On January 7th 1610, Galileo Galilei made an observation through his small telescope that changed our understanding of the universe. Using his self made refractor telescope with an aperture just one inch in diameter with a magnification of 15X, Galileo pointed his telescope to the bright planet Jupiter. The planet Jupiter showed up in the narrow of 15minutes of arc field of the telescope, accompanied by three satellites, Io was observed on 8th January as his crude instrument was unable to separate Io and Europa the previous night.
There was no previous records of observations made of Jupiter through telescope and Galileo was in uncharted territory. Galileo first took these 4 objects as stars and continued his observations. The movement of Jupiter with respect to these “stars” made Galileo to think that maybe Jupiter has a retrograde loop. As he continued to track Jupiter, he observed that Jupiter did not move much with respect to other stars apart from the 4 “stars” that were close to Jupiter. One more puzzling observation was that these 4 “stars” always stayed close to the planet.
After several nights of observations Galileo came to the conclusion that these 4 objects were carried along by Jupiter and they are moons of Jupiter. This had a profound impact on our way of looking at the universe. This observation gave the Copernican theory more ground.
Galileo gave the moons of Jupiter roman numbers I,II,III and IV. The names suggested by Simon Marius is what we use now. Io (I), Europa (II), Ganymede (III) and Callisto (IV). The moons are called Galilean moons in honour of Galileo.
Here is a copy of Galileo notes translated
Credit: NASA
Centuries later we sent spacecraft to Jupiter and also took pictures of the Jovian moons. Here are close up photos of the moons and also few facts about them.
Io
Credit: NASA/JPL/University of Arizona
Io is the closest of the Galilean moons which orbits the planet at a distance of 421000km. Io is little bigger than our earth's moon. Io has a radius of 1821km, earth's moon has a radius of 1738km. Io goes around Jupiter in 1.7days. Io is the most volcanically active moon in our solar system. One of the reason for this high activity is that Io is affected by gravitation of Jupiter and high tidal forces make molten interior escape through the gaps created from the distortions caused by the tidal forces.
Europa
Credit: NASA/JPL/Ted Stryk
Europa is the second moon from Jupiter and is little smaller than our moon. The radius of Europa is around 1560km. It orbits the planet at a distance of 670000km and completes one rotation in 3.5days. The surface is covered with frozen salt water and scientists think that with the eccentric orbit of Europa there may be places where it may be possible for water to exist.
Ganymede
Credit: NASA/JPL/Ted Stryk
Ganymede is the largest moon in the solar system, larger than the planet Mercury. The radius of Ganymede is 2631km and that of Mercury is 2439km. Ganymede orbits the planet Jupiter 1070000km and completes on orbit every 7.15days. This is the only moon to be known to have magnetosphere and also its been thought the moon may have a salt water ocean that is trapped under the ice surface.
Callisto
Credit: NASA/JPL/Ted Stryk
Callisto is the last of Galilean moons and also the farthest from the planet Jupiter. Callisto orbits the planet at a distance of 1882000km and completes on rotation around the planet in 16.6days. The moon Callisto is also the third largest moon in the solar system with a radius of 2410km almost the size of Mercury. Callisto is among the most cratered objects in solar system. Scientists think that the core could have frozen as it formed and no geological activity has taken place from the past 4 billion years.
Jupiter is easy to spot these days. Just after sunset if we look you, Jupiter will be shining bright in the Zenith. No star in the region is as bright as Jupiter. The moons of Jupiter can easily be seen even with a pair of binoculars.
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