Why there are two high tides per day
One bulge forms toward the moon and one forms on the opposite side because of differential gravitational pull.
Earth & Space
Why Do Tides Happen?
Tides are not simply the ocean chasing the moon. They happen because the moon's gravity pulls different parts of Earth by slightly different amounts.
The short answer
The moon's gravity pulls Earth's oceans unevenly. Water on the side facing the moon is pulled slightly outward, creating one high-tide bulge. A second bulge forms on the opposite side because the moon pulls Earth's center more strongly than it pulls the far-side ocean, leaving that water behind. As Earth rotates through these two bulges, a coastline usually experiences two high tides and two low tides per lunar day. Because the moon moves in its orbit, that cycle is about 24 hours and 50 minutes rather than exactly 24 hours. The sun also creates tides, though its tidal effect is weaker than the moon's. When the sun, moon, and Earth line up, their forces combine to make spring tides. When the sun and moon are at right angles, the result is weaker neap tides.
Why high tide is 50 minutes later each day
The moon advances in orbit each day, so Earth has to rotate about 50 extra minutes to catch up.
The sun causes tides too
The sun's tidal effect is about 46 percent as strong as the moon's, producing spring and neap tide patterns.
Myth: tides are water being pulled upward
Tides are a stretching effect caused by differences in gravity across Earth's diameter.
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