Motion of the Ocean

Motion of the Ocean
The National Oceanic and Atmospheric Administration (NOAA) provides daily tide predictions for several stations throughout the U.S. Sometimes the predictions are spot-on, like the one on top from February 18/19. (Blue line is the prediction; red line is the recorded tide.) Sometimes the predictions are off, like the one on bottom from July 2010. You can find past predictions from various stations at (The data for the Port Aransas station starts Oct 11, 2005.)
When the tides of life turn against you and the current upsets your boat, don't waste those tears on what might have been, just lie on your back and float.

~ Unknown

As the sun rises in the east and the stars shine at night, we expect the tides to rise and fall. Tides may seem simple on the surface, but their secrets baffled scientific minds for centuries. Even Galileo only partially succeeded in his explanation, correctly linking tides to the inertia of the oceans and the Earth's orbit around the sun, but completely leaving the moon out of the equation. Newton was the first to explain how ocean tides result from the gravitational attraction between the earth and the moon. Put simply, tides are created because the moon is constantly pulling at everything on the earth to bring it closer. This has only a small effect on Earth's land surfaces because they are less flexible, but not completely stationary, as we know. Movements within the earth's crust are called terrestrial tides. Terrestrial tides can move land surfaces up to 22 inches a day and are important for such things as radio astronomy, calculating coordinates on a GPS, and volcanology (because sometimes terrestrial tides can trigger a volcanic eruption).

Though tides do occur, to a much smaller extent, in the solid crust of the earth, as well as in large lakes and the atmosphere, the word "tides" is generally used to define the larger motion of the ocean responding to inertial and gravitational forces, distinct from wind-actuated waves and the continuous ocean currents. The moon's tidal force has a much greater effect on the surface of the ocean because water is fluid and responds more dramatically to the moon's insistent tug. This type of gravitational force is called tractive force, from the Latin tractus, a form of the verb trahere, meaning "to pull."

The sun also has an effect on the tides. Why then, does the moon always get the credit? When measuring tidal forces on Earth, the distance between two objects is more telling than their masses. Our sun is 27 million times larger than our moon and, subsequently, has a greater gravitational attraction to Earth. If tidal forces were based solely on comparative masses, the sun should have a greater tide generating force than the moon. However, the sun is 390 times further from Earth than the moon. Thus, the sun's tide-generating force is only about half that of the moon.

So we'll take the sun out of the equation, for simplification purposes. What if we also took out the moon? If there were no moon, as Earth spun on its axis, ocean waters would be kept at equal levels around the world by the earth's own gravity pulling inward and centrifugal force pushing outward. However, the gravitational force of the moon is strong enough to disrupt this balance by drawing water towards itself, causing a bulge. But it's not just pulling the water. Remember, it pulls everything on Earth, so the earth is also being pulled toward the moon (and away from the water on the far side). The water on the far side is under inertia's influence (the tendency of a body at rest to remain at rest - or in motion to remain in motion - unless acted upon by an outside force). As the earth is pulled away from the water that wants to remain in place, another bulge is formed, opposite and equal the moon's bulge. Over the rest of the planet, gravity and inertia are in relative balance. As the moon orbits and the earth rotates, the bulges stay aligned with the moon. These bulges are responsible for high tides and low tides.

You are familiar with a 24-hour day. That's a solar day, the time it takes for a specific site on Earth to rotate from a point under the sun to that same point under the sun. A lunar day is 24 hours and 50 minutes. Since the moon orbits the earth in the same direction that earth rotates around its axis, the earth needs that extra 50 minutes every day to catch up with the moon. Because the earth experiences two tidal bulges every lunar day (one with the moon, one opposite the moon), coastal areas experience two high tides and two low tides every 24 hours and 50 minutes. Add another variable: the moon's orbit, which happens to be elliptical. When the moon reaches the closest point to Earth in its orbit (the perigee), tides are slightly larger. When it reaches the farthest point (the apogee), tides are slightly smaller.

Add the sun back into the equation. The sun generates a noticeable tidal force, but solar tides are only about half as large as lunar tides and are, therefore, generally expressed as a variation of lunar tides. When the sun and the moon are at right angles to each other, with respect to the earth, the sun's gravity draws some water away from the moon's bulges, leaving only moderate tides, called neap tides. These tides occur during quarter moons. When the sun, moon, and earth are in alignment, the gravitational forces of the moon and sun combine, creating extra large bulges, which results in very high high tides and very low low tides. These occur during the full and new moons and are called spring tides, from the German springen, meaning "to leap." The Proxigean Spring Tide is a rare high tide, even higher than normal spring tides. The new moon occurs when the moon is between the sun and earth; the perigee is the point in the moon's orbit when the moon is closest to the earth; and the Proxigean Spring Tide occurs when the new moon is at its closest perigee (the proxigee). This tide occurs, at most, once in every year and a half.

The distances and positions of the sun, moon, and earth all affect the two tidal bulges. At a smaller scale, the tides can be influenced by additional non-astronomical factors, such as configuration of the coastline, local depths of the water, the shape of the ocean floor, etc.

Subtract the continents. If the earth were a perfect water-covered sphere, uninterrupted by landmasses, the globe would experience two equally proportioned high tides (and low tides) per lunar day the bulge towards the moon and the bulge opposite the moon. However, continents constantly block the bulges as the earth rotates. When these ocean tidal bulges hit wide continental margins, the height of the tides can be magnified. Unable to wash over the large landmasses, the tides find alternate routes, creating complex patterns that differ from region to region, even within the same ocean basin. *Fun fact: tides travel west.

The Bay of Fundy in Nova Scotia is a classic example of how the shapes of bays and estuaries can alter tides. Funnel-shaped bays, like this one, can dramatically increase tidal magnitudes; this one has the highest tides in the world, over 48 feet (more than four stories). FORCE, the Fundy Ocean Research Center for Energy, estimates the Bay of Fundy pushes 110 billion tons of water with every tide. Conversely, narrow inlets and shallow water tend to dissipate tides. Inland bays, such as the Laguna Madre, are even classified as non-tidal. Estuaries with powerful seasonal river flows, such as the Delaware River, can alter or even mask incoming tides.

Where rivers empty directly into the sea, instead of into a bay or estuary, a tidal bore can form. A tidal bore is a strong tide that pushes up the river, against the river's current. The Amazon River's tidal bore reaches 13 feet tall, travels up to 9 miles an hour, and invades 6 miles of the river. It is a true tidal wave, unlike a tsunami. Tsunamis are not caused by tides. Also, red tides have nothing to do with actual tides; nor, for that matter, do rip tides. All this bad press, and the tides were innocent all along...

Weather patterns can affect tides, locally. Strong winds can push water away from shore, exaggerating low tide; or drive water onto the shore, practically eliminating low tide. According to NOAA, "high-pressure systems can depress sea levels, leading to clear sunny days with exceptionally low tides; [and] low-pressure systems that contribute to cloudy, rainy conditions typically are associated with tides than are much higher than predicted."

Despite the many complex tidal patterns found in the nooks and crannies of Earth's shorelines, the major shorelines experience one of three basic tidal patterns.

1) Diurnal tide: one high tide and one low tide every lunar day. Many areas in the Gulf of Mexico have this tidal cycle.

2) Semidiurnal tide: two high tides and two low tides (where the two high tides are of approximately equal size, as are the two low tides) every lunar day. Many areas on the eastern coast of North America experience this tidal cycle.

3) Mixed semidiurnal tide: two high tides and two low tides (where the high tides are different size, as are the low tides) every lunar day. Many areas on the western coast of North America experience this tidal cycle.

Back to the nooks and crannies. Tides definitely have a global impact, but they also contribute to smaller, localized ecosystems. High tides transport sand and sediment that shapes the shorelines. Estuaries depend on tides to replenish nutrients and remove pollutants, in order to sustain biologically diverse communities. Tides also transport floating animals and plants between breeding areas in the estuaries and deeper waters.

The part of the shoreline directly affected by tides is called the intertidal zone. Many organisms have evolved to live in this particular type of ecosystem, from seals and sea otters to snails and hermit crabs. The intertidal zone can be broken down into four mini-zones.

1) Splash zone: this area is only splashed by water and mist during high tide; it is never fully submerged. Here, you can find the seals, sea otters, etc.

2) High-tide zone: this area is pounded by strong waves. Only animals with strong shells and the ability to cling tightly to rocks live here. These animals include mussels, barnacles, crabs, etc.

3) Mid-tide zone: this is where tide pools are usually found, and is the busiest mini-zone. Mobile animals from all other mini-zones come here to feed. Animals that live in this zone can have softer bodies than those that live in the high-tide zone, but they must still be strongly anchored to the substrate. These include anemones, snails, hermit crabs, etc.

4) Low-tide zone: this area is only exposed at the lowest tide. It is the home of sea slugs, and the harvest ground of humans. People with simple nets can catch fish easily here; others can collect crabs, clams, mussels, etc. It is a plentiful resource for many cultures.

Predicting tides has always been important to people who make their livelihood off the sea. Both commercial and recreational anglers use a knowledge of tides to improve their catch rates. Combined with a knowledge of local species, water depths, and underwater topographies, anglers can predict when and where fish may concentrate. Knowing when and which certain places are safe to navigate a boat through is useful as well, not only to fishing vessels, but also to merchant and cargo vessels. The swelling of marine traffic and the expansion of ship sizes over the last few centuries make predictions of water-level, tides, currents, and weather increasingly important. Coastal construction projects, such as bridges, breakwaters, channels, and docks, also require monitoring of tide levels.

But how to predict the tides? Though a definite relationship exists between the moon and the tides, providing one factor of predictability, there are so many other factors involved that it is not feasible to predict tides purely from a knowledge of the positions of the moon (and sun). A record of actual observations of tides in many areas over an extended period of time, combined with astronomical calculations, is necessary to achieve maximum accuracy. This particular period of extended time is 18.6 years; the amount of time during which all significant astronomical variations of tides will occur. To this effect, NOAA maintains a network of 140 tide gauges along the coasts of the US (and a few other places). Tidal energy is also a renewable resource that is just starting to be harnessed in places like Ireland, South Korea, and the US.

Someday we will harness the rise and fall of the tides and imprison the rays of the sun.
~ Thomas Edison (or possibly Nikola Tesla, but that's another story)

Where I learned about tides, and you can too!


Education Portal

Smithsonian National Museum of Natural History: Ocean Portal

National Geographic

Moon Connection

Enchanted Learning

Live Science

University of Alaska Fairbanks

HiWAAY Information Services