Tides are the effects produced by the rise and fall of sea levels. Spring tides most often occur when the Sun, Moon and Earth are aligned. The combined gravitational forces that these bodies exert upon Earth produce the highest tides known as spring tides. Coastal regions usually experience two high tides and two low tides throughout the day. Spring tides most often occur during full Moon when the Moon is situated opposite from the Sun, and during new Moon which occurs when the Moon lies between the Earth and the Sun. The distance at which the Moon is from the Earth influences tide height, being highest when the Moon is at perigee than at apogee.
An ocean tide is a term that refers to the rise and fall of sea water levels with respect to the coastline level. Tides are produced by the combined gravitational forces of the Sun and Moon, as well as the rotation of the Earth. The gravitational force of the Moon upon the Earth causes the oceans to bulge towards the Moon. Since the Moon also exerts its gravitational force upon solid Earth, the ocean on the opposite side of the Earth bulges, as well. Each day, there are two high tides and two low tides (semi-diurnal), although some places only experience one high and one low tide in one day (diurnal).
The Moon and the Sun are aligned every two weeks, during new Moon and full Moon. It is during these periods that the highest amplitude spring tides occur. During new Moon and full Moon, the gravitational pull of the Sun and Moon combined exert a stronger attraction upon Earth, producing spring tides. The elliptical orbit of the Moon around Earth causes the Moon to be closer (perigee) or farther (apogee) away from Earth at any given time. During perigee, the Moon approaches the Earth at its closest distance, producing even higher than normal spring tides known as perigean spring tides, which are known to produce intense flooding along coastal areas.
The local bathymetry significantly determines the exact time of a tide, as well as the tide´s height at a given shoreline point. The Bay of Fundy, on the East coast of Canada features some of the world's largest tidal amplitudes. In some parts of the Northern Gulf of Mexico and Southeast Asia, tides usually have one high and one low (diurnal) per tidal cycle. Semi-diurnal tides, which include two high and two low tides per cycle, are common along the Atlantic coast of the U.S. and Europe. Mixed states of these two tides are common along the Western coastline of the U.S. and Canada.
During the times when the Sun and Moon are not aligned (first quarter or third quarter Moon), its gravitational forces cancel each other out. During first quarter or third quarter Moon, the Sun and Moon are in a 90° angle as seen from Earth, and the gravitational pull that each body could be exerting upon Earth is partially cancelled, resulting in low amplitude tides known as neap tides. These weak tides create less extreme tidal conditions. The interval between spring and neap tides is approximately seven days.
The Sun's gravity also influences the creation of tides; however, the solar tides are about half as strong as the Moon's tides. When the Sun, the Moon and the Earth are aligned, the gravitational pull of the Sun complements that of the Moon producing the spring tides. Since the position of the Moon changes each day in the sky, the times for high and low tides varies by 50 minutes. The periodicity of ocean tides is influenced by the rotation of the Earth. The ocean floor and coastline shape influence the size and propagation of tidal waves; land masses and ocean basins act as barriers against ocean water, resulting in variation tidal patterns around the world.