Every day, the world's oceans rise and fall in vast cycles. Beaches that are dry at noon are underwater by evening. Boats float at low tide one moment and sit on mud the next. These tides aren't random — they're caused by the gravitational pull of the Moon and Sun on Earth's oceans, modified by Earth's rotation and the shape of ocean basins.
The Short Answer
Tides are caused primarily by the Moon's gravitational pull on Earth's oceans, with the Sun contributing about half as much force. The Moon's gravity creates two tidal "bulges" — one on the side of Earth facing the Moon, and one on the opposite side. As Earth rotates beneath these bulges, each point on the surface experiences two high tides and two low tides every roughly 24 hours and 50 minutes.
The Moon's Role
The Moon is the dominant tide-causing force despite being much smaller than the Sun. This seems counterintuitive — why does the smaller body dominate? The answer lies in how gravity works over distance. Tidal forces aren't about how strong gravity is at a point; they're about the difference in gravity from one side of Earth to the other. This difference grows with proximity. The Moon is much closer than the Sun (384,000 km vs 150 million km), so its gravity varies more dramatically across Earth's diameter.
The Moon's gravity pulls the ocean toward itself on the side of Earth facing the Moon, creating a bulge of higher water. Less obviously, the ocean on the far side of Earth bulges outward too — because the Moon's gravity pulls Earth itself (which is between the two oceans) toward the Moon more than it pulls the far-side ocean. The result: two bulges, on opposite sides of Earth.
The Sun's Role
The Sun also creates tides, but smaller ones. Its gravitational pull on Earth is about 178 times stronger than the Moon's, but its tidal force (the difference in pull across Earth's diameter) is only about 46% of the Moon's. So solar tides are real but weaker.
When the Sun, Moon, and Earth are aligned (full Moon and new Moon), the solar and lunar tides reinforce each other, producing the largest tides — called "spring tides" (the name has nothing to do with the season). When the Sun and Moon are at right angles relative to Earth (first and third quarter Moon), their effects partially cancel, producing smaller tides called "neap tides."
Why Two Tides Per Day
Because of the two tidal bulges (toward and away from the Moon), each point on Earth's surface passes through both bulges as Earth rotates. This creates two high tides and two low tides each day. The cycle isn't exactly 24 hours, though — it's 24 hours and 50 minutes, because the Moon itself moves about 12° around Earth each day in its 27.3-day orbit. So the high tide comes about 50 minutes later each day.
The Theoretical vs Real Tides
If Earth were a perfect sphere covered uniformly with deep ocean, tides would behave simply — water would flow toward the bulge, and we'd see clean 12.4-hour cycles everywhere. Reality is far more complex because:
- Continents block water flow. Land masses force water to flow around obstacles.
- Ocean basins have shapes. Each basin has its own resonant frequency, like a bathtub sloshing.
- Coastal bathymetry varies. Shallow shelves amplify tides; deep water reduces them.
- The Coriolis effect deflects flow. Earth's rotation makes water flow in curved paths.
- Friction with seafloor delays response. Tides don't arrive exactly when expected.
The result: some places have huge tides (the Bay of Fundy in Canada has up to 16 m tidal range), some have almost none (parts of the Mediterranean have 10–20 cm). Some places have semidiurnal tides (two equal highs and two equal lows per day); others have diurnal tides (one high and one low per day); some have mixed patterns.
Famous Tidal Ranges
The world's most extreme tides:
- Bay of Fundy (Canada) — 16 m range, the world's highest
- Ungava Bay (Canada) — 15.6 m range
- Bristol Channel (UK) — 14 m range
- Cook Inlet (Alaska) — 12 m range
- Río Gallegos (Argentina) — 13 m range
The Mediterranean and Baltic Seas, conversely, have minimal tides — typically under 30 cm. The Gulf of Mexico has small mixed tides.
Tidal Bores
In some places, the rising tide travels upstream into a narrowing river as a wave — a tidal bore. The most famous tidal bores:
- Qiantang River (China) — up to 9 m bore, world's largest
- Severn Bore (UK) — popular for surfing
- Petitcodiac River (Canada) — Bay of Fundy area
- Amazon Pororoca (Brazil) — surfable bore traveling up the Amazon
Tidal bores typically occur in places with very high tides and funneled river mouths. The Qiantang River bore has been documented for over 2,000 years and is a major tourist attraction.
Tidal Energy
Tides represent a massive renewable energy source. Tidal power plants extract energy from rising and falling water. Major examples:
- Sihwa Lake (South Korea) — 254 MW, world's largest
- La Rance (France) — 240 MW, the world's first major tidal power plant (1966)
- Annapolis Royal (Canada) — 20 MW, Bay of Fundy
- MeyGen (Scotland) — tidal turbine farm in the Pentland Firth
Tidal energy is predictable (tides are easy to forecast far into the future), but expensive to develop and constrained to a few good sites. Most renewable energy growth is in solar and wind rather than tidal.
Earth's Tides Slow Down the Moon
Tidal interactions also affect the Earth-Moon system. The friction of tidal bulges sweeping across Earth slowly transfers angular momentum to the Moon, causing the Moon to spiral away from Earth at about 3.8 cm per year. This same process slows Earth's rotation slightly — about 2 milliseconds per century. Billions of years from now, the Moon will be much further away and Earth's rotation will be much slower.
Other Bodies in the Solar System
Tides exist on many worlds, not just Earth:
- Jupiter's moon Io has the most extreme tides — Jupiter's gravity creates 100-meter tidal flexing of Io's rocky surface, generating heat that drives extensive volcanism.
- Europa has subsurface ocean tides created by Jupiter's gravity, possibly creating habitable conditions.
- Mars has very weak tides because Phobos and Deimos are tiny.
- The Sun has tides from the planets, especially Jupiter — these affect solar surface phenomena.
Tides and Marine Life
Tides shape much of marine and coastal ecology. Intertidal zones — the areas between high and low tide marks — host some of the most diverse and productive ecosystems on Earth. Tide pools contain specialized organisms that have evolved to handle dramatic conditions: exposure to air at low tide, then submersion at high tide; freshwater dilution from rain followed by salt-water inundation; intense sunlight and rapid temperature changes. Mussels, barnacles, starfish, anemones, and many other species depend on tidal cycles. Even on a global scale, tides drive ocean mixing that brings nutrients to surface waters, supporting marine food webs.
Tide Tables and Predictions
Tide predictions have been accurate for centuries. Modern tide tables can predict high and low tide times years in advance to within minutes. Major tools:
- NOAA Tides & Currents (US government)
- UK Hydrographic Office
- WillyWeather and similar apps
- Tide-prediction stations have been operating since the 19th century
Predictions account for the Moon's position, Sun's position, local basin geometry, and historical observations.
Key Facts
- Tides are caused mainly by the Moon's gravity, with the Sun contributing about half as much.
- Each location on Earth typically experiences two high tides and two low tides per day.
- The Bay of Fundy has the world's highest tides at up to 16 m.
- Spring tides (largest) happen during full and new Moon; neap tides (smallest) during quarter Moons.
- Earth's tides are slowly slowing the planet's rotation and pushing the Moon away.
Fun Facts
- The atoll's name comes from Portuguese or Spanish explorers; etymology is debated.
- Diego Garcia's lagoon is one of the largest natural sheltered harbors in the Indian Ocean.
- The surrounding Chagos waters are among the cleanest tropical seas in the world.
- The Chagossian displacement is sometimes called "the last forced exile" of a democratic government.
- The atoll is closer to India than to Africa, despite being administered with Saint Helena.
The Bottom Line
Diego Garcia is a small atoll with outsized geopolitical importance. As the host of a major US-UK military base in the heart of the Indian Ocean, it has been central to American power projection for half a century. The forced removal of its indigenous Chagossian population is one of the most controversial colonial-style displacements of the modern era, and the recent UK sovereignty transfer to Mauritius represents a partial resolution of decades of legal and ethical disputes.