1. 22.2 The Earth-Moon-Sun System

2. Rotation The spinning of a body, such as Earth, about the axis.

3. Revolution The motion of one body about another, as Earth about the sun.

4. Precession A slow motion of Earth’s axis that traces out a cone over a period of 26,000 years.

5. Perihelion The point in the orbit of a planet where it is closest to the sun.

6. Aphelion The place in the orbit of a planet where the planet is farthest from the sun.

7. Perigee The point at which the moon is closest to Earth.

8. Apogee The point where the moon is farthest from Earth.

9. Phases of the Moon The progression of changes in the moon’s appearance during the month.

10. Solar Eclipse A eclipse of the sun; this occurs when the moon moves in a line directly between Earth and the sun, casting a shadow on Earth.

11. Lunar Eclipse An eclipse of the moon; occurs when the moon is in Earth’s shadow.

12. Key Concept The two main motions of Earth are rotation and revolution.

13. Key Concept Lunar phases are a result of the motion of the moon and the sunlight that is reflected from its surface.

14. Key Concept During a new-moon or full-moon phase, the moon’s orbit must cross the plane of the ecliptic for an eclipse to take place.