Why does the Moon affect tides?

The Moon exerts a gravitational force on Earth that decreases with distance. The ocean on the side facing the Moon is pulled more strongly, creating a bulge (high tide). On the opposite side, the Earth itself is pulled away from the water, creating a second bulge. As Earth rotates, coastlines pass through these bulges, producing two high tides and two low tides every ~24 hours 50 minutes (a lunar day). The Sun also influences tides — during full and new moons, Sun-Moon-Earth alignment produces extra-large "spring tides." The average tidal range is about 54 cm in the open ocean, but in the Bay of Fundy (Canada), tidal ranges can exceed 16 meters.