Tides

The term “tides” is a generic word that is used to describe the periodic and alternating rise and fall in sea level within a day, with respect to the land. Tides are generated by the gravitational forces of the moon and the sun. They are not only confined to seas and oceans, as tides also occur in lakes and rivers, the atmosphere, and within the earth’s crust. However, tides in these areas occur at a lesser degree and are not as evident as those in large bodies of water.

The Lunar/Moon Tide

The earth and the moon have a significant gravitational pull on each other. Since the earth is larger in terms of mass, it has a stronger gravitational pull and this keeps the moon in orbit. On the other hand, the gravitational force of the moon pulls on the earth and everything in it. However, the earth’s gravity is strong enough and manages to pull back everything except water, and since water is in perpetual motion, the earth can’t hold on to it so the moon manages to pull at it. This causes the oceans to bulge out in the direction of the moon’s gravitational pull. Concurrently, another bulge takes place on the other side because there is also a force pulling water out but in a direction away from the moon.

Semidiurnal Tides

As the earth rotates on its axis in about 24 hours, a certain point on its surface moves through areas with various forces acting on it. In a single day, that point on earth moves from an area of high tide through an area of low tide, back through the area of high tide, and then through another area of low tide, before it finally returns to the point of origin of the initial high tide. These result in two high tides and two low tides each day, and they are called semidiurnal tides.

The Tidal Day

The moon is in perpetual motion as it rotates around the earth at a rate of 12° a day. That is equivalent to one rotation in a month, and the moon rotates in the same direction as the earth’s spin. This means that when the earth completes one rotation, the moon has already moved 12° farther, and it takes another 50 minutes for the moon to reach the same position corresponding to a point on the earth’s surface. Hence, the tidal cycle lasts about 24 hours and 50 minutes, and this explains the reason why high and low tides are roughly 50 minutes later each day.

The Sun Tide

As aforementioned, tides are primarily caused by the gravitational attraction of the earth and the moon. However, it is important to note that there is also a gravitational attraction between the sun and the earth, though the effect of the sun on tides is not as distinct as the moon’s effects. The sun’s gravitational pull can either reinforce the moon’s effects or negate them, depending on the moon’s position relative to the sun.

In a month’s time, the moon rotates around the earth and there will come a point wherein it will lie in between the sun and the earth. This phase is called the new moon during which the sun’s gravitational pull will be in the same direction as the moon’s, and their gravitational forces tend to reinforce one another. During this period the high tides are higher and the low tides are lower as compared to other days when the earth is subject to the moon’s gravitational pull alone. Such extreme tides are called spring tides.

The same thing occurs when the moon is located on the opposite side of the sun, or during full moon. The gravitational forces of the sun and the moon combine to produce higher highs and lower lows. At times when the sun and the moon are not approximately aligned, their gravitational pull on earth simply cancels each other out so the tides are at their lowest range. These are called neap tides and they typically occur during the first and last quarter moons.

Inclination

The moon’s rotation does not follow the earth’s equator; instead it follows an inclined orbit with respect to the earth’s axis. As a result, the northern and southern latitudes normally experience only one high tide and one low tide in a day. These are called diurnal tides. In the same manner, the earth’s inclination relative to the sun also has an effect on tides. The sun’s inclination follows a cycle that is about a year long, with its highest inclination corresponding to the summer and winter months. During this period the bulges in the ocean are least apparent in the equatorial region, and it is most likely to have only one tidal cycle per day.