Tides, Waves, and Sea Level

Ocean tides are the periodic rise and fall of sea level produced chiefly by the gravitational pull of the moon, with a smaller but still significant contribution from the sun. The moon's gravity pulls the ocean on the side of Earth nearest to it into a bulge, while inertial effects produce a second, corresponding bulge on the opposite side of the planet; as Earth rotates beneath these two bulges, most coastlines experience two high tides and two low tides over roughly a 24-hour-and-50-minute period, known as a semidiurnal tidal cycle, though some regions experience only one high and one low tide daily, a diurnal pattern, or a mixed pattern combining both. Because the moon's gravitational influence on the tides is roughly twice that of the more distant sun, lunar tides dominate the overall pattern, but solar tides modulate their amplitude. When the sun, Earth, and moon align during the new and full moon phases, their gravitational effects reinforce one another, producing the unusually large spring tides; during the first and third quarter moons, the sun and moon pull at right angles, partially canceling each other and producing the smaller neap tides.