Grades 6–8 · Earth & Space
Why does an ideal tidal ocean have two bulges?
Tides depend on differences in gravitational pull across Earth, not simply on water being pulled toward the Moon. The Moon attracts the whole Earth, including its center and both sides. The near side is closer to the Moon and feels a stronger attraction; the far side feels a weaker one.
In the diagramTides depend on differences in gravityFarCenterNearEarthMoonAll three pull arrows point toward the Moon.Marked position: ideal higher water levelNear pull is stronger; far pull is weaker than center.
The center's acceleration, its change in motion under the pull, gives a reference for comparing those pulls. The near side accelerates toward the Moon more strongly than the center does. The far side also accelerates toward the Moon, but less strongly than the center. Relative to that center, the near and far sides are stretched apart.
An ideal global ocean responds with two opposite higher-water regions along the Earth–Moon direction and lower-water regions to the sides. The far bulge does not require the Moon to repel water. Nor does the picture mean that all water piles up on the near side.
The drawn bulges are greatly exaggerated and represent a simplified equilibrium tendency. Actual coastal tides also depend on the Sun, land boundaries, ocean depth and how water moves. The marked position explains a place within the ideal pattern, not a prediction of a beach's next high tide.
About this explanation
Starting knowledge: Know gravity attracts masses and distinguish a comparison relative to Earth's center from absolute motion.
Sources
- National Oceanic and Atmospheric Administration — Tides