Figure 20.4Tides are caused by the gravitational attraction of Earth, the Moon, and the Sun. (a) The Moon’s gravitational pull forms two bulges of ocean water, one on the side of Earth nearest the Moon and the other on the side farthest from the Moon. As Earth rotates, these bulges remain aligned with the Moon and pass over Earth’s surface, creating the high tides. (b) At the new and full Moon, Sun and Moon tides reinforce each other, causing the highest (spring) tides. At the first- and third-quarter Moon, Sun and Moon tides are in opposition, causing the lowest (neap) tides.