The Moons of Jupiter – Then & Now



The View from Galileo’s Telescope

On January 7, 1610 Galileo first viewed Jupiter through his telescope.  What caught his eye was not the planet itself, but three bright stars that were arranged in a perfect line on either side of the planet.  Galileo sketched Jupiter and the three stars, thinking at first they were simply a chance alignment. Some of his original sketches are below.




Above: Galileo’s sketches of the moons of Jupiter made on the nights of February 3 and 4, 1610. (Ori means Orient, or East, Occ Occident, or West).  Image Credit: Octavo Corp./Warnock Library)



The next night Galileo decided to check in on Jupiter again.  There were the three stars, but now in different positions relative to the planet itself.  The next night was cloudy, but on the 10th, he saw a fourth star in the same line.  As he continued to study Jupiter on successive nights, Galileo came to the realization that these four points of light were not background stars, but mini planets orbiting around Jupiter.  The existence of four new worlds was amazing in itself, but the discovery would help bring about a revolution in astronomy and our understanding of the cosmos. 


Was the Earth or the Sun the center of the Universe?  The debate was raging in the first decade of the 17th century.  Does the Sun move around a stationary Earth, or does the Earth, as simply another planet, move around a stationary Sun?  Neither camp had any hard evidence to back its case, but the Earth-centered supporters had a strong argument.  If the Earth moved, wouldn’t it leave the Moon behind? (Remember, this is before we had an understanding of gravity).  Galileo, a supporter of the Sun-centered universe, could now counter that argument:  If Jupiter can move and take its moons with it, then surely the Earth can carry its Moon through space as well.  Although not proof that the Earth moved, it was one significant piece of evidence that would help pave the way for acceptance of a heliocentric (Sun-centered) universe.




Above: MicroObservatory image of Jupiter’s moons.  Notice that the planet itself is over-exposed to show the fainter moons.


What did YOU See with MicroObservatory?

How do the Moons of Jupiter appear in your images taken by MicroObservatory? To see the images in more detail, you may want to open your image in our MicroObservatory image processing software.  


In the archive image above, the planet itself looks smeared out because it is overexposed to show the fainter moons, Can you see the positions of the moons change with respect to Jupiter?  The inner moons move in their orbits faster, so they will show more of a change.  If you requested more than one image of Jupiter’s Moons, you can combine the images into an animation to see how the moons move.


The Moons of Jupiter - 400 years later

The moons of Jupiter are seen today as unique worlds in their own right. The Jovian system, comprising the four Galilean and (at last count) 23 smaller moons, has been visited by seven space probes since1973.  The most significant mission, fittingly named Galileo, studied Jupiter and its moons for eight years between 1995 and 2003. What do we know about the moons of Jupiter now?



Io the innermost Galilean moon, is perhaps the most amazing moon in the solar system. Looking like a messy pizza, its orange-yellow surface is covered in sulfur-belching volcanoes and lava lakes.  Io’s geology is so active because the poor little moon is stretched and squeezed by the gravity of Jupiter and the other outer Galilean moons, heating and melting its surface. (Image credit: NASA)










Callisto is the second largest moon of Jupiter and is about the same size as the planet Mercury. Whereas Io has the youngest surface in the Solar System, Callisto has the oldest. Its crust dates back 4 billion years, just shortly after the solar system was formed.  Its ancient history is evident by having the most cratered surface of any moon in the solar system. (Image credit: NASA)











Ganymede is the largest moon of Jupiter.  If it orbited the Sun instead of Jupiter it would be classified as a planet. Like Callisto, Ganymede is most likely composed of a rocky core with crust of rock and ice. Ganymede has had a complex geological history. It has mountains, valleys, craters and lava flows. Ganymede’s dark crust is mottled with bright spots where recent meteorite impacts have exposed clean bright ice from below the surface. (Image credit: NASA)









Europa is among the brightest moons of the solar system, a consequence of sunlight reflecting off a relatively young icy crust. Its surface is also among the smoothest, lacking the heavily cratered appearance characteristic of Callisto and Ganymede. Lines and cracks wrap the exterior suggest a solid layer of ice above a liquid water ocean. Liquid water is found on only one other body in the solar system: Earth. (Image credit: NASA)











Find Out More


2009 is International Year of Astronomy, chosen to commemorate the 400th anniversary of Galileo’s discoveries with the telescope.  Find out what else is happening by visiting these web sites:


The World site for International Year of Astronomy.


The United States national site for International Year of Astronomy.


NASA’s International Year of Astronomy website.


Learn more about the Galileo Mission, at:


For more information on the planets of the Solar System, go to:


The space probes that have visited Jupiter are Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 and Voyager 2 (1979); Galileo (1995) and New Horizons (2007).  To read about all NASA’s missions, go to


Take a look at the full list of objects in MicroObservatory’s Galileo activity, and see how our understanding has evolved over the last four centuries.