JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Neap and spring tides. Diagram showing the Sun and Moon and their alignments to produce spring (high) and neap (low) tides. Tidal effects act to stretch a spherical body into an ovoid shape, and they act along the line connecting the centre of the two celestial bodies. The strength of a tide depends mostly upon the mass exerting a tidal force, and its distance. Despite the Sun's greater distance, its large mass means that its tidal force on the Earth is still about half that of the Moon, and so the Sun appreciably affects the tides. At first & third quarter (left), the Sun and Moon oppose each other to produce the neap tides. At new and full Moon (right), they combine for the spring tides.
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