The interaction between the gravitational forces of Earth, the Moon, and the Sun causes tides. The rhythmic movement of the water associated with the tides causes successive high and low waters in the coastal areas.

This Reading elaborates on the concepts presented in the Activities: Water: The Sum of Its Parts; The Ocean; The Tides: A Balance of Forces; Waves; and The Ocean: A Global View from Project Earth Science: Physical Oceanography, Revised 2nd Edition. Although the Reading was written especially for this volume with the teacher in mind, students should also be encouraged to read them for both interest and additional study.