The regular rise and fall in the surface level of the Earth's oceans, seas, and bays caused by the gravitational attraction of the Moon and to a lesser extent of the Sun. The maximum high tides (or spring tides) occur when the Moon and Sun are directly aligned with Earth, so that their gravitational pull on Earth's waters is along the same line and is reinforced. The lowest high tides (or neap tides) occur when the Moon and Sun are at right angles to each other, so that their gravitational pull on Earth's waters originates from two different directions and is mitigated. Tides vary greatly by region and are influenced by sea-floor topography, storms, and water currents. See also ebb tide, flood tide, neap tide, spring tide.