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.
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.