Ocean tides are the regular rise and fall of sea level caused mainly by the Moon’s gravity. The Moon pulls more strongly on the side of Earth facing it than on Earth’s center, and it pulls least strongly on the far side. This difference in gravitational pull stretches Earth’s oceans into two broad tidal bulges.
Tides matter for coastal life, navigation, fishing, and tidal energy systems.
Understanding How Tides Are Caused by the Moon
Gravity acts on every part of Earth, but its strength changes with distance. This small change is the important part for tides. Earth and the Moon move around a shared balance point in space.
As Earth turns, each location moves through the regions where ocean water is slightly more spread out. A high tide is not a giant wall of water racing across the ocean.
It is usually a slow change in water level that can take hours. The water rises because the whole ocean surface is being reshaped by the Earth Moon system.
Real coastlines make tides much less simple than a basic globe diagram suggests. Continents block the movement of water. Ocean basins have different shapes and depths.
Narrow bays can concentrate incoming water, while wide open coasts may show a smaller change. Friction with the sea floor slows the flow. Water can keep moving after the strongest gravitational effect has passed, much like water sloshing in a bath after it is pushed.
This delay helps explain why local high tide does not always happen when the Moon is highest in the sky. Tide tables are calculated for particular places because one town can have very different tide times from another town only a short distance away.
The Sun affects tides too, even though it is much farther away. Its gravity pulls on Earth strongly, but the difference in its pull across Earth is weaker than the Moon's tidal effect. At certain points in the lunar cycle, the Sun and Moon pull in roughly the same direction.
Their effects combine and the difference between high water and low water becomes larger. At other points, their pulls act at right angles, so the tidal range is smaller. The word spring in spring tide does not refer to the season.
It means the water seems to spring farther up the shore. These larger ranges can make coastal currents stronger and expose more beach or mudflat at low water.
Students can observe tides through tide charts, harbour records, or repeated photos of the same shoreline. Pay attention to the difference between tide height and wave height. Waves are mainly made by wind, while tides are long, regular changes in sea level.
Weather can still change the observed level. Strong onshore winds can pile water against a coast, and low air pressure can allow sea level to rise. During storms, these effects can combine with a high tide to cause flooding.
For boats, the depth of water may decide whether a channel is safe. For shore animals, the changing waterline controls feeding time, shelter, and exposure to air.
Key Facts
- Newton’s law of gravitation: F = Gm1m2/r^2
- Tidal force depends on differences in gravity across Earth, not just the Moon’s total pull.
- The Moon creates two main tidal bulges, one facing the Moon and one on the opposite side of Earth.
- Most coastlines experience about two high tides and two low tides each lunar day.
- A lunar day is about 24 hours 50 minutes, so high tides usually occur about 50 minutes later each day.
- Spring tides occur near new moon and full moon, while neap tides occur near first quarter and third quarter moon.
Vocabulary
- Tide
- A tide is the periodic rise and fall of ocean water level caused mainly by the gravitational effects of the Moon and Sun.
- Tidal force
- Tidal force is the stretching effect caused by a difference in gravitational pull across an object such as Earth.
- High tide
- High tide is the time when ocean water reaches a local maximum height along a coast.
- Low tide
- Low tide is the time when ocean water reaches a local minimum height along a coast.
- Spring tide
- A spring tide is an especially large tidal range that occurs when the Sun, Moon, and Earth are nearly aligned.
Common Mistakes to Avoid
- Thinking the Moon only pulls water on the near side of Earth is wrong because the Moon also pulls on Earth itself and pulls least on the far side, producing two tidal bulges.
- Confusing gravitational force with tidal force is wrong because tides depend on how much gravity changes across Earth, not simply on the total strength of the Moon’s attraction.
- Assuming high tide happens exactly when the Moon is overhead is wrong because continents, ocean basins, friction, and local coastline shape can delay or reshape the tide.
- Believing the Sun has no effect on tides is wrong because solar gravity modifies the lunar tides, creating stronger spring tides and weaker neap tides.
Practice Questions
- 1 A coastal town has a high tide at 6:10 a.m. If the next high tide is about 12 hours 25 minutes later, what time is the next high tide?
- 2 If high tide occurs about 50 minutes later each day, and today’s high tide is at 3:20 p.m., what time should you expect the comparable high tide tomorrow?
- 3 Explain why there is usually a high tide on the side of Earth opposite the Moon, even though that side is farther from the Moon.