Tides are created by the gravitational forces of the Moon and the Sun on the Earth's oceans. The gravitational force causes the ocean to bulge, creating high tides where it bulges and low tides where it does not. The Moon's gravitational force has a greater effect on tides than the Sun's, and the alignment of the Sun, Moon, and Earth affects the size of the tides. Full and new moons cause higher high tides and lower low tides (spring tides), while quarter moons cause smaller differences between high and low tides (neap tides).