Lesson 5 of 5 · Tides & tidal locking
Gravity stretches and reshapes
7 minute lesson · 3 checks along the way
Your goal for this lessonExplain tides through differences in gravity, connect lunar and solar effects, and describe why a tidally locked Moon still rotates.
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Compare the pull across a world
The Moon attracts the whole Earth, not only the ocean nearest it. Because gravity varies with distance, that attraction differs across our planet. The near side is accelerated toward the Moon more strongly than Earth’s center; the far side is accelerated less strongly. Relative to the center’s motion, this difference tends to stretch Earth along the Earth–Moon direction.
That difference in attraction is a tidal effect. A simplified equilibrium picture shows two ocean bulges, one on each side, with lower water levels between them. It is not a picture of the Moon lifting one isolated pile of water while the rest of Earth stays still. Solid ground also deforms slightly under , even though ocean tides are more familiar.
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The Sun joins the pattern
The Sun contributes to Earth’s tides too. Its overall gravitational pull on Earth is large, but tides depend on the difference in pull across Earth, not simply the total attraction at our location. The much closer Moon makes the larger contribution to Earth’s ocean tides. Distance and the scale over which the attraction changes both matter to that comparison.
Near new and full Moon, the solar and lunar tidal patterns reinforce each other, producing the generally larger tidal ranges called spring tides. Near the quarter phases, their combination generally gives smaller ranges called neap tides. Spring here does not mean the season. A tidal range is the difference between high and low water, rather than the height of a single wave.
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A familiar face that is still turning
Tidal interactions also change rotation over long periods. As a body deforms, internal friction can convert mechanical energy into heat. In the Moon’s history, this interaction helped slow its rotation into synchronous tidal locking: one turn on its axis in the same time as one orbit around Earth. Approximately the same lunar hemisphere therefore faces us throughout the orbit.
Keeping one side toward Earth requires rotation. Picture carrying a globe around a central marker while keeping a marked face pointed inward: the globe turns once during the trip. A globe that kept one unchanging orientation relative to distant stars would show different sides to the marker. Tidal locking is coordinated rotation and revolution, not an absence of spinning.
Work through a decision
A coastal forecast needs more than two bulges
A diagram can isolate the cause of tides without predicting a particular harbor’s water level. Continents, ocean depth, coast shape, Earth’s rotation, and weather affect the water’s response. Many shores have roughly two high tides in a lunar day, but local patterns vary. The ideal bulges do not mean that high water must occur exactly when the Moon is overhead.
Imagine an exhibit trying to predict tomorrow’s tide from a Moon picture alone. Improve it by adding a local tide prediction and explaining the information behind it. The Moon’s phase helps describe the broad spring–neap pattern; it does not supply an exact time or height for every coast. Keep the physical cause and the detailed local prediction connected, while recognizing their different levels of detail.
The oceans in this July 6, 2015 EPIC view of Earth connect gravity to everyday life. This composite shows broad ocean and weather patterns; its scale does not let us read the tide level along a coastline.
Look closely
Find a coastline. What observations would you add before predicting when its next high tide will occur?
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Put it together
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