Tides

There are several kinds of tides. The ones that break upon a beach every 10 seconds to a minute are caused by sea level disturbances out in the ocean produced by such things as storms. Also, the various circulation currents of sea water can have velocity components directed towards the land which will bring water up onto the beach. As this water travels towards the beach from deep water to shallow water, its amplitude will increase until it finally 'breaks' as a full-fledged breaker, suitable for surfing etc. Now I have never surfed anything except the internet, but I have watched plenty of people enjoy this sport. I have done my fair share of bodysurfing, which may not be as exhilarating, but most certainly beats the boogie boarders who just go out onto the waves and run people over. The key to body surfing is to know when to get into the wave, how to streamline with your head up and eyes open, and get all the way onto the beach, thus making a favorable impression on anyone who might be watching. Closing your eyes is bad, because you might inadvertently run into some foolish junior high kid who is in your way. Then you jam your fingers into his gut, and dislocate the ring finger of your left hand, as I did when I was in my thirties, riding the 10 foot waves of Huntington Beach, CA. Okay, back to tides. Oh, body surfing in the total nighttime darkness is really cool. Huntington Beach has these campfire pits. You go to some local business, buy a bunch of wooded pallets, and make a big bonfire. Then you ride the waves in the dark. Hopefully, you do not mind the sight of some local fisherman hauling in a small nurse shark, 50 yards from where you are bodysurfing in the dark.

Now, underlying this minute to minute wave activity is a slower water wave which causes an alternating pattern of high-tide, low-tide, high-tide, low-tide in most places on the Earth that are directly on the ocean. This roughly 6 hour cycle is caused by the gravitational tugging of the Moon upon the Earth. This 'tidal' pull causes the shape of the solid Earth to be not perfectly round by something like a few dozen yards over its entire 27,000 mile circumference. The Earth gets distorted a small bit, but because it is solid rock its a small effect. The water in the oceans, however, gets distorted into a roughly ellipsoidal ( football-like) shape with a much larger amplitude. The orientation of this shape changes from minute to minute as the Moon orbits the Earth, which is why the high and low tide times change all the time. The Moon's gravity causes these tides by deforming the oceans, and as the Earth rotates under this ocean bulge, it causes a high tide to propagate onto beaches. Because there are two bulges, we get two high tides, and also two low tides each day. If you will recall from our history lesson, Isaac Newton was the first to give a mathematical description of the force of Gravity. Gravity is this invisible force that acts between any two bodies of mass. The force of gravity, otherwise called the force of attraction, is proportional to the square of the distance between them. Understanding the tides is an exceedingly complicated manner, well beyond the scope of this introductory level Astronomy course. For those mathematical details, I would direct you to the NOAA Tides website. For now, I will give a more simplified explanation.

The Moon Tide

The earth and the moon are two great masses that have a significant gravitational pull on each other. This is what keeps the moon in orbit around the earth, and it is also what causes tides to occur in the ocean. Picture the earth with a uniform level of water all around it. The moonís gravity pulls on the earth, and pulls the rock and water towards it. (Yes, I know that my fellow Physics teachers abhor the use of the word "pull" in reference to tides and gravity, when we should be describing "mutual attraction," but the word "pull" will suffice for our purposes right now.) So, I did say that the gravitational force pulls the Earth's rock toward the moon, slightly deforming the entire planet. But since the rock is solid, obviously, the tidal distortion is barely perceptible. However, the liquid water ocean can be deformed more markedly. The water moves up into a slight bulge on the side of the earth that faces the moon. At the same time, there is a force pulling water out in the opposite direction of the moon.

To understand this force, you need to picture the earth and the moon as one unit. Picture two unequal balls on the ends of a stick. The stick can represent the gravitational attraction between the two objects of mass. The barycenter is the term used to described the center of gravitational mass between the two bodies of unequal mass. By plugging the appropriate values into Newton's gravity formula, you will discover that the center of the earth-moon system is over 4700 km from the center of the Earth ... still inside of the Earth's crust. Therefore, the Moon technically does not simply orbit the Earth, but the Earth-Moon system is spinning around this barycenter of mass, some 4700 km from the center of the Earth's core.

 

 

 

 

 

If you look at the system from a distance, you will discern the Earth appearing to move back and forth 9400 km per complete lunar orbit. It is this wobble of the Earth that caused astronomers to try looking for similar wobbles at distant stars, for if a star wobbles, it means that there must be a planet or even planets orbiting it.

 

 

 

 

 

 

 

Now, back to the stick and two ball model. If you spin this stick around, you can imagine the force that a particle might feel if it were on the far end of either the moon or the earth. It would feel a force outward, away from the center of the spin. This is called the centrifugal force. The water on the far end of the earth, away from the moon is always being pulled out from the center of the spinning earth-moon unit. The gravitational and centrifugal forces are constant, always pulling water towards the moon and directly away from the moon. The forces in either direction are equal to each other. The bodies of water that feel these forces change constantly as the earth rotates within these forces, but the force directions are always toward and away from the moon.

But there is more to the story than simple centrifugal force, accounting for the high tide opposite the Moon. There is a change in the gravitational force across the body of the Earth. If you were to plot the pattern of the Moon's 'tidal' gravitational force added to the Earth's own gravitational force, at the Earth's surface, you would be able to resolve the force vectors at different latitudes and longitudes into a radial component directed towards the Earth's center, and a component tangential to the Earth's surface. On the side nearest the moon, the 'differential' gravitational force is directed toward the Moon showing that for particles on the Earth's surface, they are being tugged slightly towards the Moon because the force of the Moon is slightly stronger at the Earth's surface than at the Earth's center which is an additional 6300 kilometers from the Moon. On the far side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it is on the far surface, so the resultant force vector is directed away from the Earth's center.

 

The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal shape due to these tidal forces. This happens because if we resolve the tidal forces at each point on the Earth into a local vertical and horizontal component, the horizontal components are not zero, and are directed towards the two points along the line connecting the Earth and the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two of these bulges. At exactly the positions of the tidal bulges where the Moon is at the zenith and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water gets piled up, and the only effect is to slightly lower the weight of the water along the vertical direction.

Another way of thinking about this is that the gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth.

 

Semidiurnal tides

As the earth turns upon its own axis in about 24 hours, a point on the earth moves through areas with these different forces acting on it. In one rotation (one day), a point on earth travels from an area of high tide (where there is a force pulling water outward), through an area of low tide, through an area of high tide again (the opposite pull), and through another area of low tide, before it returns to the point of origin at high tide. This results in two high tides and two low tides in a day (called semidiurnal tides).

The Tidal Day

The moon does not stay put, but rotates around the earth at a rate of about 12į a day, or one rotation a month. The rotation is in the same direction as the earthís spin, so by the time the earth has done one rotation, the moon has shifted 12į further, and it takes an extra 50 minutes for the moon to be in the same position relative to a point on the earth. Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and low tides are about 50 minutes later every day.

The Sun Tide

The tides are caused mainly by the gravitational attraction of the moon and the earth, but there is also a gravitational attraction between the earth and the sun. The effect of the sun upon the tides is not as significant as the moonís effects. Basically, the sunís pull can heighten the moonís effects or counteract them, depending on where the moon is in relation to the sun. In one month, the moon rotates around the earth. When the moon is between the sun and the earth (at new moon), the sunís gravitational pull is in the same direction as the moonís. During these days the high tides are higher and the low tides are lower than they'd be with just the moonís pull alone. This is called spring tide. The term has nothing to do with a season of the year, but is derived from the German "der springen" which means to leap up. The larger tides leap up more than other tides. The same thing happens when the moon is on the direct opposite side of the sun (full moon). The two gravitational forces work together to make high high tides and low low tides.

"Courtesy of Windows to the Universe, http://www.windows.ucar.edu".

 

 

When the moon is in its first quarter or its last quarter, the sunís gravitational pull is in perpendicular direction to that of the moon. The sun pulls water away from the areas of high tide to the areas of low tides, resulting in lower high tides and higher low tides. These are called neap tides.

"Courtesy of Windows to the Universe, http://www.windows.ucar.edu".

 

 

There may be even weaker tides caused by the gravitational influences of the planets Mars and Venus, but they are probably lost in the daily noise of individual tides.

Inclination

The moon does not rotate around the earthís equator, but follows an orbit that is inclined in relation to the earthís axis. Because of this, northern and southern latitudes commonly face only one high tide and one low tide in a day, called diurnal tides. The inclination of the moon changes in relation to the earth on a 19 year cycle. The earthís inclination in relation to the sun also effects the tides. The sunís inclination follows a year-long cycle, and is in highest inclination in the summer and winter months. During these months the "bulges" in the ocean are offset the most from the equator, and it is most likely to encounter only one tide cycle per day, or diurnal tides.

References: Duxbury and Duxbury (1994) An introduction to the World's Oceans, Wm. C. Brown Publishers,4th edition: Dubuque: Iowa. Pinet, Paul (1998) Invitation to Oceanography, Jones and Bartlett Publishers: Sudbury, Massachusetts. Check out this great site: NOAA's tide page - explanation of the tides, and a great glossary of terms related to tides. Home | OceanInfo | Career Info | Ask a Scientist/Answer Archive AquaFacts | OceanNews | Records | Links

Source for this tide information is OceanLink

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When the Earth, Moon and Sun are aligned for Spring Tides, are they highest at Full or New Moon?
Spring tides are about the same height whether at New or Full Moon, because the tidal bulge occurs on both sides of the Earth...the side toward the Moon ( or sun) and the side away from the Moon (or Sun). They will not be equally high because the distance between the Earth and Sun, and the Earth and Moon both vary and so will their tide producing effectiveness. The highest Spring tides occur when the Moon is at its closest to the Earth...the so-called Perigee Tide.

Something if great interest to Astronomy students living along coastal areas is knowledge of the tides in greater detail. This Perigee Tide can be amplified to produce the maximum possible high tide. In a very rare occurrence, New Moon at Perigee, as well as Perihelion will produce the ultimate high tide. It is this particular tide which Patrick Swazy escaped from the tail of Keannu Reeves in the movie "Point Break." Patrick went to Australia, because it is summer down under when Earth is at perihelion. The great wave was amplified by a New Moon at perigee. You can watch the movie to see what happens. Although I have no certainty about the movie director's knowledge of tides, or where the director was aware of this chance alignment, but it makes for good storytelling. In reality, the Billabong Surfing Championship is always held in Australia in December or January, during their summer, and when the Moon is New ... a simple and predictable event to schedule. I am not downplaying the Bonzai Pipeline of Hawaii's North Shore, but Australia is the place to go. The wide Pacific Ocean has lots of time for the tidal bulge to build before it rams into the East Coast and creates big surf ... Kowabunga, dudes!

Although the Sun exerts a gravitational force 180 times as strong as does the Moon on Earth, because the Moon is so much closer, the variation in Moon's force across Earth's diameter is about 2.2 times larger than the variation in the Sun's force. As noted above, it is this variation that produces tides, thus the pair of bulges raised by the Moon are considerably larger than the pair of tidal bulges get in and out of step, combining in step to produce "spring" tides (no connection with the season) when the Moon is new or full, and out of step to produce "neap" tides when the Moon is at first or last quarter. Another factor having a substantial influence on tidal ranges is the elliptical shape of the Moon's orbit. Although the Moon is only 9 to 14% closer at its close point to Earth (perigee) than at its far point (apogee), because the variation in its gravitational force varies inversely as the cube of its distance (the force itself varies inversely as the square of the distance), the Moon's tidal influence is 30 to 48% greater at perigee than at apogee. In the Bay of Fundy the perigee-apogee influence is greater than the spring-neap influence. Although the variation of the Moon's distance is not readily apparent to observers viewing the Moon directly, to observers near the shores of Minas Basin, the three to six meter increase in the vertical tidal range makes it obvious when the Moon is near perigee, clear skies or cloudy!


The Best Places on Earth to See the Highest Tides!! Minas Basin and Bay of Fundy in Nova Scotia. Click on the map to learn more!!!

Anchorage Alaska boasts the world's second highest tides: varying over 40 feet, low to high tide! Bore tide (one of the three highest in the world, and a weird phenomenon: capillary action on a gigantic scale!) occurs 2 hours 15 minutes after low tide; best viewed between Mileposts 101 and 90 Seward Highway (26 to 37 miles from Anchorage). For details see the website http://hometown.aol.com/akvisinfo/data/general/scak.htm.

I have been up in Alaska twice in my life, and one of those times was a memorable 18 day wilderness camping experience with my best friend who I grew up with in Milwaukee. We were looking out over the Anchorage bay on the first day in Alaska, and saw a flock of shorebirds on the gray silt. Curious as to whether these birds were puffins (my absolute favorite bird, but one I have never seen), I began to walk onto the tidal flats to get a closer look. Within a few steps, my foot was sucked into the silt to a depth of 1 foot (no play on words intended). I was stuck, and Marc had to get a long stick to yank me out of the mud. I thought nothing more of the experience until the end of our trip, when we were down by the Portage Glacier. I noticed a sign posted by the tidal flats warning people to stay of the silt, and if they were somehow stuck in the silt, as I was earlier, they were to use the nearby emergency phone to dial 911 and get immediate help. The tides there come in so fast and furiously that people who get their feet stuck in the mud often will drown in place before a rescue team can extract them.

Here are two questions I found while surfing the net for tide information.

How is it that the Sun tide is 46 percent of the Moon tide?
The tidal force difference is the mass ratio times the cube of the distance ratio. For M(sun) = 2 x 10^30 kilograms, M(Moon) = 7.4 x 10^22 kilograms and d(moon) = 380,000 kilometers and d(sun) = 150 million kilometers, this gives you

(2x10^30)/(7.4x10^22) X (380,000)^3/(150,000,000)^3 = 5/11 = 0.46

Copyright 1997 Dr. Sten Odenwald

Why aren't the Atlantic and Pacific coast tides the same?
The nature of tides on the Earth's oceans is very complex. The oceans are, of course, being periodically 'forced' by a number of tidal sources including the Moon and the Sun, but this forcing has a number of different periods and harmonics. The two dominant periods are sue to the Sun and Moon, these are referred to as the S1 and M2 'modes' which have roughly 12 hour periods because they raise TWO water tides on the ocean diametrically opposite each other. But, for a variety of reasons, any given port will not have two high and two low tides each day; also called 'semi-diurnal tides'. A careful monitoring of the tides at any port for several years will show that in addition to the major modes, there are as many as 300 minor or 'harmonic' modes as well.

The World Ocean is a complex dynamic system. The natural velocity of a water disturbance depends on the depth and salinity of the water at each point it passes. When bodies of land circumscribe bodies of water, they produce a collection of resonating systems that favor water oscillations with certain frequencies over others. From among the 300+ harmonics that can be measured, every port and coastal location has its own unique signature depending on its latitude, longitude, water depth and salinity. The result is that the 'two high two low' tide rule can be strongly modified so that the time between successive high tides can be greater than or less that 12 hours in many cases. The result is that for some locations, there can be days when only one high tide occurs. Looking at the Atlantic and Pacific Coast tide tables for 1995, the data for the various 'Standard Ports' showed that virtually all days had two high tides and two low tides in San Diego, San Francisco, New York and Charleston. There were, however a few days every few months when only a single high tide occurred.

Copyright 1997 Dr. Sten Odenwald

Okay, you are finished with tides, and have perhaps "had it up to here" with this information, but hopefully you found some of the material interesting. It is now time to move on to more practical learning opportunities about the Moon. You have covered Structure, Phases, and Tides. Please move forward to Eclipses to learn the most fantastic celestial sights visible to the naked eye, or return to the Moon Introduction, or the Syllabus.

 


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