The breathing ocean
If you spend a day at the seashore, you’ll notice the water creeping up the beach and then pulling back. These regular changes in sea level are called tides. Most coasts have two high tides and two low tides about every 24 hours and 50 minutes.
What causes tides?
Tides are caused mainly by the Moon’s gravity, with help from the Sun.
- The Moon’s gravity pulls on Earth’s oceans, creating a bulge of water on the side facing the Moon.
- A second bulge forms on the opposite side of Earth, where the Moon’s pull is weakest and the ocean is "left behind."
- As Earth rotates, each coastline passes through both bulges (high tides) and the low areas between them (low tides).
Because the Moon also moves along its orbit, it takes about 24 hours and 50 minutes for a spot on Earth to line up with the Moon again. That’s why high tide comes about 50 minutes later each day.
The Sun’s role
The Sun is much bigger than the Moon, but it’s so far away that its tidal effect is only about half as strong.
| Tide | When | Moon phase | Tidal range |
|---|---|---|---|
| Spring tide | Sun, Moon and Earth line up | New moon and full moon | Largest: highest highs and lowest lows |
| Neap tide | Sun and Moon are at right angles | First and third quarter | Smallest |
Spring tides have nothing to do with the season. The name comes from the water "springing up." They happen twice a month, all year round.
Tidal range
The tidal range is the difference between high and low tide. It varies a lot from place to place depending on the shape of the coast and sea floor.
| Location | Typical tidal range |
|---|---|
| Mediterranean Sea | Less than 1 m |
| Open ocean | About 0.5–1 m |
| Vancouver | About 3–5 m |
| Bay of Fundy, NS and NB | Up to about 16 m, the highest in the world |
The Bay of Fundy
The Bay of Fundy, between Nova Scotia and New Brunswick, has the world’s highest tides. Twice a day, about 100 billion tonnes of water flow in and out, more than the flow of all the world’s rivers combined. The bay’s length and funnel shape make the water slosh back and forth in rhythm with the tides, a kind of resonance.
At low tide, you can walk on the ocean floor around the Hopewell Rocks. At high tide, the same spot is under more than 10 m of water. Fundy is also home to a tidal bore, a wave that travels up rivers like the Petitcodiac when the tide comes in.
The intertidal zone
The intertidal zone is the strip of shore that’s underwater at high tide and exposed at low tide. Living things there face big challenges:
| Challenge | Adaptation |
|---|---|
| Drying out at low tide | Barnacles and mussels close their shells tightly |
| Crashing waves | Seaweed has strong holdfasts; sea stars grip with tube feet |
| Changing temperature and saltiness | Many animals hide in tide pools or under rocks |
| Predators from land and sea | Crabs hide under rocks; snails have thick shells |
Tide pools are rocky pools that hold water at low tide, full of sea anemones, crabs, small fish and sea stars.
Why tides matter
| Use | Details |
|---|---|
| Navigation | Ships need deep enough water to enter harbours |
| Fishing | Many fish feed and move with the tides |
| Clamming and seaweed harvesting | Done at low tide |
| Tidal power | The Annapolis Royal station in Nova Scotia (1984–2019) was one of the first tidal power plants in North America; tidal turbines are being tested in the Bay of Fundy |
| Safety | People can be stranded by rising tides on beaches and sandbars |
Always check tide tables before exploring the shore.
Tides on other worlds
Tides happen anywhere gravity pulls unevenly. Earth’s gravity creates tides in the Moon’s rocks, and over billions of years, these tidal forces slowed the Moon’s spin until it kept one face toward Earth. Tides are also slowly lengthening Earth’s day by about 2 milliseconds per century.
Common mistakes
- Thinking spring tides happen only in spring.
- Thinking the Sun has no effect on tides.
- Thinking there’s only one high tide a day. Most places have two.
Try it yourself
- Look up tide tables for the Bay of Fundy and a coast near you. Compare the tidal ranges.
- Use a ball (Earth) wrapped in a loose plastic bag (ocean) and a magnet or your hand (Moon) to model the bulges.
- Track the Moon’s phase for two weeks and compare it with tide tables.
Quick recap
- Tides are the regular rise and fall of sea level, caused mainly by the Moon’s gravity.
- Most coasts have two high and two low tides each day.
- Spring tides (largest) happen at new and full moon; neap tides (smallest) at quarter moons.
- The Bay of Fundy has the world’s highest tides.
- Intertidal life is adapted to changing water levels.