Relive the Discoveries of Galileo September 2014 | Page 10

Relive the Discoveries of Galileo

Discovery No. 3
The Moons of Jupiter
The discovery of objects orbiting Jupiter may
be Galileo’s most famous discovery, so famous that
what he discovered are now known as the Galilean
Moons. (They are individually named Io, Europa,
Ganymede, and Callisto.) While using his small (2inch diameter) telescope in January 1610, Galileo
wrote that he found, “four planets never seen from the
beginning of the world right up to this day” in tight
proximity to Jupiter. Throughout that year, he made
repeated observations and recorded, by date, the
position of these tiny but bright “planets” that always
stayed with Jupiter, but on alternating sides. It would
take two years of study through the telescope to deduce
that the “planets” were actually in orbit around Jupiter.
In orbit around Jupiter?! The idea was, well, Earthmoving. According to Ptolemy, Earth stood still and
all the heavens, both the wandering planets and the
fixed stars, were in orbit around us. That puts Earth in
a very special place, uniquely stationary, and in the
centre of the universe.

Galileo’s notes on the moons of Jupiter

But if there could be new planets found in orbit around
Jupiter, then Jupiter can claim just as much status as Earth as a place other bodies orbit. Earth is not that
special, no longer unique, not necessarily
in the centre. In fact, Galileo wondered, if
moons could be in orbit around Jupiter,
and if all the phases of Venus indicate
Venus is in orbit about the Sun, then
maybe Earth is not the centre around
which all bodies are in orbit. Perhaps
Copernicus was right. Maybe Earth is in
orbit around the Sun. It was an earthmoving idea!
The discovery of the moons orbiting
Jupiter was one of the announcements
Galileo made in Sidereus Nuncius (The
Starry Messenger). It launched him into
celebrity status (you could say “stardom”)
and made him the focus of attention
throughout Europe.
The pages for January 7 to 14, 1610 in Galileo’s notebook
where he describes the changing positions of Jupiter’s moons

- 10 -

On the facing page, let’s relive the
discovery of Jupiter’s moons.
The
sketching is easy!

For more information about DRAA publications, please visit our website: www.drastronomy.com