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Unlock This CourseEvery day, billions of tonnes of seawater move up and down our coastlines in a never-ending dance. This movement creates high tides when water levels rise and low tides when they fall. But what causes this incredible natural phenomenon that affects surfers, fishermen and coastal communities worldwide?
Tides are one of the most predictable forces in nature, yet they're caused by invisible gravitational forces acting across hundreds of thousands of kilometres of space. Understanding how tides work helps us predict when beaches will be underwater, when harbours will be accessible and when marine life will be most active.
Key Definitions:
The Moon is the primary driver of Earth's tides. Despite being 384,400 km away, its gravitational pull is strong enough to lift entire oceans. The side of Earth facing the Moon experiences the strongest pull, creating a bulge of water - this is high tide.
The Sun also affects tides, but its influence is about half as strong as the Moon's. When the Sun and Moon align, their combined pull creates extra-high tides called spring tides. When they're at right angles, we get smaller neap tides.
Imagine Earth as a ball covered in water, spinning in space with the Moon orbiting around it. The Moon's gravity pulls hardest on the water closest to it, creating a bulge on the Moon-facing side. But there's also a bulge on the opposite side of Earth - this happens because the Moon pulls Earth itself slightly away from the water on the far side.
Earth has two tidal bulges at any given time, positioned roughly opposite each other. As our planet rotates once every 24 hours, different coastlines move through these bulges, experiencing high tide when they pass through a bulge and low tide when they're between bulges.
Occurs when a location rotates into one of the two tidal bulges. Water level rises gradually over about 6 hours, reaching maximum height.
Happens when a location is between the two bulges. Water level falls for about 6 hours, reaching minimum height before rising again.
Most places experience two high tides and two low tides every 24 hours and 50 minutes - the time it takes for the Moon to return to the same position.
The height of tides changes throughout the month depending on the positions of the Moon and Sun relative to Earth. This creates a predictable pattern of stronger and weaker tides that repeats roughly every two weeks.
Spring tides occur during new moon and full moon phases when the Sun, Moon and Earth align. The combined gravitational pull creates the highest high tides and lowest low tides, with the greatest tidal range. Despite the name, spring tides happen year-round, not just in spring!
Neap tides happen during first and third quarter moon phases when the Sun and Moon are at right angles to Earth. Their gravitational forces partially cancel each other out, creating smaller tidal ranges with lower high tides and higher low tides.
Not all coastlines experience the same tidal patterns. The shape of coastlines, depth of water and underwater geography all influence how tides behave in different locations.
Narrow channels and funnel-shaped bays can amplify tides dramatically. As water gets squeezed into smaller spaces, it has nowhere to go but up, creating some of the world's most extreme tidal ranges.
Home to the world's highest tides, with ranges up to 16 metres. The funnel shape of the bay amplifies the incoming tide, creating a natural tidal bore that can be heard from kilometres away.
Experiences very small tides, typically less than 30cm, because it's almost completely enclosed. The narrow connection to the Atlantic Ocean limits tidal flow.
Different locations experience different tidal patterns based on their geography and position on Earth. Understanding these patterns is crucial for navigation, fishing and coastal management.
Oceanographers classify tides into three main types based on how many high and low tides occur each day and their relative heights.
Two high tides and two low tides of similar height each day. Common along Atlantic coasts of Europe and North America.
One high tide and one low tide per day. Found in parts of the Gulf of Mexico and some Pacific locations.
Two unequal high tides and two unequal low tides per day. Common along the Pacific coast of North America.
The Severn Estuary between England and Wales has the second-highest tidal range in the world, reaching up to 14 metres during spring tides. The funnel shape of the estuary concentrates the incoming tide, creating the famous Severn Bore - a wave that travels up the river for over 40 kilometres. This extreme tidal range has been harnessed for renewable energy, with plans for tidal barrages that could generate electricity for millions of homes.
Because tides are caused by the predictable movements of the Moon and Sun, we can calculate tide times and heights years in advance. This predictability makes tides incredibly useful for human activities.
Tide tables show predicted times and heights of high and low tides for specific locations. These predictions are based on astronomical calculations combined with local geographic factors. Modern computer models can predict tides with remarkable accuracy, usually within minutes for timing and centimetres for height.
Ships use tide predictions to plan harbour entries, especially in shallow ports. Many harbours can only be accessed at high tide, making accurate predictions essential for maritime commerce.
Surfers, fishermen and beachgoers all rely on tide predictions. Many fish species feed more actively during tidal changes, while surfers need to know when beaches will have the right water depth.
Tides create unique ecosystems and influence marine life in countless ways. The constant movement of water brings nutrients, oxygen and food to coastal areas while creating challenging conditions that only specially adapted organisms can survive.
The area between high and low tide marks creates one of Earth's most challenging habitats. Organisms here must survive being underwater for hours, then exposed to air, sun and temperature changes. Creatures like barnacles, mussels and sea anemones have evolved remarkable adaptations to thrive in these conditions, creating some of the most biodiverse ecosystems on our planet.
The immense power of moving tidal water represents a clean, renewable energy source. Unlike wind or solar power, tides are completely predictable, making them an attractive option for sustainable electricity generation.
Tidal power stations work by capturing water at high tide, then releasing it through turbines as the tide falls. France's La Rance tidal power station has been generating clean electricity since 1966, proving that tidal energy can be both practical and environmentally friendly.