What causes the ocean tides?

Tides are the slow, regular rise and fall of sea level, driven mainly by the Moon's gravity and, to a lesser degree, the Sun's. The Moon pulls hardest on the ocean nearest it and least on the ocean farthest away, while Earth's center feels something in between. This uneven pull stretches the ocean into two bulges, one facing the Moon and one on the opposite side. As Earth rotates beneath these bulges, each coastline passes through them, producing in most places two high tides and two low tides roughly every 24 hours and 50 minutes. That interval, the lunar day, runs slightly longer than a solar day because the Moon keeps moving along its own orbit while Earth spins.

The key idea is that tides come from a difference in gravity, not its absolute strength. Gravity weakens with distance, so the Moon tugs the near side of Earth more firmly than the far side. Subtract the pull felt at Earth's center, and what remains, the tidal force, points toward the Moon on the near side and away from it on the far side, raising both bulges at once. This differential nature explains a genuine surprise: the Sun's gravitational pull on Earth is far stronger than the Moon's, yet the Moon dominates the tides. Tidal force falls off with the cube of distance rather than the square, so the Moon's closeness beats the Sun's greater mass. The Sun's own tide-raising force is still about 46 percent as strong as the Moon's.

When the Sun and Moon line up, at new and full Moon, their pulls combine into large spring tides; when they sit at right angles near the quarter Moons, they partly cancel into gentler neap tides. But real coastlines rarely match the tidy two-bulge picture. Ocean basins have their own shapes, depths, and natural rhythms, and Earth's rotation swirls the water around fixed points, called amphidromic points, where tide height barely changes. The result is enormous variety: the Bay of Fundy in Canada swings more than 15 meters between high and low, while parts of the Mediterranean barely move. Some coasts get two tides a day, others just one. People use these predictable patterns for navigation, fishing, coastal safety, and tidal power.

Myth: the Moon lifts only one bulge, on the side facing it. Reality: there are two, because tides arise from the difference in the Moon's pull across Earth, so the far side bulges as well. Myth: because the Sun is so massive, it should control the tides. Reality: tidal force depends on distance cubed, so the nearer Moon wins, though the Sun's tidal pull is still nearly half the Moon's. Myth: high tide always comes when the Moon is directly overhead. Reality: basin shape, friction, and resonance can delay high tide by hours, and some places see only one tide each day. Myth: Earth's spin flings the water outward to make tides. Reality: rotation sets the timing of tides, not their underlying cause.