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Each day the tides at any given place on Earth occur about an hour later then on the previous day. Since the Moon passes overhead about an hour later each day, it was long ago thought that the Moon had an effect on the tides. Differential Forces One should consider a simple water molecule in the ocean. It is attracted gravitationally by the Earth, but it also "feels" an attraction from the Moon (the attraction …

…are lined up on opposite sides of the Earth (full Lunar phase) or on the same side (new Lunar phase). Conversely, when the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides. The figure shown above illustrates spring and neap tides.