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Regular, periodic rise and fall of the surface of the sea, occurring in most places twice a day. Tides result from differences in the gravitational forces exerted at different points on the Earth's surface by another body (such as the Moon). Although any celestial body (e.g., Jupiter) produces minute tidal effects, the majority of the tidal forces on the Earth are raised by the Sun (because of its enormous mass) and the Moon (because of its proximity to Earth). In fact, the tidal forces from the Moon are about twice as strong as those from the Sun. The largest tides (spring tides, exhibiting very large change in sea level between high and low tides) occur at the new and full moon, when the Earth, the Moon, and the Sun are aligned and the Sun's tidal forces are added to those of the Moon. The smallest tides (neap tides) occur when the Sun and Moon are at right angles (from Earth), when the tidal forces from the Sun partially cancel those from the Moon. The geometry of the coastline and of the water's basin also affects the range of the tides.
This entry comes from Encyclopædia Britannica Concise. For the full entry on tide, visit Britannica.com.