Tides
🎯 In this topic you will
- Explain what tides are and how they occur
- Describe the role of tidal forces and their origins
- Explore how tidal forces affect the oceans and the land
🧠 Key Words
- coastal
- depth
- earthquake
- earth tide
- force of attraction
- harbour
- tidal force
- tidal range
- tide
Show Definitions
- coastal: Relating to land that is near or next to the ocean or sea.
- depth: The vertical distance from the surface of the water to the bottom of the ocean or sea.
- earthquake: A sudden shaking of the Earth's surface caused by movements in the Earth’s crust.
- earth tide: A small change in the height of the land caused by gravitational forces from the Moon and the Sun.
- force of attraction: The pull that one object exerts on another due to gravity.
- harbour: A sheltered area along the coast where boats and ships can dock and unload.
- tidal force: The gravitational force that causes tides, mainly from the Moon and the Sun.
- tidal range: The vertical difference in water level between high tide and low tide.
- tide: The regular rise and fall of sea level caused by gravitational pulls from the Moon and the Sun.
🌊 What Are Tides?
In some parts of the world, the depth of the ocean changes by several metres during the day. The depth is the distance from the surface of the water to the bottom of the ocean.
📸 Observing the Tides
The picture shows the same place at two different times. The pictures were taken six hours apart. This change in depth of the water is called a tide.
🧪 Did you know?
In some places, the difference in ocean depth between high and low tide can be greater than the height of a four-story building!
📏 Understanding Tidal Range
The difference in depth of the water between high and low tides is the tidal range. The largest tidal range in the world is 16.3 m in the Bay of Fundy in Canada. Some of the smallest tidal ranges in the world are less than in the Caribbean and Mediterranean seas.
🌍 Earth Tides and Timing
Tides also cause the land to change in height through the day. This is called earth tide. The tidal range due to earth tide is about 30 cm. High tides are about 12 hours apart. Low tides are also about 12 hours apart. The time between high and low tide is six hours.
📌 Important Concept
Tidal Range: The tidal range is the difference in water depth between high tide and low tide, and it varies depending on location and lunar alignment. Even the solid Earth experiences tidal changes called earth tides, with smaller but measurable vertical shifts.
🌖 What Causes Tides?
The Moon orbits the Earth. The Moon stays in orbit because of the force of gravity from the Earth, but the Moon also has gravity, and this gravity pulls on the Earth.
🌕 Tidal Forces from the Moon
As the oceans are made from water, the gravity from the Moon can pull the water more easily than the land. The pull from the Moon’s gravity is called a tidal force. The diagram shows how this happens.
⚠️ Common Mistake
Be careful not to assume that only the Earth’s gravity affects tides — the Moon’s gravity plays the main role in pulling ocean water to create tidal bulges.
🌎 Rotation and Tidal Timing
The side of the Earth closer to the Moon will have high tide. The Earth takes 24 hours to spin on its axis. This means that 12 hours later, the side that was closest to the Moon is now furthest away. You can see from the drawing that the side furthest away also has a high tide. This is why the time between high tides is 12 hours.
☀️ Role of the Sun in Tides
The Sun also produces a tidal force on Earth, but this is weaker as the Sun is further away than the Moon. When the Sun and the Moon are in line with Earth, this produces a larger tidal force.
🌊 Effects of Tides
Some harbours can only be used at certain times of the day. If the water in the harbour is not deep enough, boats cannot move safely. Harbours are places where boats and ships can load and unload passengers and cargo.
🌬️ Coastal Hazards
In weather with strong winds, coastal areas are more likely to have flooding at times of high tides. Coastal areas are parts of the land that are close to the oceans. The flow of water in and out of some coastal areas can be dangerous for small boats.
🐟 Tides and Ecosystems
In some places, tides affect food chains, including the human food chain. For example, at low tide birds can eat some types of shellfish when they are not covered with water. Some types of fish move to find food according to tides in coastal areas.
🌋 Tides and Geohazards
Volcano eruptions have been linked with earth tides. By studying Earth tides, scientists may be able to predict when a volcano will become dangerous. Earthquakes may also be linked with earth tides. Movement of water with tides can be used to generate electricity.
❓ QUESTIONS
1. Which of these causes the force of gravity for tides on Earth?
the Sun only the Moon only the Sun and Moon the Sun, Moon and other stars
👀 Show answer
2. What is the name given to the pull of gravity that causes tides?
high tide low tide tidal range tidal force
👀 Show answer
3. State the time between:
a one high tide and the next high tide
b one low tide and the next low tide
c a high tide and the next low tide
👀 Show answer
b) 12 hours
c) 6 hours
4. Explain why some harbours cannot be used at low tide.
👀 Show answer
5. Explain why the largest tidal ranges happen when there is either a full moon or a new moon.
👀 Show answer
6. The average depth of water in a place near the coast of the Pacific Ocean is ______.
The largest tidal range in that place is ______.
Calculate the maximum depth of water at that place.
👀 Show answer
- Average depth = 5.0 m
- Tidal range = 2.0 m
- Maximum depth = 5.0 m + 1.0 m = 6.0 m (assuming 1 m rise from average level during high tide)
🔬 Think like a Scientist
Discovering the causes of tides
In this task, you will find out about how scientists used evidence to discover what causes tides.
In 330 BCE, a sailor from Greece noticed that the depth of water in some parts of the oceans changed regularly.
He noticed that the depth increased to a maximum twice every day.
He thought that this was because of the Moon.
People did not know about gravity until much later.
1. Use words from the list to copy and complete the sentences.
a conclusion, an observation, a prediction, a measurement, an explanation, a model
The sailor noticed that the depth of water changed. This was ____________.
The sailor thought that the change was caused by the Moon. This was ____________.
The sailor did not know about gravity, so could not give ____________ for the tides.
👀 Show answer
- an observation
- an explanation
- a model
Around the year 1600, a scientist from Germany suggested that there was a force of attraction between the Moon and water. He thought this force caused the tides.
People in 1600 did not know about gravity.
This German scientist said that the force of attraction was magnetic.
We now know that the attractive force between the Moon and the water in the oceans is not magnetic.
2. Describe what could be done to show that there is no magnetic force between the Moon and water in oceans.
👀 Show answer
Use a magnetometer or test water samples to show they are not affected by magnetic fields. This shows the Moon's pull is not magnetic.
People did not believe that the Moon or the Sun could have an effect on the oceans because gravity had not been described.
The problem of what causes tides was finally solved by Newton in the year 1687.
Newton had already described the effects of gravity.
He then used his ideas about gravity to calculate the tidal forces, without the need for experiments.
These calculations were accurate enough to show people that gravity from the Moon and the Sun caused the tides.
People then accepted that tidal forces were caused by gravity from the Moon and the Sun.
3. Which two statements explain why people accepted Newton’s ideas about gravity?
- Newton did experiments on the tides that were fair tests.
- Newton provided evidence to support an hypothesis.
- Newton made observations whereas previous scientists did not.
- Newton made predictions that were shown to be accurate.
👀 Show answer
- Newton provided evidence to support an hypothesis
- Newton made predictions that were shown to be accurate