Isn't it strange that of all the many pieces of our Solar System, it is the smallest ones – comets, meteors, and an eclipse of the Moon, that attracted the attention of civilizations right up to Shakespeare's time 400 years ago? While the great worlds orbit the Sun, their motions are predictable and are not a cause for alarm. But a meteor shower did attract notice, along with an increasing interest in the workings of our Solar System. Our solar neighborhood is a big place, with several different types of object in its domain, ranging in size from one star, the Sun, to tiny micrometeoroids that are even smaller than the meteors this book is about. These objects include the Sun, planets, moons, comets, asteroids, meteoroids, and micrometeoroids. It seems a simple division, but recent debates tell us that the boundaries among these types of objects are fuzzy at best. See Figure 4.1.

Categorizing objects in our Solar System

The best known debate concerns the definition of a planet. In 2006 the International Astronomical Union defined a planet in a most controversial way.