soon he observed a fourth light
near the planet that was moving in the same strange way.
Galileo continued his patient watch for seven straight nights, and
on January he finally solved the puzzle. The four lights

(02:12):
weren't stars at all. They were moons or natural satellites
orbiting the planet Jupiter in much the same way as
our moon orbits the Earth. This discovery was notable for
many reasons, but the most important was that it provided
strong evidence of a heliocentric universe. Up until then, most humans,

(02:35):
except for Copernicus I, believed the Earth was the fixed
center of the universe and that all the other celestial
bodies revolved around it. But the discovery of Jupiter's moons
through a wrench in that geocentric theory. If moons could
orbit another planet instead of the Earth, then maybe planets

(02:55):
could orbit something besides the Earth as well, and if
that were true, then perhaps Copernicus had been right all along,
and the Earth really does orbit the Sun. Galileo published
his discoveries a year later in a book titled Siderius Nuncius,
or the Starry Messenger. This was the work that set

(03:18):
Galileo on his path to believing in a Copernican system,
and his other discoveries of sun spots and solar rotations
pushed him the rest of the way there. However, when
Galileo later published his confirmation of the heliocentric theory, he
was charged with heresies by the Inquisition of the Catholic Church.

(03:39):
He was found guilty in sixteen thirty three and sentenced
to life in prison. However, since he was nearly seventy
years old by that time and in relatively poor health,
Galileo was permitted to serve out his sentence at home.
He remained there until January eighth, sixteen forty two, when
he succumbed to fever and heart palpitations and died at

(04:03):
the age of seventy seven. In his own time, Galileo
was unjustly punished for his discoveries, but he's honored for
them today. The four moons of Jupiter that he discovered
are now known as the Galilean satellites. In his notebooks,
Galileo had only referred to the moons numerically as one

(04:24):
to three, and four, but the German astronomer Johannes Kepler
recommended naming them after figures from Roman mythology who were
closely tied to Jupiter, the king of the Roman gods.
It took more than two hundred years for Kepler's idea
to gain traction, but he eventually got his wish, and
the four large moons are now called Io Europa, Ganymede,

(04:50):
and Callisto. Of course, Jupiter, being the show off planet
that it is, couldn't content itself with only four moons.
In the or since Galileo, astronomers have discovered seventy five
additional moons, bringing Jupiter's grand total to seventy nine moons
as of two So if you didn't have moon envy before,

(05:13):
you probably do now. But don't let it get you down.
Jupiter was just made for moons. Because its mass is
more than three hundred times that of Earth, Jupiter's gravitational
field is much larger and stronger too. That enables it
to capture and hold a lot more satellites in its orbit.

(05:34):
And gosh, we're all really impressed down here, I can
tell you. Most of Jupiter's moons are quite small, only
around six miles or less in diameter. The four moons
discovered by Galileo or the planet's largest, which is how
he spotted them through his twenty power telescope. Each of
the four has its own unique features. For instance, the

(05:58):
largest of the group, Ganymede, is the only known moon
to have its own magnetic field, while Callisto is the
most heavily cratered object in the entire Solar System. As
for Io, that moon's distinctive feature is the many volcanoes
that dot its surface, making it one of just a

(06:19):
few celestial bodies to have confirmed volcanic activity. Last, but
definitely not least, there's Europa, the smallest of the Galilean satellites.
Because its entire surface is smooth, uncratered ice, Europa has
a high degree of reflectivity, making it one of the

(06:40):
brightest moons in the Solar System. Still, Europa's biggest claim
to fame is the enticing possibility that a vast ocean
lies beneath its icy surface, one that might even be
hospitable to life. Future robotic explorations like NASA's upcoming Europa
Clip per Mission could supply the answer to that crucial question.

(07:04):
But like Galileo all those centuries ago, we'll just have
to be patient and wait and see. I'm Gaye Lousier
and hopefully you now know a little more about history
today than you did yesterday. You can learn even more
about history by following us on Twitter, Facebook, and Instagram

(07:25):
at t d I HC Show, And if you want
to commiserate about being stuck on old one Moon Earth,
you can always drop me a line at This Day
at i heart media dot com. Thanks as always to
Chandler Mays for producing the show, and thank you for listening.
I'll see you back here again tomorrow for another day

(07:47):
in History Class. For more podcasts from my Heart Radio,
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