They move around the sun in almost circular paths, rather like circus horses trotting or galloping round the ring-master ... the solar system has only one-way traffic - like Piccadilly Circus. The traffic nearest the centre moves fastest; that further out more slowly, while that at the extreme edge merely crawls....It is true that even the furthest and slowest of the planets ts covers nearly three miles every second, which is about 200 times the speed of an express train, but this is a mere crawl in astronomy....
Before we leave Piccadilly Circus, it should be understood that we cannot represent the solar system by putting up a statue of Eros in the middle to represent the sun, and letting nine taxicabs gyrate round it to represent the nine planets. The statue is far too big to represent the sun and the taxicabs are enormously too big to represent planets. If we want to make a model to scale, we must take a very tiny object such as a pea, to represent the sun. On the same scale the nine planets will be small seeds, grains of sand and specks of dust. Even so, Piccadilly Circus is only just big enough to contain the orbit of Pluto, the outermost planet of all. Think of a pea and nine tiny seeds, grains of sand and specks of dust in Piccadilly Circus, and we see that the solar system consists mainly of empty space. It is easy to understand why the planets look such tiny objects in the sky.
Yet the solar system is crowded compared with most of space. If a pea and nine smaller objects in Piccadilly Circus represent the sun and planets, the nearest of the stars will be represented by a small seed somewhere near Birmingham-all in between is empty space.
Thursday, 3 July 2008
Before the sort of YouTube illustrations linked to in my last post, people had to resort to rather more homely ways to illustrate the vastness of the Universe. But some of them were still highly effective - I still love this, from Sir James Jeans' book The Stars in Their Courses (1931) - he's talking about the planets in our solar system: