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What actually causes tides? The force of gravity pulls inward toward Earth's center of mass and holds water on our planet's surface. However, gravitational forces of the moon and Sun also "tug" upon our ocean waters. Both these "extraterrestrial" ("outside-Earth") bodies draw ocean waters to positions on the earth's surface directly beneath them, as seen in the image above.
From Virtual Vacationland, http://www.bigelow.org