THE PHYSICS OF TIDES

Porter Johnson


This lesson was created as a part of the SMART website and is hosted by the Illinois Institute of Technology.

Mt. St. Michel on the north coast of France at low tide (left) and high tide (right).
The water surrounding this island is the Gulf of Sant-Malo.
(Click on the images for a larger photograph.)

Low tide: High Tide:


The tides vary along the shores of the oceans of the earth with a period of about 12.5 hours.  Curiously, the circadian rhythm period of about 25 hours, corresponding to two tidal periods, subconsciously affects a large variety of animal life, including humans.  The period of tides provided our ancestors with a primitive clock.  In many modern languages the words for "tide" and "time" are identical --- in Dutch the word "tijd" has both meanings, and in modern English, the word "tide" is frequently used as a synonym for "time".  Both meanings are interplayed in these famous words:

"There is a tide in the affairs of men, which taken at the flood, leads on to fortune.
Omitted, all the voyage of their life is bound in shallows and in miseries."
Julius Caesar (Act IV Scene 3 line 218 -Brutus) by William Shakespeare

For additional information see the website http://hea-www.harvard.edu/ECT/Words/#time.

The T'lingit tribe of Southeastern Alaska believe that the Raven caused the tides to rise and fall.  The natural rhythm of the tides is felt instinctually by seafaring people everywhere, since they need to know when they can launch their ships,  to go to sea,  or to return home.  Consider this selection from History of the Gallic Wars: [Latin Text or English Translation by Julius Caesar [Book 4, Chapter 29: Caesar in Britain. Heavy Damage to the Fleet].

Eadem nocte accidit ut esset luna plena, qui dies a maritimos aestus maximos in Oceano efficere consuevit, nostrisque id erat incognitum. Ita uno tempore et longas naves, [quibus Caesar exercitum transportandum curaverat,] quas Caesar in aridum subduxerat, aestus complebat, et onerarias, quae ad ancoras erant deligatae, tempestas adflictabat, neque ulla nostris facultas aut administrandi ` aut auxiliandi dabatur. 

...i. e. ...

It happened that night to be full moon, which usually occasions very high tides in that ocean; and that circumstance was unknown to our men. Thus, at the same time, the tide began to fill the ships of war which Caesar had provided to convey over his army, and which he had drawn up on the strand; and the storm began to dash the ships of burden which were riding at anchor against each other; nor was any means afforded our men of either managing them or of rendering any service.

Here is a comment about the tides taken from the  film The Prince of Tides:

"Tides mean everything, Lowenstein. They measure everything. Moods, seasons, the time to plant... ...to fish, to mate. It's primitive. Biological clocks ticking all over the place.""

The tides are the regular rising and falling of sea level. They are caused by the gravitational attraction of the moon and the sun. These celestial bodies pull on the Earth. The solid Earth moves only a tad toward the moon and sun, but the liquid ocean moves more. For more details see the website http://en.wikipedia.org/wiki/Earth_tide,


Questions Concerning Tides

What is the effect of the moon on tides??

A moon raises tides on the planet it orbits. Because of internal friction, the maximum tide height lags the applied force and the bulges are displaced. Tidal torques on bulges oppose and slow down rotation on the moon (until it becomes tidally locked) and the planet When rotation of the planet slows, angular momentum is conserved and the orbital radius of the satellite increases for a prograde satellite, but decreases for a retrograde satellite.  For more details see the website http://en.wikipedia.org/wiki/Tide


Why are there two tides per day?

Qualitative Explanation of Tides:  http://wiki.answers.com/Q/Why_are_there_two_high_tides_in_one_day

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Where are the highest tides in the world?  

The Guinness Book of World Records states the world's highest tides to be in the Minas Basin, NS, with the maximum tidal range recorded at 16.8 meters (54.6 ft).  Parrsboro, the largest Seaport on the Minas Basin, affords the best view of this tidal phenomenon. At this point the tide floods and ebbs over 3.2 kilometers (2 mi.) of tidal flat from the low watery mark to the head of the harbor. Each phase of the cycle takes approximately 5hrs\40 min. which results in each succeeding high or low water mark range an average 14m. (45.5 ft.) while the harbor heights are about 7.5 m (24 ft.). For more details see the websites http://www.town.parrsboro.ns.ca/worlds-highest-tides.html and http://en.wikipedia.org/wiki/Bay_of_Fundy

Historical tidal gauge at Anchorage Alaska. 

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://en.wikipedia.org/wiki/Tidal_bore

For 11 months of the year, the water levels of the two oceans on the ends of the Panama Canal differ because of tides and climatic conditions; only in February are they about the same. While the Pacific tides are very high (up to 7 meters) those in the Atlantic are quite low. For details see the website http://www.czbrats.com/PanCanal/Canal_Transit.htm
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What about Earth Tides?

Recent studies have shown that not only are there tides in the oceans, but also in the earth and the air. Only recently, with very highly sensitive instruments, have scientists been able to see a tidal range of about twenty two inches. Compared to the ocean's tides, that amount is very small, but still a change is occurring every day. Also, this crustal tide is due to the same factor as the ocean tides, the sun and the moon. We can neither see, nor feel it, because like on a ship on the sea, we move with the crust's flood and ebb. How did scientists then figure out the earth was moving, when even the instruments on it would be moving as well? Highly sensitive instruments were used, such as a horizontal pendulum, and a gravimeter, which are used to measure the amount of gravitational pull from the moon. Pierre Simon de Laplace, a scientist, discovered tides in the air. It took him eight years of reading his barometer four times every day to come to the conclusion that the air also flows similarly to tides in the ocean. Puzzlingly, though, these tides did not follow the moon, as do the ocean, and earth tides, but instead these tides followed the clock. High tides were at 10:00 am and pm, and low tides were at 4:00 am and pm. Unlike ocean tides, which flow an hour later each day, these times remained fixed. Scientists now believe that they are in phase with the heat of the sun, which caused the pressure to rise and fall. There is some tidal movement like that of the oceans, in the air, but to a very small extent.  Source: http://members.tripod.com/~CovenK/index-5.html.

 

Click for Chart on Gravity and Tidal Action

 


Tide Amplitudes Around the World

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What are Tidal Bores?  

Tidal Bore at Mt St Michel

Sometimes, when the tide enters a river which has just the right geometry, an isolated wave can be produced called a tidal bore, physicists refer to this type of wave as a "soliton". This is a wall of water often moving upstream at speeds of 10-15 miles per hour. The Severn Bore and the Aegir on the River Trent are such examples. 

For more details see these websites:

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What about tides on the moon?

The generally accepted theory is that the Moon's gravitational field causes the Earth to become tidally deformed. This ellipsoidal deformation in the mass of the Earth then applies a gravitational force upon the Moon which accelerates the Moon in its orbit and the Earth-Moon distance gradually increases. The Moon's action on the bulge of the Earth causes a torque that slows the Earth's rate of rotation. The net affect is that, to conserve angular momentum, the Moon's orbit increases at the expense of the Earth's spin slowing down. Ultimately, some energy is also lost from the system as the ocean tides on the Earth slosh against the continental shelves and tidal basins, dissipating energy by piling up sand and other things.  See the website http://www.badastronomy.com/bad/misc/tides.html

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Frequently Asked Questions Concerning Tides: For additional information see the websites http://www.astronomycafe.net/qadir/amoont.html and http://www.pol.ac.uk/home/insight/tidefaq.html.

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Tidal Trivia.

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References

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