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Diving Deeper: Episode 6 (Apr. 8, 2009) —
What are Tides?


HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host Kate Nielsen.

Today’s question is….What are tides?

Tides are basically very long-period waves that move through the oceans in response to the forces exerted by the moon and the sun. Tides begin in the oceans and then move towards the coast where they appear as the regular rise and fall of the sea surface.

To help us dive a little deeper into this question, we will talk with Steve Gill on tides – what they are, what causes them, and the factors that affect them. Steve is the Senior Scientist with the Center for Operational Oceanographic Products and Services. Hi Steve, welcome to our show.

STEVE GILL: Hi Kate, thanks, it’s good to be here to talk about a topic that I have studied and worked on for over 33 years. Much of the practical application of tides is something that comes from on-the-job training and not learned in text books, so what the NOAA Tides and Currents program does is fairly unique.

HOST: Steve, first, what is the difference between a tide and a current?

STEVE GILL: Well, Kate, that’s a good question and typically the first thing I cover in many of my talks with students. The word “tides” is a general term used to define the alternating rise and fall in sea level with respect to the land. So, tides are characterized by water moving up and down during the day. Currents on the other hand move horizontally rather than vertically. Currents describe the horizontal motion of the water and are driven by several factors, one of those is tides; another is the wind.  The horizontal movement of water that accompanies the rising and falling of the daily tides is called the tidal current.

HOST: Thanks Steve, so basically tides move up and down and currents move back and forth. What causes tides?

STEVE GILL: Gravity is one of the major forces that causes tides. Tides are caused by the gravitational pull of the moon and the sun. The gravitational forces are counterbalanced by the outward force of inertia from the moon revolving around the Earth and Earth revolving around the sun in their orbital paths. The combination of these two forces results in the tide-producing forces. So, ocean tides are a combination of lunar tides (lunar meaning the moon) and solar tides (solar meaning the sun).  

HOST: So what does this mean exactly, the gravitational pull of the moon and the sun and the outward force of inertia?

STEVE GILL: Well, that can be a little confusing, but let me provide a little more background. In 1687, Sir Issac Newton first found that ocean tides can be explained by the gravitational attraction of the sun and the moon on the oceans of the Earth. In simpler terms, Newton’s law of universal gravitation states that the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them.   The outward inertial forces counterbalance gravity; that is why the moon doesn’t fall towards the Earth in its monthly orbit and why the Earth doesn’t fall towards the sun in its yearly orbit. Put together, these forces result in the distance between the two objects being much more critical than their masses in forming the tide-producing forces on the Earth.

HOST: Steve, can you break this down for us a little bit more?

STEVE GILL: Certainly. Our sun is about 27 million times larger than our moon. Based on its mass, the sun’s gravitational attraction to the Earth is more than 177 times greater than that of the moon to the Earth. If tidal forces were based only on these masses, the sun should have a tide-generating force that is much greater than that of the moon. However, the sun is 390 times further from the Earth than the moon is, reducing its tide-generation force. Because of this, the sun’s tide-generation force is only about half that of the moon.

HOST: OK, so mass and distance combined are needed to fully understand the gravitational pull of the moon and the sun. And the orbits of the Earth around the sun and the moon around the Earth are needed to understand inertia. Is there anything else we need to know about tide-producing forces?

STEVE GILL: Actually, yes. The tide-producing forces can be thought of in terms of a tide-generation force envelope surrounding the Earth. This tidal-force envelope has bulges; one facing the moon and one on the opposite side of the Earth from the moon. There is a similar tidal-force envelope for the sun. On the near side of the Earth, the gravitational forces are greater than the outward forces of inertia, resulting in a bulge in the envelope toward the moon and the one for the sun. On the far side of the Earth, the forces of inertia exceed the gravitational forces, resulting in an equal, but opposite facing bulge. These forces are not actually strong enough to pull the ocean away from the surface of the Earth however. The tides are caused by the oceans being moved back and forth in their basins as they rotate underneath the tide-generating force bulges. The back and forth motion results in a tide wave, not a tidal wave or tsunami that you sometimes hear about. This tide wave is set up in each of the ocean basins.

HOST: So, how is this back and forth motion of a tide wave different than a tsunami?

STEVE GILL: A tsunami is set up by underwater seismic earthquake and they’re actually much higher frequency wave. They happen with peaks and troughs every several minutes as opposed to a tide wave which is every 12 hours or every 24 hours. This is a very high speed wave in the open ocean and you really don’t see that wave until it reaches shore.

HOST: Steve, what is the difference between high tide and low tide?

STEVE GILL: When the highest part, or crest, of the tide wave reaches a particular location, high tide occurs; low tide is the lowest part of the tide wave or trough. The difference between high tide and low tide is called the tidal range. Most people experience this difference when they are walking along the beach and perhaps notice either more or less beach area for a place to stop, sit down, or rest. I know my children would have fun building a series of sand castles further and further up the beach throughout the day as the tide came in and washed them out. Tides on all coasts originate in the oceans and travel onto shore and up into the estuaries, bays, and rivers.

HOST: Thanks Steve, it sounds like there are a lot of factors that do affect tides. You just mentioned that these tidal bulges have a direct effect on tidal heights. Do these tidal bulges impact how often tides occur?

STEVE GILL: Kate, yes they do. Tidal bulges do play a role in the frequency of tides. Remember that there are lunar and solar tides because there are separate tidal-force bulges for both the moon and the sun. Most coastal areas experience two high tides and two low tides every lunar day. A lunar day is the time it takes for a specific site on Earth to rotate from an exact point under the moon to the same point under the moon the next day. Unlike a solar day which is 24 hours, a lunar day is 24 hours and 50 minutes because the moon revolves around the Earth in the same direction that the Earth rotates on its axis; so it takes the Earth an extra 50 minutes to catch up to the moon. 

Because the Earth rotates through two opposite lunar tidal bulges every lunar day, most coastal areas experience these two highs and two low tides every 24 hours and 50 minutes.  High tides occur approximately 12 hours and 25 minutes apart and it takes about six hours and 12 minutes for the water at the shore to go from a high to low, or from a low to high. The high and low tides tend to occur about 25 minutes later each calendar day because the lunar day is longer than our 24-hour clock day. However, there are a few areas at which the solar tide-producing forces dominate those of the moon. In those very few areas, the high and low tides tend to occur 12 hours apart and at around the same time each day.

HOST: Steve, why does the difference between high and low tides vary more in some places than in others?

STEVE GILL: Well, Kate, if the Earth were a perfect sphere covered by one ocean without large continents, all areas on the planet would experience two equally proportioned high and low tides every day. However, these large areas of land divide the oceans into large ocean basins, each reacting to the tide-producing forces in their own way, depending upon their size and depth. This also brings up another important point on how tides can vary over time and by location. The lunar and solar tidal bulges are not always aligned with the plane of the Earth’s equator, and their alignment tracks the moon and the sun as they change their declinations. The declination can be thought of as angle in the sky of the moon or sun above the horizon. The moon has a maximum and minimum declination each month because the plane of the moon’s orbit around the Earth is not the same as the plane of the equator.  Similarly for the sun – the sun has a minimum and maximum declination each year because the plane of the Earth’s orbit around the sun is not the same as the plane of the equator.   This solar declination is also what causes the seasons on Earth. The different responses to the tidal bulges and their declinations cause the ocean basins to have varying tidal patterns.

There are three basic tidal patterns that occur along the Earth’s major shorelines. Most areas have two high and two low tides each day. If an area has two highs and two low tides each day that are about the same height, the pattern is called a semi-daily or a semidiurnal tide. This is true for most of the East Coast of the United States. If the two high and two low tides each day differ in height, the pattern is called a mixed semidiurnal tide, like you may see along the West Coast of the United States. Some areas, such as the northern Gulf of Mexico, have only one high and one low each day. This is called a diurnal tide.

HOST: Thanks Steve for your explanations on the causes, frequency, and intensity of tides as well as the role of gravity with the position of the Earth, moon, and sun and how these cause tides. Are there other factors that affect tides?

STEVE GILL: Kate, yes there sure are. Again, remember that there are lunar tides and solar tides and the tides we observe on the coasts are a combination of the two. So each month there are full moons and new moons in which the sun and moon are aligned and their gravitational attraction on the Earth acts together to cause stronger tides than normal.  These are called spring tides. And there are two times each month when the moon and the sun are at right angles to each other with respect to the Earth to cause weaker tides than normal, these are at half moons, and these are called neap tides. The orbits of the moon around the Earth and the Earth around the sun are also not perfect circles, but they’re elliptical or oval in shape. This results in stronger lunar tides each month when the moon is closer to the Earth in its orbit and stronger solar tides occur each year when the Earth is closest to the sun, that typically occurs in early January. 

So, while the gravitational pull of the moon and the sun is the main factor, on a much smaller scale, the magnitude of local tides can be strongly influenced by the shape of the shoreline. When oceanic tides hit wide continental margins, the height of the tide can be magnified when compared to very small tides at ocean islands not near the continental margins. Also, the shapes of bays and estuaries can magnify the intensity of tides and some shallow bays, lagoons, and rivers can also lessen the intensity. The Bay of Fundy in Nova Scotia in Canada is a classic example of the magnification effect and has the highest tides in the world at more than 15 meters or approximately 49 feet. Cook Inlet in Alaska is a similar example with the highest tides in the United States.

And finally, local wind and weather patterns can also affect tides. Strong offshore winds can move water away from coastline, exaggerating low-tide exposures. In many areas with very weak tides, such as the shallow Chesapeake Bay and areas of the Gulf of Mexico, the weather changes in wind and barometric pressure can affect the water levels as much or more than the tides.

HOST: So Steve, why do we study tides?

STEVE GILL: Well, we study tides for a variety of reasons. If we know the times, heights, and extents of both the inflow and outflow of the tidal waters we can better navigate through the intracoastal waterways and within the estuaries, bays, harbors; and we can work on harbor engineering projects such as the construction of bridges and docks; and we can collect data critical to fishing, boating, surfing, and many other water-related sports. We put in tide stations to measure the tides and analyze the data so that we can predict the tides and publish tide tables. And this is just to name a few of the ways that we use tidal data to help us in our daily lives. 

HOST: How can the public access tidal data and information to plan upcoming recreational activities?

STEVE GILL: There are a few Web sites out there that provide local and regional tidal information. You can go to to see current conditions, especially during storms or go to to get historical data and much more information on tides and currents and sea levels. You can go to to see how real-time tides and currents measurements and forecast models are used by the maritime community. 

HOST: Thanks Steve. We also have listeners from many different regions of the U.S. How are some of our non-coastal listeners impacted by tides?

STEVE GILL: Well, by measuring and analyzing the tides, we can produce very accurate tide and tidal current predictions and estuarine models that are used to make sure that our ports are used in the safest and most efficient manner. This affects the whole U.S. economy, as most of the U.S. trade and commerce comes into and out of the U.S. through the major coastal ports. So that affects everyone’s lives.

HOST: Steve, what is the role of the National Ocean Service in studying tides?

STEVE GILL: Kate, the Center for Operational Oceanographic Products and Services in the National Ocean Service is responsible for maintaining a national network of 205 long-term continuously operating water-level stations around the country, including the U.S. Great Lakes, which are non-tidal. We put in many more short-term water-level stations each year for various surveying, engineering, or habitat restoration projects. In addition, our office is also responsible for predicting and monitoring tides and tidal currents, computing tidal datums and sea level elevations, and for computing long-term relative sea level trends along our coasts.

So, the study, measurement, and analysis of tides by NOAA continue to be an important program for our nation so that all maritime users can enjoy use of our coastal resources.

HOST: Thanks Steve for joining us on today’s episode of Diving Deeper and exploring what tides are and what causes them. To learn more about tides and some of the products that Steve mentioned today, please visit the Center for Operational Oceanographic Products and Services Web site at

That’s all for this week’s show. Please tune in on April 22nd for our next episode on estuaries.