What Causes Tides?
Tides are one of the most reliable phenomena on Earth — twice a day, coastlines around the world experience a dramatic rise and fall of sea level. But what exactly drives this cycle? The answer lies in gravity, specifically the gravitational interplay between the Earth, the Moon, and the Sun.
The Moon's Dominant Role
Despite being much smaller than the Sun, the Moon has a far greater influence on Earth's tides because it is so much closer. The Moon's gravity pulls at the ocean water on the side of Earth nearest to it, creating a tidal bulge — a dome of water that follows the Moon as Earth rotates.
Simultaneously, a second tidal bulge forms on the opposite side of Earth. This may seem counterintuitive, but it occurs because the centrifugal effect of the Earth-Moon system pushes water outward on the far side. The result is two high tides and two low tides per day — a pattern called semidiurnal tides.
Spring Tides vs. Neap Tides
The Sun also exerts a gravitational pull on the oceans, though it's only about 46% as strong as the Moon's tidal force. When the Sun and Moon align — during a new moon or full moon — their forces combine to produce spring tides: exceptionally high high tides and unusually low low tides.
When the Sun and Moon are at right angles to each other (during a first or third quarter moon), their forces partially cancel out, creating neap tides — tides with a much smaller range between high and low.
| Tide Type | Moon Phase | Tidal Range |
|---|---|---|
| Spring Tide | New Moon / Full Moon | Greatest (highest highs, lowest lows) |
| Neap Tide | First / Third Quarter | Smallest (moderate highs and lows) |
Why Tides Vary Around the World
Not every coastline experiences the same tidal pattern. Geography plays a huge role:
- Diurnal tides — only one high and one low per day (e.g., Gulf of Mexico)
- Semidiurnal tides — two roughly equal highs and lows per day (e.g., UK coasts)
- Mixed semidiurnal tides — two highs and lows of unequal height per day (e.g., US West Coast)
The shape of ocean basins, the depth of the water, and the resonance of bays and inlets all amplify or dampen the tidal signal. The Bay of Fundy in Canada, for example, sees tidal ranges exceeding 16 metres — the largest in the world — due to its funnel shape and natural resonance frequency.
The Tidal Cycle and the Lunar Day
A common misconception is that tides follow a 24-hour clock. In fact, they follow a lunar day of approximately 24 hours and 50 minutes, because the Moon moves eastward in its orbit while Earth rotates. This is why high tides occur roughly 50 minutes later each day — a fact that's essential for anyone reading a tide table.
Why Understanding Tidal Science Matters
Tidal science is not just academic. It has real-world applications for:
- Coastal engineers designing sea defences and harbours
- Navigators timing passages through shallow waters
- Fishermen identifying optimal feeding windows
- Surfers reading how tide height will shape a wave break
- Ecologists studying intertidal habitats
Understanding the forces behind tides gives you a richer appreciation of the ocean — and the tools to work with it rather than against it.