Tides

Tides

Tides refer to the vertical motion of water caused by the gravitational effects of the sun and moon. Currents refer to the horizontal movement of water. Tidal currents, then, refer to the horizontal movement of water caused by the vertical movement of tides.

Tides and the resulting tidal currents are to a sailor as winds are to an airplane pilot. Without a thorough understanding of them, navigation becomes inefficient, or even dangerous, and the simplest landing can end in tragedy.

Every boat must eventually come to shore, where the tides and their currrents most influence navigation. Entering in-shore waters without knowledge of the local effects of tidal currents can be a recipe for disaster. For example, a cruise ship was recently driven into a bridge over Portsmouth Harbor (NH) by tidal currents because the captain did not appreciate how swift the ebb tidal currents can get in that harbor.

Why tides occur and their periodicity is the result of a complex set of interactions of gravitational and centrifugal forces exerted by several bodies at once. Moreover, tides at any given place are also effected by factors such as land masses, coastal features, weather, oceanic currents, sea bottom topography, and many more that exceed the scope of this page. Clearly this is a complicated topic. Much of what follows is a simplification of the actual phenomena, but I hope it is sufficient to illustrate the origin and behavior of tidal motion.To understand why tides can be so devastating (as well as why they can be so great), you must first understand how they work. Basically, tides are caused by the gravitational pull of the moon, sun, and earth. According to Rachel Carson, "In theory, there is a gravitational attraction between every drop of seawater and even the outmost sea in the universe." "What Makes Tides" says that high tides happen when the moon's attraction is the strongest. And low tides happen when it's the weakest. The pull of the moon causes oceans to "bulge out" in the direction of the moon and this causes an equal and opposite bulge on the opposite side of the earth. Centrifugal force also causes the second bulge. Imagine the earth and moon as two balls (earth being the larger one,) on the ends of a stick. If you spin this stick around, the far side of earth is pulled outward. This is how centrifugal force works and how it causes the second bulge. This means that high tide happens at the same time on opposite sides of the earth.("Tides"). From its lowest point, the tides rise for about six hours until it reaches "high tide." Then it falls for about six hours until it reaches "low tide." (World Book Encyclopedia). To see some current tide levels, you can go to the NOAA tide website. This happens two times a day (this is called a semi-diurnal tide,) unless, in rare cases, a location experiences a diurnal tide which is when there's only one high tide and one low tide a day. "Diurnal tides are caused by the varying declination of the moon's orbit around the earth," says "Intertidal Ecology." The difference between high tide and low tide is called the tide range. The range changes every day due to the position of the sun and moon. Shape, size, and formation of the seacoast also make a difference in tide range. (World Book Encyclopedia). There are a few different types of tides that occur at different times depending on the stage of the moon. When there is a full or new moon, a spring tide occurs, because the moon has an especially strong pull during its new and full stages. A spring tide has nothing to do with the spring; it's just an especially strong (or high,) tide. A neap tide occurs when the gravitational pull of the sun and the moon are perpendicular to each other (during quarter moons,) because the pull of the moon is weak. A neap tide is a very low tide. ("Tides.") Sometimes after storms, tides wash pollution onto the shore. If we stop polluting, like the this will no longer be a problem. Organizations like the Environmental Protection Agency and the EPA's Spill Prevention and Control centers have been working for over two decades to prevent water pollution. IntroductionAn ocean tide refers to the cyclic rise and fall of seawater. Tides are caused by slight variations in gravitational attraction between the Earth and the moon and the sun in geometric relationship with locations on the Earth's surface. Tides are periodic primarily because of the cyclical influence of the Earth's rotation.The moon is the primary factor controlling the temporal rhythm and height of tides (Figure 8r-1). The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction. The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth. At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction. On the opposite side of the Earth, another tidal bulge is produced away from the moon. However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest. Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period. The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the tidal period is 24 hours and 50 minutes in length.The second factor controlling tides on the Earth's surface is the sun's gravity. The height of the average solar tide is about 50 % the average lunar tide. At certain times during the moon's revolution