The ocean is a gigantic body of salt water that covers 70.8% of the earth's surface. The movements of oceanic waters result from a complex interplay of winds and gravitational forces caused by the interaction of the earth, sun and moon. Just as the moon orbits the earth, they both orbit the sun and the combined gravitational effects are the primary cause of the rise and fall of water levels, known to us as tides.
As the moon rotates around the earth, its gravitational force acts on the part of the earth directly opposite from it. Since the land on earth is solid, it is unaffected by gravity but the oceanic waters are impacted, resulting in the ocean "bulging" away from the earth as it is drawn toward the moon by gravitational force. Water is also displaced to the opposite side of the earth, facing away from the moon by centrifugal force. This force results from the rotation of the earth and moon about a common centre of mass. Both gravitational and centrifugal forces join together to produce two tidal bulges of approximately the same size. High tide occurs as the bulge hits land, while low tide occurs between bulges. Tides occur 50 minutes later each day because the moon takes 24 hours and 50 minutes to rotate the earth.
The sun is much larger than the moon but since it is so much farther away from the earth, it has a smaller effect on tides that the moon. During a new moon and full moon, the gravitational effects of the sun and moon are combined resulting in extremely high and low tides, known as spring tides. By contrast, during the first and third quarters of the lunar cycle, the sun and moon are not aligned, causing tides of a small magnitude, known as neap tides.
The height of tides vary as a result of geographical location, shape of the ocean basin, and many other factors. Tides at the end of a long inlet are considerably amplified because the large volume of inflowing water is pressed into a narrow space. By comparison tides on a mid-ocean island are often less than 1 m. The Bay of Fundy on Canada's east coast is a 250-km-long basin which tapers inward, producing tidal ranges of 15 m.
Variation in the slope of ocean basins cause different tidal frequencies in different areas. The northern Canadian coastline is subjected to two equal high and low tides everyday, called semidiurnal, but the Gulf of Mexico has only one high and one low tide each day, called diurnal.