SPENCER SUTTON / SCIENCE PHOTO LIBRARY SPENCER SUTTON / SCIENCE PHOTO LIBRARY
Tides, the rise and fall of sea levels, are caused by the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The moon is the largest influence on the Earth's tides. When the sun, moon, and Earth are aligned (at the time of the new or full moon), the solar tide combines with the lunar tide to create extra-high high tides, and very low, low tides (both called spring tides). When the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces more moderate tides (called neap tides). During each lunar month, two sets of spring tides and two sets of neap tides occur.
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