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The Discovery of the Galilean Satellites
The Discovery of the Galilean Satellites
Written By Ron Baalke
Probably the most significent contribution that Galileo Galilei made to
science was the discovery of the four satellites around Jupiter that are
now named in his honor.
Galileo first observed the moons of Jupiter on January 7, 1610 through
a homemade telescope. He originally thought he saw
three stars near Jupiter, strung out in a line through the planet. The next evening, these stars seemed to have moved the wrong way, which caught his attention.
Galileo continued to observe the stars and Jupiter for the next week. On
January 13, a fourth star appeared.
After a few weeks, Galileo had observed that the four stars never
left the vicinity of Jupiter and appeared to be carried along with the planet, and that they
changed their position with respect to each other and Jupiter. Finally, Galileo
determined that what he was observing were not stars, but planetary bodies that were
in orbit around Jupiter.
This discovery provided evidence in support of the Copernican system and
showed that everything did not revolve around the Earth.
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Galileo's Letter to Prince of Venice
Diagram Translation
Galileo realized that the telescope had applications that reached beyond astronomy:
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"Most Serene Prince. Galileo Galilei most humbly prostrates himself
before Your Highness, watching carefully, and with all spirit of willingness,
not only to satisfy what concerns the reading of mathematics in the study
of Padua, but to write of having decided to present to Your Highness
a telescope ("Occhiale") that will be a great help in maritime and land
enterprises. I assure you I shall keep this new invention a great secret
and show it only to Your Highness. The telescope was made for the most
accurate study of distances. This telescope has the advantage of discovering
the ships of the enemy two hours before they can be seen with the natural
vision and to distinguish the number and quality of the ships and to judge
their strength and be ready to chase them, to fight them, or to flee from them;
or, in the open country to see all details and to distinguish every movement
and preparation."
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Galileo's Observations of Jupiter's Moons
Galileo published his observations in Sidereus Nuncius in March 1610:
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"I should disclose and publish to the world the occasion of discovering and
observing four Planets, never seen from the beginning of the world up to
our own times, their positions, and the observations made during the last
two months about their movements and their changes of magnitude; and I
summon all astronomers to apply themselves to examine and determine their
periodic times, which it has not been permitted me to achieve up to this
day . . . On the 7th day of January in the present year, 1610, in the first
hour of the following night, when I was viewing the constellations of the
heavons through a telescope, the planet Jupiter presented itself to my view,
and as I had prepared for myself a very excellent instrument, I noticed
a circumstance which I had never been able to notice before, namely that
three little stars, small but very bright, were near the planet; and although
I believed them to belong to a number of the fixed stars, yet they made me
somewhat wonder, because they seemed to be arranged exactly in a straight
line, parallel to the ecliptic, and to be brighter than the rest of the
stars, equal to them in magnitude . . .When on January 8th, led by some
fatality, I turned again to look at the same part of the heavens, I found
a very different state of things, for there were three little stars all
west of Jupiter, and nearer together than on the previous night."
"I therefore concluded, and decided unhesitatingly, that there are
three stars in the heavens moving about Jupiter, as Venus and
Mercury around the Sun; which was at length established as clear as
daylight by numerous other subsequent observations. These observations
also established that there are not only three, but four, erratic
sidereal bodies performing their revolutions around Jupiter."
The Galilean Moons (Voyager Images)
Io, Europa, Ganymede and Callisto
Simon Marius claimed to have observed Jupiter's moon as early as
late November 1609 (about five weeks prior to Galileo) and had begun
recording his observations in January 1610 at about the same time
Galileo was first making his observations. However, since Marius did not
publish his observations right away as Galileo had done, his claims were impossible
to verify. Since Galileo's work was more reliable and extensive, he
is generally given the credit for discovering the moons of Jupiter.
In 1614, Marius did provide the names of the Jupiter's moons that we are
familiar with today, based on a suggestion from Johannes Kepler:
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- "Jupiter is much blamed by the poets on account of his irregular
loves. Three maidens are especially mentioned as having been
clandestinely courted by Jupiter with success. Io, daughter of the
River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there
was Ganymede, the handsome son of King Tros, whom Jupiter, having
taken the form of an eagle, transported to heaven on his back, as
poets fabulously tell . . . . I think, therefore, that I shall not
have done amiss if the First is called by me Io, the Second
Europa, the Third, on account of its majesty of light, Ganymede,
the Fourth Callisto . . . ."
"This fancy, and the particular names given, were suggested to me
by Kepler, Imperial Astronomer, when we met at Ratisbon fair in
October 1613. So if, as a jest, and in memory of our friendship
then begun, I hail him as joint father of these four stars, again
I shall not be doing wrong."
The Galilean Moons (Galileo Images)
Ganymede, Callisto, Io and Europa
Galileo originally called the Jupiter's moons the "Medicean
planets", after the Medici family and referred to the individual moons
numerically as
I, II, III and IV. Galileo's naming system would be used for a couple
of centuries. It wouldn't be until the mid-1800's
that the names of the Galilean moons, Io, Europa, Ganymede and Callisto,
would be officially adopted, and only after it became very apparent that
naming moons by number would be very confusing as new additional moons were being
discovered.
![rule.gif](https://webarchive.library.unt.edu/eot2008/20090514133414im_/http://www2.jpl.nasa.gov/galileo/rule.gif)
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