Astronomy 2e · Earth, Moon, and Sky
Ocean Tides and the Moon
On this page 9 sections
In 30 seconds
Twice a day on most ocean coasts, the water rises and falls. These tides come from the differential gravity of the Moon (and, less strongly, the Sun): the Moon pulls Earth's near side harder than its center, and its center harder than the far side, stretching Earth into a slight bulge along the Moon–Earth line. The oceans respond visibly, forming two bulges — one facing the Moon, one opposite. As Earth rotates, each point passes through both bulges, producing two high tides and two low tides about every 24 h 50 m (the Tidal day Time between successive high tides of the same kind, about 24 h 50 m. Full entry →).
The Moon dominates: tidal force depends on the difference in gravity across Earth's diameter, which falls off faster with distance than gravity itself; the Sun's effect is real but about half as strong. When the Sun and Moon align (new and full moon), their bulges combine into extra-large spring tides; at right angles (quarter phases), they partially cancel into smaller neap tides. The friction of the moving water slowly brakes Earth's rotation and pushes the Moon away.
Why this matters
- Coastal life and safety: harbors, shipping, fishing, and emergency planning depend on Tide The daily rise and fall of sea level driven by the Moon's and Sun's gravity. Full entry → predictions; spring tides raise flood risk.
- Marine ecology: intertidal life runs on the daily wet/dry schedule set by the tides.
- Planetary science: the same tidal physics explains locked moons, Io's volcanism, and why Earth's day is lengthening.
- Exams: the two-bulge explanation, two daily high tides, spring vs. neap, and Moon-vs.-Sun strength are classic items.
The college version
Core Concepts
Differential gravity: why the Moon stretches Earth
Gravity weakens with distance. The Moon pulls Earth's near side harder than its center, and its center harder than the far side. Because the pull is not uniform, Earth stretches along the Moon–Earth line: near-side water is pulled toward the Moon; far-side water is "left behind" relative to the center, bulging outward. The result: two tidal bulges. This is differential (tidal) gravity, not total gravity — the Moon's overall pull is tiny compared with Earth's own; the difference across Earth's diameter stretches, rather than lifts, the ocean.
The 24-hour-50-minute tidal cycle
If the Moon stayed fixed, a point on the equator would pass through both bulges once per rotation — two high tides in 24 hours. But the Moon orbits eastward about 13° per day, shifting the bulges, so any location must rotate a little extra to catch the next one. Successive high tides are therefore about 12 h 25 m apart, and the two-high-tides cycle runs about 24 h 50 m — mirroring the ~50-minute daily moonrise delay. Most coasts see two roughly equal high tides a day (semidiurnal), but basins and coastlines reshape the timing — some places get "mixed" tides, and a few get only one high tide a day (diurnal).
Spring and neap tides: the Sun joins in
The Sun raises its own bulges about half as large as the Moon's; the two sets combine by geometry:
- Spring tides (new and full moon): Sun, Moon, and Earth aligned; the bulges add, giving the largest range — very high highs, very low lows. The name means "springing up," not the season.
- Neap tides (first and third quarter): the Sun and Moon pull at right angles; the bulges partially cancel, giving the smallest range.
The spring–neap cycle follows the phases: about two weeks from spring to neap and back, with spring tides at the eclipse-hosting phases — new and full moon.
Local geography controls what you actually see
The global bulge model predicts the pattern, but the size and timing of tides at any harbor are shaped by local sea floor and coastline: continents block the bulges, bays funnel the water, resonant basins amplify. The Bay of Fundy in Canada has some of the largest tides on Earth; an open coast may show only a few tens of centimeters. Tide tables are therefore local — there is no single "tide height" for the planet.
Tidal friction: the long-term story
The moving water drags across the ocean floor, and Earth's rotation carries the bulges slightly ahead of the Moon–Earth line. That offset lets the Moon's pull brake Earth's spin — Tidal friction The braking of Earth's rotation by the moving water and its drag on the sea floor. Full entry → — slowly lengthening the day (commonly taught: ~1.7 ms per century). Conserving angular momentum, the Moon gains energy and recedes at a commonly cited ~3.8 cm per year. The same physics in reverse explains tidal locking of the Moon and other moons.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| The Moon's total gravity | The Moon's tidal (differential) gravity | Total gravity binds the Earth–Moon system; the difference in pull across Earth's diameter stretches the oceans. |
| One high tide per day | Two high tides per day | The two bulges sweep past each coast each tidal day, giving ~two highs and ~two lows every 24 h 50 m. |
| Spring tide | Spring season | "Spring" comes from the water springing up; spring tides happen year-round, twice per month. |
| Neap tide = "no tide" | Small tide | Neap tides still rise and fall — just with a smaller range than spring tides. |
| The Moon alone drives tides | Sun + Moon | The Sun's tidal effect is about half the Moon's; the two combine and partially cancel through the month. |
| Tides are the same everywhere | Local tides | Continents, bays, and basins amplify or damp the global bulges, so every coast has its own tides. |
| The far-side bulge is "pushed by nothing" | Differential gravity | The far-side bulge forms because the Moon pulls the far side less than the center, so that water lags behind. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The Moon pulls the water on the near side harder than the water on the far side, stretching Earth like a balloon pulled at two opposite spots. That stretching makes two bumps of water; as Earth spins, the bumps sweep past the coasts, giving two high tides a day. When the Sun lines up with the Moon the bumps team up — extra-big tides; when they pull from the sides, the bumps partly cancel — extra-small tides.
Worked example
A boater in a harbor with a shallow entrance wants to know when the channel is deep enough. The tide table shows high at 6:10 a.m., low at 12:25 p.m., next high at 6:35 p.m. — the familiar 12 h 25 m rhythm. It is a full moon weekend — the month's spring tides: today's high is unusually high (great for launching, risky for low boats) and today's low unusually low. Two weeks later, at first quarter, the same harbor will see neap tides with a much smaller range. The boater launches early during the spring-tide high and never assumes last week's tide times still apply — both shift ~50 minutes per day and follow the two-week spring–neap cycle.
Key takeaways
- Tides = differential (tidal) gravity: the Moon's pull differs across Earth's diameter, stretching Earth into two bulges — toward the Moon and away.
- Two high tides per day, ~12 h 25 m apart (tidal day ≈ 24 h 50 m), because Earth must catch up with the Moon's eastward motion.
- The Moon dominates: its tidal effect is about twice the Sun's, because tidal force falls off steeply with distance.
- Spring tides (new/full moon): aligned Sun and Moon → largest range. Neap tides (quarter phases): right angles → smallest range. "Spring" ≠ the season.
- Local geography rules the details: basins and coastlines amplify or suppress tides (Bay of Fundy, e.g.).
- Tidal friction brakes Earth's rotation and pushes the Moon away — the physics that tidally locks moons.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why does the Moon create two tidal bulges rather than one?
Show answer
Because tidal force is differential: the Moon pulls near-side water more than Earth's center, and the center more than far-side water, stretching the planet into bulges on both sides of the Moon–Earth line.
Why are high tides about 12 h 25 m apart rather than exactly 12 hours?
Show answer
The Moon moves eastward ~13° per day, shifting the bulges; a location must rotate an extra ~25 minutes per half-rotation to catch the next one — hence ~12 h 25 m between high tides.
At which lunar phases do spring and neap tides occur, and why?
Show answer
Spring tides at new and full moon (Sun and Moon aligned, bulges add); neap tides at first and third quarter (Sun and Moon at right angles, bulges partially cancel).
Why is the Moon's tidal effect stronger than the Sun's even though the Sun's gravity on Earth is far stronger?
Show answer
Tidal force depends on the difference in gravity across Earth's diameter, which falls off with the cube of distance; the Sun is ~390 times farther away, so despite its far larger total gravity, its tidal effect is only about half the Moon's.
What is tidal friction doing to Earth's rotation and to the Moon's orbit?
Show answer
Tidal friction brakes Earth's rotation (the day lengthens) and, by conservation of angular momentum, transfers energy to the Moon, pushing it into a wider orbit (it recedes a few centimeters per year, a commonly cited estimate).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Tide
- The daily rise and fall of sea level driven by the Moon's and Sun's gravity.
- Differential (tidal) gravity
- The difference in gravitational pull across Earth's diameter, not the total pull.
- Tidal bulge
- The elongated bump of water on the side toward the Moon and the side away from it.
- Tidal day
- Time between successive high tides of the same kind, about 24 h 50 m.
- Spring tide
- Extra-large tidal range when the Sun and Moon align (new or full moon).
- Neap tide
- Small tidal range when the Sun and Moon pull at right angles (quarter phases).
- Tidal friction
- The braking of Earth's rotation by the moving water and its drag on the sea floor.
- Semidiurnal / diurnal tides
- Two / one high tides per tidal day.
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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