Tides are caused by the Moon's gravitational pull on the Earth and its oceans. Because the force gravity of an object is stronger the closer you are to it, the Moon pulls more strongly on the water on the side of the Earth closer to the Moon than it does on the water on the far side (pink arrows). (The Sun's gravity also creates tidal bulges on the Earth, but its effect is much smaller than the Moon's. However, when the Moon and Sun are lined up, such as at New and Full Moon, their combined effect creates tides that are more extreme . . . what technically are called Spring Tides.)

This difference in force draws the water into two tidal bulges. One bulge is on the side of the Earth nearest the Moon while the other bulge is on the far side of the Earth.

In the sketch of the Earth and the Moon, we have ignored the Earth's rotation. The Moon's gravity holds the tidal bulges in approximately a fixed position. But the Earth rotates "under" the bulges and each day carries a person through the bulges, as the animation below shows. Click for movie