Slide 1
The Moon’s gravitational pull varies across Earth
Tides arise from differences in gravitational pull across an extended body. The Moon pulls more strongly on Earth’s near side than on its far side.
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The relevant quantity is the difference in acceleration, not just the total attraction. Earth and its ocean are already moving under the Moon’s gravity together, so tides concern their relative deformation.
Slide 2
The difference tends to stretch Earth in two directions
Relative to Earth’s center, material on the near side is pulled moonward more strongly, while far-side material is pulled less strongly. This produces a stretching tendency.
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This idealized pattern helps explain two tidal bulges without treating the far side as being repelled by the Moon. Gravity remains attractive on both sides; its strength is what differs.
Slide 3
The Sun adds a tide that depends on alignment
The Sun also produces a tidal effect. Lunar and solar contributions combine differently as the Sun, Earth and Moon change alignment.
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Near new and full Moon, their effects tend to reinforce larger tidal ranges; near quarter phases, the range tends to be smaller. The term spring tide refers to that larger range, not the season of spring.
Slide 4
Coasts and basins reshape the simple tidal picture
Coastlines, basin shape and water depth alter how tidal motion travels. Different places can have different daily tide patterns and heights.
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Some coasts have two similar high tides, others have unequal pairs, and some have one dominant daily high tide. Local geography turns the simple astronomical forcing into a more complex response.
Slide 5
Tidal friction also changes rotation and orbit
Tidal friction transfers energy and angular momentum within the Earth–Moon system. Over long periods it affects Earth’s rotation and the Moon’s orbit.
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The same gravitational interaction that drives ocean rhythms participates in long-term orbital evolution. A tide table addresses local timing, while planetary dynamics studies the slower changes accumulating across many cycles.
Sources
- Ocean Tides and the Moon
OpenStax, Rice University
- Types and Causes of Tidal Cycles
NOAA