The physical forces that cause the tides have not always been understood. Long before science attempted to explain them, most people believed either that the gods were responsible or that Earth itself was "breathing" and causing ocean waters to rise and fall in regular intervals. Many early scientists -- the most prominent being Galileo -- tried and failed to find a clear and accurate explanation for tides.
We now know that tides are caused by the Moon's gravitational pull on Earth, which causes ocean waters to bulge on the side of Earth closest to the Moon and on the opposite side and gives rise to two high and two low tides per day in most coastal regions around the globe. As this article from the NOVA Web site explains, however, astronomical influences create a variety of tidal curiosities.
