01
Near side: pulled harder
Ocean water on the side facing the Moon is pulled more strongly than Earth's center, stretching a bulge toward the Moon.
Moon · Question
Why are there two high tides a day, not one?
If tides were just the Moon "pulling" water toward it, there would be one high tide a day, on the side facing the Moon. In a simple ocean-covered-Earth model there are two — because tides come from differences in gravitational attraction, not simple pulling.
Visual model
Spring tides vs. neap tides.
Sun and Moon pull along the same line, reinforcing the stretch on both sides of Earth — the highest high tides and lowest low tides of the month.
Sun and Moon pull at right angles to each other, partially canceling out — the smallest difference between high and low tide in the month.
Why does the tidal stretch change with alignment?
Move the Moon relative to a fixed Sun direction. The model combines two directional tidal-force patterns, with the Sun's contribution set to approximately half the Moon's. The stretched shape is greatly exaggerated and is not an ocean-height forecast.
0° · aligned90° · right angle
0° — Spring configuration: lunar and solar tidal patterns reinforce in this model.
The combined equilibrium quadrupole is a teaching approximation using a constant lunar contribution and a solar contribution half as large (NOAA). It excludes continents, ocean-basin resonance, local delays, the Moon's distance changes and unequal high tides. Shape distortion has no sea-level units.
Sources: NOAA — Spring and neap tides and NOAA — differential tidal forces and local limits.
What's simplified
Why two bulges, not one?
02
Far side: left behind
Water on the far side is pulled less than Earth's center is, so it's effectively "left behind" — stretching a second bulge away from the Moon.
03
Earth rotates through both
As Earth spins once a day, most coastlines pass through both bulges — many places experience two high tides in a lunar day of about 24 hours 50 minutes. Actual local patterns also depend on geography and ocean dynamics.
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Next in this series →
Earth's seasons ↗
Why tilt creates seasons, not distance.
Moon
Moon phases
Spring tides happen at new moon and full moon — see why.
Moon
Tidal locking
The Moon's own rotation was shaped by this same tidal force.