Tides

A periodic variation in the surface level of the oceans, caused by the gravitational attraction of the moon and the sun.

The ocean tides are formed by the gravitational attraction between the Earth, the Moon, and the Sun. The Earth is also pushed and pulled by this force, but its movement is so slight that it is hard to detect. The effect of the Sun is only about half as strong as that of the Moon, even though the Sun is about eight million times greater. The proximity of the Moon to the Earth makes up for the greater mass of the Sun.

When the Moon is directly over a point on Earth, its pull causes water there to pile up, producing a high tide. As water is drawn toward this point, it drops in other areas, causing a low tide. At the same time, water piles up on the opposite side of the Earth because the attraction of the Moon and the Earth's solid crust in this region is greater than it is for the water, which has less mass and is farther away. The lithosphere is drawn toward the Moon, leaving the water at this point farther from the Earth's center.

As the Earth rotates, different parts are being attracted to the Moon so the locations of high and low tides changes. About 24 hours and 50 minutes elapses between risings of the moon, which is known as a lunar day. As the Moon rotates slowly in the same direction as the Earth, it appears over different parts of the planet throughout the lunar day, causing the tides in different places.

Explanation

Tides aren't the same everywhere because there are many factors involved. The local geography, the prevailing weather conditions, the width and depth of the water subject to tidal forces, and much more contributes to the tides.

Tides are affected by the distance of the Moon as it orbits Earth. When the Moon is nearest to Earth, at perigee, the Moon's force is 40 percent greater than when it is farthest away, at apogee. Also, tides are affected by the position of the Moon in relation to the equator. When it is lined up with the equator, like it does every month, the tidal bulges aren't evenly placed and the tides show various irregularities, called diurnal inequalities. The position of the Sun also influences tidal bulges, but not as much. Tides are also affected by natural fluctuations in water levels, called oscillation. The attraction between the Earth and the Moon and between the Earth and the Sun causes the water in the oceans to move up and down, or oscillate, around a central point where there is practically no movement. Tides are higher if the period of the tide is the same as the natural period of oscillation because the two forces reinforce each other. This is called tidal resonance.

What You'll Need:

• ruler
• scissors
• paper cup
• string
• measuring cup
• pencil

Directions:

1. Punch two holes across from each other beneath the rim of the paper cup, using the pencil point.
2. Cut a piece of string 24 inches (60 centimeters).
3. Tie the ends of the string in these holes.
4. Fill the paper cup 1/2 full with water.
5. Go outside without spilling the water.
6. Hold the string and swing the cup around in a horizontal circle above your head several times.

What's Happening?

Even though there is water in the cup, none spills out. The cup turns sideways, but the water stays inside the spinning cup. The moon causes the ocean water to bulge on the side of the earth facing the moon because of its pull. The other side of the earth, opposite the moon, also has a bulge which results because of the spinning earth. The spinning causes a centrifugal force that causes the revolving object to fly away from the center that it turns around. The water in the cup moves outward because of the centrifugal force, but the paper cup keeps it from flying away. In the same way, the earth has a centrifugal force from revolving around the sun and rotating on its own axis as well. The spinning causes the bulging of the ocean waters, called high tides. The bulging water is prevented from spinning out by the earth's gravitational force.

• Spring Tides: During a full moon or a new moon, the Sun, Moon, and Earth are basically in a straight line, so the tidal forces of the Sun are reinforced by the Moon. During this time, the high tides are highest and the low tides are lowest.
• Neap Tides: When the moon is at the first quarter or last quarter, a straight line drawn from Earth to the Moon is approximately at right angles to a straight line drawn from the Earth to the Sun. The tides produced by the Moon during this time are partly neutralized by those produced by the Sun, so the difference between depths at high and low tide, called the tidal range, is small.
• Semidiurnal Tides, or Semidaily: Two practically equal high tides occur each lunar day, and the low tides are also pretty close as well. These tides are common in the Atlantic Ocean.
• Mixed Tides: The two high tides of the lunar day may be equal while the low tides are very different, or vice versa. This is common in the Pacific and Indian oceans.
• Diurnal Tides, or Daily: There is only one set of low and high tides each lunar day. The second set more or less disappears. This happens in the Gulf of Mexico, the China Sea, and other bodies of water.
• Tidal Bore: The incoming tide in estuaries, inlets, and narrow bays that transforms into a rapidly advancing wall of water.
• Tidal Waves: Destructive waves that are actually caused by earthquakes, volcanic eruptions, or violent storms at sea, also known as tsunamis. (see Waves)

Tides can cause currents to form. Look at some common currents caused by tides.

• Slack Current: At high tide, the harbor basin is filled to its maximum with water and the current at the harbor entrance is generally weak and variable.
• Ebb Current: As the high tide recedes, the current at the harbor entrance is directed away from shore and reaches its maximum strength about halfway between high and low tide.
• Flood Current: After low tide, the water begins to flow from the sea back into the harbor, and this current gradually increases in strength until it reaches a maximum about halfway between high and low tides. This current then gradually diminishes in strength until the cycle is completed when another slack current begins after the high tide.
• Races: Rapid and powerful currents that are caused by tides, especially during ebb tide.
• Whirlpool: Two opposing tidal currents meet. This can be very destructive by entrapping and wrecking small boats, or even making a large boat impossible to steer until the whirlpool dies down.

We can use the tides to get power. The periodic movement of a great amount of water, such as tides, can provide a lot of energy for us. In Europe they use this energy to generate electricity by building dams with turbines that are moved by the tides. The United States experimented with this energy, as well. They used it to power mill wheels for grinding corn and spices, and tried to use it for power plants, but they didn't because of lack of funds. In Canada, a huge hydroelectric power plant was constructed that generates about 14,800 megawatts of electricity and has five reservoirs. Unfortunately, there has been many environmental and cultural costs. Reversed rivers caused death and confusion among wildlife, and the increased flow of rivers caused mudflats and destruction of plants and animals.

There are several methods for measuring tides, and the results help us to predict changes. A device called the tide staff measures the rise and fall of tides. A plant,marked in units, is fastened vertically to a pile, dock, or cliff. The top marking extends above the highest tide, and the bottom marking below the lowest. This measures the tides accurately in protected areas where the waves are small, but is ineffective on waveswept coasts. In these places, they use a tape gauge. A large pipe with openings below the lowest possible level at low tide is set vertically in the water of a well, with a tape attached that has measurements on it. The tape passes over a pulley, which has a counterweight attached at its other end, and observers can measure the tide. The standard automatic tide gauge used by the United States National Ocean Survey, uses a float in the well that is attached to a wire that passes over a pulley and causes it to turn if the water level rises or falls. The pulley is mounted on a rod and turns to make a pencil move along the threads of the rod. The pencil makes a continuous line on paper wrapped around a roller that is turned. The high water and low water are recorded on the paper, along with the hourly levels.

Mariners need to know what's happening with the tides to help them in their travels. A helpful book is the Tide Tables, which is a handbook published each year by the United States Ocean Survey. There is a volume for the Pacific and Indian oceans, and another for the Atlantic. It includes a listing of the times of high and low water for every day of the year at certain stations along the coast, as well as a table for calculating the times and height of high and low water for certain stations and a table for calculating the height of the tide at any time between high and low water.