An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Either of the two swells of land or water (on Earth) created by the pull of another object (Moon) orbiting around it. The → gravitational attraction between the → Earth and the → Moon is strongest on the side of the Earth that happens to be facing the Moon. This attraction causes the water on this "near side" of Earth to be pulled toward the Moon. As gravitational force acts to draw the water closer to the Moon, → inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the Moon, causing a "bulge" of water on the near side toward the Moon. On the opposite side of the Earth, or the "far side," the gravitational attraction of the Moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge. In this way the combination of gravity and inertia creates two bulges of water (Ross, D.A. 1995. Introduction to Oceanography. New York, NY: Harper Collins. pp. 236-242).

→ tidal; → bulge.