The solar system is one of the most wonderful examples of this beautiful harmony to be witnessed. There are nine planets with fifty-four known satellites and an unknown number of smaller bodies. The major planets counting outward from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Earth is the only one on which life is known to exist. It is surely the only one on which human beings can live and survive unaided thanks to abundant land and water and to a breathable atmosphere. 

In the structure of the solar system, we encounter another beautiful example of equilibrium: the balance between a planet's centrifugal force countered by the gravitational attraction of its primary. (In astronomy, a primary is something that another body revolves about. Earth's primary is the sun; the moon's primary is Earth.) Without this balance, everything in the solar system would fly off into the chilling depths of outer space. The balance between these two forces results in paths (orbits) that the planets and other bodies follow around their primaries. If a body moved at too slow a speed, it would plunge into the primary; if it moved at too fast a speed, the primary would be unable to hold onto it, and it would fly off into space. Instead, every body moves at just the right speed to keep it in orbit. Moreover, this equilibrium has to be different for each body because the distance of planets to the sun differs. So do their masses. Therefore, they have to have different orbital speeds not to plunge into the sun or not to fly off into space.