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.
Galileo published his observations in Sidereus Nuncius in March 1610:
“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.”
We now know that Jupiter has 95 moons
that have been officially recognized by the International Astronomical Union.
But the number doesn’t capture the complexity of the Jovian system of moons,
rings and asteroids. The giant planet has thousands of small objects in its
orbit. Scientists are getting so good at spotting tiny moons orbiting distant,
giant planets that the International Astronomical Union has decided the
smallest will no longer be given mythological names unless they are of
“significant” scientific interest.
Info via JPL NASA
Know more about Jupiter and her moons
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