Ocean tides are caused by the pull of gravity of the moon and the sun on the ocean's surface. As the moon is much closer than the sun to earth, its influence is far greater. The moon's gravitational force causes a bulge in the oceans surface on the side of the earth facing the current position of the moon. Due to the law of inertia, a bulge also forms on the opposite side of the earth. At the peaks of each of these bulges is high tide, at the troughs, low tide. We experience high and low tides at the beach when these peaks and troughs reach our shores.
The Lunar Day
A solar day is a 24-hour period, the amount of time it takes for the earth to rotate 360 degrees so that sunlight travels around the world and returns to the same spot. The moon rotates around the earth the same direction that the earth rotates around its axis. For this reason, a lunar day, the amount of time for the moon to make a full trip around the earth, is slightly longer than a solar day: 24 hours and 50 minutes.
The Moon and the Tides
Since the gravity of the moon causes the bulges in the ocean, it also takes 24 hours and 50 minutes for the bulges to make their way around the world. Since there are two bulges, there are two high tides and two low tides per 24-hour and 50-minute period. Thus, high tide occurs every 12 hours and 25 minutes. This is why the high and low tides do not occur at the same time each day.
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Types of Tides
If the earth was completely covered in ocean, with no continents to block the movement of water, there would be two high tides and two low tides per lunar day. However, in the real world, continents block the movement of water, complicating the tidal patterns. Due to this interference, there are three types of ocean tides. These are diurnal, semi-diurnal and mixed tides.
Semi-Diurnal and Mixed Tides
Most tides are semi-diurnal or mixed. Semi-diurnal tides are when the two high and two low tides are the same height. In mixed tides, the two high and two low tides are different heights.
Diurnal tides occur when there is so much interference by continents, only one high tide and one low tide occur per day. In the Americas, diurnal tides only occur in the Gulf of Mexico and the coast of Alaska.