Jupiter is the largest planet and the giant of our Solar System.
It was a very important part of Galileo’s early telescope work. His discovery of four moons of Jupiter, led directly to massive progress in the accepted understanding of the structure of the solar system
Jupiter’s diameter is eleven times that of the Earth, meaning that its volume is over thirteen hundred times as great. But, it is only 300 times as heavy.
So Jupiter’s average density is only one quarter that of the Earth. It would nearly float in water, if you had a large enough bath!
How can we explain this?
The answer is that Jupiter is made up of gas, not rock like the Earth. It is called a “Gas Giant”, together with Saturn, Uranus and Neptune.
Jupiter moves in an orbit that averages 483 million miles from the Sun.
Jupiter takes 12 earth year to complete one orbit. This is excellent news for astronomers because it means that it reaches opposition (its closest point to us) every single Earth year. And, it is excellently placed in our sky for observation, for several months of each of our years.
If you observe Jupiter through a telescope, it is fairly obvious that it is flattened at the north and south poles. This is because it spins very quickly on its axis. Each rotation, and therefore each Jupiter day, takes only 10 hours, with the equator moving at 28,000 miles an hour.
This causes the gaseous material to bulge at the equator and the equator actually rotates faster than other areas.
There are many highly interesting features of the surface, that you can see through a modest telescope.
The surface has a characterisctic pattern of “belts” that adjust and swirl. Sometimes wider, sometimes narrower…
But the most famous feature is the “Great Red Spot”. This has been observed for more than 300 years and there has been much debate about what causes it. For some years, it has even disappeared, but then come back.
Various space missions to Jupiter have proved that it is a massive, persistent storm in Jupiter’s atmosphere system. But at some time, it may just disappear for good.
There have been many space missions to Jupiter. Notably successful were the two Voyagers of 1979 and the most aptly named, Galileo probe of 2003.
These probes captured wonderful images, especially Galileo, and not least of the four main moons that had been so important to Galileo’s work four hundred years before.
These four main moons are all very different.
With the exception of Europa, they are bigger than the Earth’s Moon.
The main Jupiter moons are so large and so bright, that any telescope and even decent binoculars, will show them fantastically.
Everytime I see the four Jovian satellites, I think of Galileo four hundred years ago. He must have been so excited, but also so fearful, about what he had just discovered.
From innermost orbit to outermost, the four main moons of Jupiter are…
Io – a sulphurous, actively volcanic world. It has very hot volcanic plumes, shooting out of this otherwise cold world.
Ganymede – a rocky, icy, pock-marked and ridged body, that is the largest moon in our solar system. It is actually bigger than the planet Mercury.
Europa – an icy but smooth, cracked surfaced world
Callisto – a heavily cratered place. Goodness knows what happened here, in years gone by!
