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If you are willing to disregard the Sun for a moment, most of our Solar System is Jupiter. Jupiter is considerably more massive than all the other planets, including its Jovian neighbors, put together. It also is very large with a radius over 11 times that of the Earth. All of the terrestrial planets huddle together close to the Sun, all within a radius of about one-and-a-half a.u. Jupiter is on the average 5.2 a.u. from the Sun.
The great distance between the Sun and Jupiter gives the giant planet a very different path through the sky from the terrestrial planets. Remember that the greater the distance of a planet from the Sun, the slower it revolves around the Sun. Because of this slow movement, Jupiter will usually remain within one constellation, or at most two, during any entire year on Earth. Unlike the terrestrial planets which you might have to check on the location several times a month, once you know where Jupiter is it will stay in the same area, if not the same spot for some time. Jupiter, for example, has spent most of 2001 in the constellation Gemini. You were able to see Jupiter as long as Gemini was above the horizon. Not that Jupiter was really "in" Gemini whose stars are many light years from the Earth. Jupiter was merely located in the same area of the sky as the stars of Gemini, and if star and Jupiter were at exactly the same spot of sky, Jupiter would block the star, not the other way around.
During Jupiter's synodic period of 399 days, it exhibits retrograde motion for 121 days. The retrograde motion of Jupiter is not as obvious as that of the terrestrial planets, and also not nearly as important to us. Jupiter is so distant from both the Sun and the Earth that its brightness is does not vary much throughout the year. Mars can vary from a bright star whose disk can easily be seen through binoculars to a very dim object depending on both the time of year and which year. Jupiter is much steadier and its brightness varies no more than by half; its brightest time is at opposition. Although not as bright as Venus, Jupiter is generally brighter than any star in the sky.
What type of planet is Jupiter? Despite its huge size, it is not very dense. This means that if you fill up a jar with Earth and fill up a jar the same size with Jupiter, the Earth jar will be over four times heavier than the Jupiter jar. Although that information is from a very reliable source, if anybody succeeds in carrying out the experiment, I certainly would like to hear about it. Jupiter is mostly made up of two gases, hydrogen and helium. There is very little of either of these gases on Earth. Jupiter's tremendous gravity can retain these light gases. Jupiter's atmosphere also contains methane and ammonia. I am emphasizing Jupiter's atmosphere because most of the planet is atmosphere and we are unable to see its surface. Below the hundreds of miles of thick atmosphere is a liquid core and probably in the very center is a small solid core.
Thanks to its size, no other planet has as much for us to see from Earth as Jupiter. Jupiter appears as a slightly flattened disk through binoculars. The flattening is a result of Jupiter’s gaseous composition and its rapid rotation; it rotates around its axis in less than 10 hours.
When looking at Jupiter through binoculars, you will se from one to four small dots in a straight line towards either side of the planet. These are Jupiter's four largest satellites. Galileo first saw them when he directed his primitive telescope towards Jupiter. His telescope was roughly as powerful as a typical modern pair of binoculars. All 15 satellites of Jupiter are known by a Roman numeral indicating their order of discover, and a name. The four satellites that can be seen through binoculars are I Io, II Europa, III Ganymede and IV Callisto. We have already come across the name of Europa as the object of Zeus' (Jupiter's) affection I the legend of Taurus the Bull. It turns out that almost all of Jupiter's satellites are named after Zeus' paramours. Fortunately Jupiter has quite a few satellites.
Most of Jupiter's satellites are only a few miles across. Only the four largest can be seen through binoculars. Actually, if it were not for the glare of Jupiter, some of its satellites would be visible to the naked eye. The two inner satellites of Jupiter (except for a small satellite named V Amalthea) are II Europa and I Io. They both have a radius of about 1,000 miles, which is similar to that of the Moon. Unlike the Moon, they revolve around their parent planet in only a few days. Their relative closeness to Jupiter makes them sometimes difficult to see. The next two satellites, III Ganymede and IV Callisto, both have a radius of 1,500 miles. They also orbit farther away from Jupiter and are much easier to see. III Ganymede takes over seven days to circle Jupiter, while the more distant IV Callisto revolves around Jupiter in a leisurely 17 days. By the way, some of Jupiter's smaller satellites are millions of miles away from the planet and take up to two years to complete an orbit. The rapid motion of the four large satellites creates a fast-changing picture of Jupiter. From night to night and even within a single night, there is a continuous shifting and rearranging of the satellites.
Everything described above is available to anyone with binoculars, while a telescope provides even more. Running parallel to Jupite's equator is a series of alternating dark and light bands that cover the entire planet. The brighter regions that may be white or pale yellow are called zones. The slightly brownish or reddish darker regions are known as belts. The zones and belts are not on the planet's surface, but its atmosphere; they are part of the weather of Jupiter. At times, the striped pattern is broken by temporary disturbances. The one permanent disturbance is the Great Red Spot. This is an oval shaped reddish or pinkish area just south of the equator. It may be a storm, which happens to be going for at least several centuries.
The four large satellites are easily seen through a telescope. Jupiter casts a shadow one can see its satellites enter and leave the area of the shadow. It is also possible to see the shadow cast by a satellite on to Jupiter. A viewer hovering on top of Jupiter's atmosphere (a viewer at the bottom of the atmosphere would be unable to see anything) under such a shadow would witness an eclipse.
Thanks to its huge size and fascinating satellite system, Jupiter is the planet created for skygazing. With just minimal equipment and Jupiter above the horizon, you have a ticket to one of the best shows in the Solar System.