The planets closest to the sun, often referred to as the inner planets, are the focus of this program. Colorful animations, along with images captured by space probes, illustrate the characteristics of Mercury, Venus, Earth, and Mars. The fundamental structure of the solar system and the process by which planets revolve around the sun are discussed. Special attention is given to the features that make Earth unique in the solar system. Important terminology includes: sun, orbit, revolve, gravity, inertia, rotation, axis, atmosphere, and spacecraft.