Continuing with my series of the five top facts about Jupiter which BBC Radio 5 posted around the time of my interview on the morning show a few weeks ago, the third fact that I listed was
it [Jupiter] takes only 10 hours to rotate but 12 years to orbit the Sun
As I mentioned in my blog on the 2nd fact last week, when we look at Jupiter we are seeing the tops of the clouds. So, when we talk about its rotation, we are talking about how long it takes for e.g. the red spot to go around once and come back to the same place as we look at Jupiter. The rotation rate of Jupiter is about 10 hours, much quicker than the Earth and the quickest of all of the planets. However, the picture is not quite as simple as this, because it rotates differentially, the gases at the equator go around faster than the gases near the poles. There is not much difference in the rotation periods, only about 5 minutes, but there is a difference.
Also, because of this rapid rotation, Jupiter exhibits what we call an equatorial bulge which is basically the flattening out of a sphere due to rotation, so its diameter at the equator is more than its diameter at the poles, and it is flattened out by its rotation away from being a perfect sphere.
Jupiter is about five times further away from the Sun than the Earth is (compare this to Mars, which is about 1.5 times further away than the Earth). As it is further away the force of gravity from the Sun is weaker, and so it moves more slowly in its orbit than the Earth does. Added to this, it has further to go (its orbit is longer, as it is further from the Sun), so these two things combined lead to Jupiter taking much longer to go around the Sun than the Earth does. It doesn’t take five times longer, but rather about twelve times longer.
As I mentioned in my blog about the position of Jupiter and the Moon in early February, Jupiter appears to wander through the background stars from week to week and month to month. As it takes about 12 years to go once around the Sun, in one year it will move about one twelfth of the way around the sky, and as there are twelve constellations in the zodiac, it moves from one constellation to the next in a 1-year period.
At the moment Jupiter is in Cancer moving into Leo. By this time next year it will be in Leo and moving into the next constellation to the East of Leo, which is Virgo. In 6-years’ time it will have moved halfway around the 12 zodiac constellations, and so will be visible in the summer months and not the winter months as it is presently. The series of diagrams below show Jupiter’s position at 8:24pm on the 5th of March 2015, 2016 and 2017 as seen from London. As you can see, the stars stay in the same place at the same time each year, but Jupiter has moved eastwards, roughly one constellation further East in each twelve month period.