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The Solar System consists for the most part of the Sun and nine planets revolving around it. The vast majority of the Matter of the Solar System is concentrated in the Sun. This is so much so that a skygazer looking towards us from the nearby star of alpha Centauri would see only a solitary star – our Sun. This is the place in the book where I'm supposed to mention how ordinary and unremarkable our Sun is compared to the other 300,000,000,00 stars in the galaxy. While it is true that the Sun's characteristics are midway between those of the very largest and very smallest stars, among the 32 stars closest to us, there are only three stars inherently brighter than the Sun. So even if the Sun can not be classified as a giant, it probably is an above-average star. If you happen to find yourself a cocktail party with denizens of alpha Centauri, you will have noting to be embarrassed about.
Of the nine planets revolving around the Sun, the Earth is the third one from the center. Starting from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter and Saturn. All these can be seen with the naked eye, while the following have been discovered only within the last few centuries with the aid of telescopes: Uranus, Neptune and finally Pluto. In between Mars and Jupiter are about 100,000 minor planets called asteroids. The Solar System is shaped like a very thin old-fashioned phonograph, in that all the planets and the Sun are locate in the same plane. This of course is the basis of ecliptic. When you look at a record from its edge it appears as a straight line. If you looked at a record from within the record (not an easy though, I admit), it will appear as a continuous straight line that joins up with itself.
Distances between tars are measured by light years. Compared to interstellar distances, the planets are practically in the Sun's lap. It takes a mere nine hours for the Sun’s rays to travel to the farthest planet, Pluto. Still, the numbers are impressive enough to create a need for a convenient measure of distance. The average distance between the Earth and the Sun is defined as one a.u. astronomical unit. One a.u. is equal to about 93,000,000 miles. It is common to show a diagram of all the planets together to give an indication of their comparative sizes. Generally, there is a separate diagram to show the comparative distances from the Sun. These two charts give the mistaken impression that the Solar System is quite a crowded place. You would need to plot both the sizes of the planets and their distances from the Sun on the same scale to provide an accurate picture of the roominess of the Solar System. Instead of drawing this chart out, which not only would expand this book to encyclopedic size, but also make it very boring, let us imagine shrinking the Earth down to the size of a pea. At the same time, let the rest of the Solar System be proportionally reduced in size.
The Sun will be the size of a beach ball. The first planet Mercury be 200 feet from the Sun and will be the size of a pinhead. Then, 300 feet away from the Sun will be Venus, represented by a pea only slight small than that representing the Earth. In this scheme of things, the Earth is 500 feet from the Sun. About 20 inches from our pea revolves the Moon. Mars is the size of a grape pit and is 700 feet away from the Sun. Skipping over the asteroid, at a distance of over 2,000 feet from the Sun we come to Jupiter, the largest planet. A tennis ball represents Jupiter, while Saturn, which is over 4,000 feet away from the center, is only slightly smaller than a baseball. The next planet Uranus is over one and a half miles from the Sun and is the size of a ping pong ball. Neptune also is a ping pong ball, but one that is over two and a half miles from the Sun. Finally, Pluto is over three miles away from the Sun and the size of a sesame seed.
The general idea is that we have within a radius of over three miles a space filled by only an occasional golf ball and a few widely separated peas. Most of the Solar System is empty space. By the way, on this scale, the nearest star is 25,000 miles away.