Waves depend on the wind and thus can be unpredictable. But tides result from the highly predictable gravitational pull of the moon and the sun on the earth. So high and low tides are known with great precision.

To understand how tides form, consider that the moon's pull on the earth is slightly stronger on the side facing the moon. This causes the Earth to elongate ever-so-slightly toward the moon. Because water is more flexible than rock, most of that stretching occurs in the oceans. This raises a small "tidal bulge" on the side of earth facing the moon and a second tidal bulge on the opposite side of earth. Now, imagine the earth rotating, but keep the tidal bulges in place: any location on earth will move alternately into deeper and then shallower water, giving two high tides and two low tides each day.

The sun is so far away that its tide-generating potential is less than half the moon's, but the sun does change the details of the tides. For example, when the moon is full or new, the sun's and moon's gravitational pulls combine. This creates the so-called "spring tides," with higher high tides and lower low tides.

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