Focuses on the planets closest to the sun, often referred to as the inner planets. 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. Focuses on the features that make Earth unique in the solar system. Important terminology includes: sun, orbit, revolve, gravity, inertia, rotation, axis, atmosphere, and spacecraft. .