MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Diagram showing the Sun and Moon and their alignments to produce neap tides and spring tides on Earth. 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. Despite the Sun's greater distance, its large mass means that its tidal force on the Earth is about half that of the Moon, and so the Sun appreciably affects the tides. At first and third quarter (Moon at left and right), the Sun and Moon oppose each other to produce the lower-amplitude neap tides. At new and full Moon (Moon at top and bottom), they combine for the higher-amplitude spring tides.
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