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Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean (see figure 4) and by the shape of the coastline and near-shore bathymetry. The surface of the Earth on the Moon’s side is closer to the Moon than the center of Earth is and thus there is a slight excess in gravitational attraction, upwards toward the Moon, whereas the surface on the far side from the Moon is further away and thus there is a slight deficit in force, downwards. The result is an upwards tidal bulge on both sides of the Earth, so there are two tide cycles per day. The movement of the equilibrium ocean surface is only decimetres, but the period of oscillation of the water in the ocean basins is about the same as the twice-per-day cycle of the variation and so the mass of moving water accumulates the effects of the varying forces (see figure 6) over many days according to the shapes of each ocean basin. The resulting water surface variation exceeds the movement of the equilibrium surface of the gravitational fields and its timing at particular locations differs also. If the period of the Moon’s orbit and of the Earth’s revolution were the same, the Earth’s surface would settle to follow the equilibrium surface exactly, rather than on average, and there would be no tides. The Moon orbits the Earth in the same direction as the Earth rotates on its axis, so it takes slightly more than a day about 24 hours and 50 minutes for the Moon to return to the same location in the sky. During this time, it has passed overhead once and underfoot once, so in many places the period of strongest tidal forcing is 12 hours and 25 minutes. The high tides do not necessarily occur when the Moon is overhead or underfoot, but the period of the forcing still determines the time between high tides.