"The squares of the times of revolution of the planets round the Sun (the periodic times) are proportional to the cubes of their mean distances from the Sun."
This is enough to alarm the boldest reader. And yet, if we unravel this somewhat incomprehensible phrase, we are struck with its simplicity.
What is a square? We all know this much; it is taught to children of ten years old. But lest it has slipped your memory: a square is simply a number multiplied by itself.
Thus: 2 × 2 = 4; 4 is the square of 2.
Four times 4 is 16; 16 is the square of 4.
And so on, indefinitely.
Now, what is a cube? It is no more difficult. It is a number multiplied twice by itself.
For instance: 2 multiplied by 2 and again by 2 equals 8. So 8 is the cube of 2. 3 × 3 × 3 = 27; 27 is the cube of 3, and so on.
Now let us take an example that will show the simplicity and precision of the formula enunciated above. Let us choose a planet, no matter which. Say, Jupiter, the giant of the worlds. He is the Lord of our planetary group. This colossal star is five times (precisely, 5.2) as far from us as the Sun.