Continuing my blogs on the five top facts about Jupiter which were posted as a tweet during my appearance on BBC Radio 5’s morning show a few weeks ago, at number 4 in my list was –
It [Jupiter] has more than 60 moons, four of which were discovered by Galileo in 1610
The four Galilean moons were the first moons to be discovered orbiting another planet, back in January 1610 when Galileo first turned his newly-fashioned telescope to look at Jupiter. Initially he thought the four bright dots he could see near Jupiter were background stars, but as he observed Jupiter over a period of several weeks he saw that not only did these bright dots follow Jupiter as Jupiter moved against the background stars, but they appeared to “dance” around it. He realised quite quickly that he was seeing moons orbiting another planet.
The four Galilean moons are called (in order of distance from Jupiter) Io, Europa, Ganymede and Callisto. The usual pneumonic for remembering this is I Eat Green Carrots, or In Every Good Class 🙂 I will do a series of blogs about each of these moons, as each one is fascinating and have been studied in detail by the Galileo space probe in the 1990s. But, one thing I will mention here is that you can see this four moons with just a pair of binoculars or a low powered telescope, you do not need any highly sophisticated equipment. If you are trying to see them with binoculars then the trick is to steady your elbows on something like a wall or the roof of a car, and lean against something to reduce any wobbling.
Another thing I will mention in this blog is that Io only takes about 2 days to orbit Jupiter, and so in the matter of just a few hours you can see a change in its position. If you look at Jupiter at e.g. 8pm and then again at e.g. 2am, or even midnight, you will see that Io has moved. The best way to know which moons are where is to go online and do a search for “jupiter moons positions” or something similar, and you should be able to find a chart which shows which moons are where (east of west of Jupiter, or behind or in front of it) on which nights.
Jupiter has many other moons, and more are being discovered, but they are all tiny compared to the Galilean moons. The Galilean moons and Jupiter form what is, in many ways, a mini Solar System, and I will talk more about that when discuss Io and Europa in more detail in future blogs.