# The Naked Scientists Forum

### Author Topic: Why do we have two high tides a day?  (Read 17165 times)

#### ProjectSailor

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##### Re: Why do we have two high tides a day?
« Reply #50 on: 01/10/2015 11:34:13 »
yes the space elevator is science fiction.. however i dont see what it has to do with tides.. which are caused by gravitational attraction from the sun and moon... for something that has been known for centuries, if not millenia, I have a book that accurately tells me in advance the heights of all the tides in a year and the times.. and are accurate within reason (weather effects have a great deal to play in some areas)

If you think we don't understand them at all.. how is this possible? we can even predict tides in areas where the land plays a massive part (southampton is a great example)

you are right in one thought though, centrifugal forces will play a part but since this is constant it will have little effect on TIDES but mostly on CURRENTS but are not the only effect on currents!

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #51 on: 03/10/2015 12:31:27 »
ProjectSailor, #49
Thank you. Sorry if I didn´t put it clearly enough.
Regarding your last point, of course currents are due not only to tides, but also to local geographical conditions.
And I brought up the space elevator article just to show the use of centrifugal force concept as a real thing, by scientists, and in a similar way to mine on my posts #20 and following ones, when trying to explain why there are two high tides per day.
You know, there are people who strongly refuse even the use of the term "centrifugal force". They consider it ONLY as an inertial, kind of ficticious force. For them ONLY centripetal force actually exists, producing the centripetal acceleration that changes the direction of the considered object, making it rotate …
Please have a look at first thread posts. F. e., at #9 where it is said:
"I was lead to believe that "centrifugal" was a dirty word in physics?”
For me without the presence of centrifugal forces it is not actually possible to understand the existence of not only a high tide “following” Moon´s location (as you say "caused by gravitational attraction from …), but also another at antipodes (not considering Sun´s effect for the sake of simplicity).
In several posts I tried to explain my ideas, as you can see if some available time (posts 20, 26, 28 and 31 are suggested). There was a rather nasty discussion with “PmbPhy”, some comment/question (#43 and 46), and then long silence (until my #48).
Had you any doubt and/or opposite idea, please kindly post it. A rational discussion is always enriching for both sides.

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #52 on: 13/10/2015 12:26:56 »
(Sorry I wrote #49 instead of #51 in last post first line)
Listening to something about detection of exoplanets, I´ve met something that should be useful to those who don´t accept “my” theory of Earth´s wobbling due to its rotation, together with our Moon, around their barycentral axis, being that what causes the high tide at the Earth´s side opposite to the Moon (with its subsequent centrifugal forces)
Even very, very distant stars, if a planet is turning around them, experience that wobbling movement. And that is being used trying to detect exoplanets:

"...the fact that a star does not remain completely stationary when it is orbited by a planet. It moves, ever so slightly, in a small circle or ellipse, responding to the gravitational tug of its smaller companion".

http://www.planetary.org/explore/space-topics/exoplanets/radial-velocity.html

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #53 on: 01/11/2015 12:22:34 »
Apart from the issue of the wobbling I referred to in my last post, I previously exposed the other "leg" of my argument: the reality of centrifugal force.
Several people argued against it. A link to a scientific paper showed that the author even had made a "bizarre" (at least to me) physical and mathematical explanation just because he had apparently forbidden himself the use of the discussed term (centrifugal f.).
I also said (#42) that in a wikipedia page something similar happens ...
Am I wrong, alone against everybody?
No. Recently, when discussing other related item (tidal locking), I found in wikipedia an explanation similarly erroneous to me, which I consider could also be properly exposed considering centrifugal forces ... I decided to check thoroughly more internet pages.
RESULT: even in the wikipedia page titled "centrifugal force", where initially the inertial, kind of ficticious character of the concept is exposed, going down one can find:
"In another instance the term refers to the reaction force to a centripetal force. A body undergoing curved motion, such as circular motion, is accelerating toward a center at any particular point in time. This centripetal acceleration is provided by a centripetal force, which is exerted on the body in curved motion by some other body. In accordance with Newton's third law of motion, the body in curved motion exerts an equal and opposite force on the other body. This reactive force is exerted by the body in curved motion on the other body that provides the centripetal force and its direction is from that other body toward the body in curved motion"
AS I ALSO SAID, if the body has a relatively big size, and centripetal acceleration is exerted by gravity from other celestial object, different parts of it experience different gravitational forces, not matching with the required centripetal acceleration. The closer the part the bigger the gravitational attraction (to the square of the distance), BUT the smaller the required centripetal acceleration (all parts are obliged to rotate at same angular speed).
That HAS TO BE compensated by internal stresses/forces, all in "couples" action/reaction (following Newton´s third law), being half of them centrifugal, and not ficticious but REAL. And the other half centripetal (in addition to gravitational ones).
The distribution of all those internal forces produces the deformation of the body, being one of the results the two opposite sea surface bulges (high tides), that move around the planet due to its rotation, one always trying to catch up with Moon´s relative position and the other with the point furthest to the Moon (not considering Sun´s effect, for the sake of exposition clearness)

#### Wiz

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« Reply #54 on: 28/07/2016 11:39:30 »
This answer is wrong. The bulge on the opposite side of the earth is caused by a centripetal force created by the earth's rotation, which is in effect trying to throw the oceans off into space.

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #55 on: 03/10/2016 10:56:33 »
I´m afraid you haven´t carefully read my last post, let alone many others of last year, when I gave many examples and explanations.
Don´t try to read them now. It would be too time consuming.
But please kindly read what I happened to post just yesterday, because last month, once again, another discussion relative to centrifugal force started. And without agreeing in the basics of that concept, it would be useless to discuss the four tides a day subject:
"Imagine you are rotating a weight, with the help of your hand (and wrist) and a string. Somebody has already put this case.
Let us put a dynamometer between weight and string. It will show the centripetal f. that is producing the rotational movement (the dynamometer pulling the weight).
But the ACTION AND REACTION principle says that if that mentioned force exist,  the weight is also pulling the dynamometer with another opposite and equal force. That is a REAL force, and CENTRIFUGAL.
By the way, the same could be said in relation with the knot between the string and the dynamometer ... And this instrument functions with two opposite forces applied at its extremes. At the inner one it would be the centripetal force (the string pulls inwards the dynamometer ), and at the outer one the centripetal force (the weight pulls outwards the dynamometer)"
Should you accept this for me cristal clear question, please let me know. Then I would suggest which of my last year post could more easily make you understand my vision of sea tides, because when the pull is a "tele-pull" (gravity), instead of direct through a string, the application of the action and reaction principle is rather trickier.

#### evan_au

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##### Re: Why do we have two high tides a day?
« Reply #56 on: 03/10/2016 23:04:30 »
Quote from: ProjectSailor
I have a book that accurately tells me in advance the heights of all the tides in a year and the times.. and are accurate within reason (weather effects have a great deal to play in some areas). If you think we don't understand them at all.. how is this possible?
They don't generate those books by calculating the tides from first principles.

Observers use Fourier Analysis, and by observation over a long period they work out the relative frequency, amplitude and phase of each contributor to the tide at a particular port; some observers have identified over 300 contributors (of which the position of Sun and Moon are major contributors), although fewer are relevant at any individual port. By using these historical Fourier coefficients, they can predict future tides.

See: https://en.wikipedia.org/wiki/Theory_of_tides#Harmonic_analysis

Quote from: Wiz
The bulge on the opposite side of the earth
The bulge of ocean water does not stay on the opposite side of the Earth from Moon or the Sun, for two reasons:
1) The North-South continents are in the way, and
2) Even if the Earth had no continents, and was an ocean of uniform depth, calculations suggest it would take 30 hours for this bulge to propagate around the Earth, not the slightly over 12 or 24 hours as observed.

In fact, the tidal bulges (there are several) remain in their own ocean basin, and rotate (roughly) once per day or twice per day, depending on the local geography.

Attraction of Sun & Moon drive these tidal bulges; whether it is primarily driven by the diurnal (roughly 24 hours) or semi-diurnal period (roughly 12 hours) depends on the resonant frequency of the individual ocean basin.

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #57 on: 04/10/2016 11:43:54 »
#56
I consider you are right that tidal bulges depend on many more facts than Moon and Sun interactions with Earth, and that N-S large continents are very important global factors.
But within E-W large water basins the (I would say) nº1 bulge, due to lunar attraction, logically happens bellow the Moon. But the very high tangential speed due to Earth´s rotation (40,000 km/24h) makes impossible that position, and it puts the bulge some distance from Moon´s vertical, towards East.
And Moons attraction makes that bulge to be always changing its longitudinal position. We could say the bulge is continuosly trying to get a bellow Moon position (not considering coastal effects)
That´s why, f.e., with new Moon (Sun´s effect adds to Moon´s) there is always a high tide some time after 12:00 solar time.
So I consider that:
"2) Even if the Earth had no continents, and was an ocean of uniform depth, calculations suggest it would take 30 hours for this bulge to propagate around the Earth, not the slightly over 12 or 24 hours as observed"
could not be correct. For mentioned "nº1" bulge it would take 24 h + app. 50 min., this delay due to Moon´s eastward movement in 24 h. (by the way, the same dayly delay of tides in not rare locations).

#### evan_au

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##### Re: Why do we have two high tides a day?
« Reply #58 on: 04/10/2016 21:52:51 »
Quote from: rmolnav
nº1 bulge, due to lunar attraction, logically happens below the Moon
This implies that the nº1 tidal bulge should start from the Eastern end of an ocean basin and move to the Western end, following the Moon.
- But what happens when this tidal bulge do when it reaches the western end? Where does all the water go?

They really didn't know the details, until the arrival of global ocean radar mapping from satellites*.

What actually happens is that if you look on a world map, the tidal bulge circulation tends to go anticlockwise in the Northern hemisphere, and clockwise in the Southern hemisphere. (This might suggest some influence from the Coriolis effect?) The bulge returns to the place where it started.

These tidal bulges circulate around a region in the open ocean that has a very low tidal range, or around an island (there is a particularly nice circulation around New Zealand).

The tidal variation in the Indian ocean actually splits in two, with half going east-to-west, and half going west-to-east. Nothing quite so simple as following the Moon...

If that doesn't work, try www.youtube.com/watch?v=5zi7N06JXD4
*Ocean-watching radar may have been promoted by the desire of the military to detect submarines via gravity waves (gravity waves are much easier to generate and detect than gravitational waves). But the rest of us benefit from having better visibility of weather at sea, and better weather prediction on land.
« Last Edit: 05/10/2016 20:37:08 by evan_au »

#### chris

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##### Why are there two high tides a day?
« Reply #59 on: 04/10/2016 23:25:50 »
This answer is wrong. The bulge on the opposite side of the earth is caused by a centripetal force created by the earth's rotation, which is in effect trying to throw the oceans off into space.

What?

A centripetal force acts towards the centre of rotation...

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #60 on: 05/10/2016 18:39:56 »
#59
You are quite right. Centripetal force obliges considered objects to follow a circular path, instead of a straight line.
But those objects (o parts of matter, such as water particles) somehow have to exert an opposite and equal force on the prime mover (the one that causes the centripetal force), following action and reaction principle.
I say "somehow" because there are many ways, depending on each particular case. In the case of the 2nd high tide (opposite to the Moon) is rather tricky.
I would suggest you to read my reply #55, where a very simple case is exposed.

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #61 on: 05/10/2016 19:17:21 »
#58
"- But what happens when this tidal bulge do when it reaches the western end? Where does all the water go?"
Tides are kind of very soft waves. High tide "excess" of water is not moving (relative to earth) horizontally, water level basically just oscillate vertically. Forgetting local effects, wether included, that is the result of all acting forces, mainly weight, lunar and solar attraction, and centrifugal forces.
When western basin ends, that physical limitation changes the scenario. Tidal effects continue on solid earth, but deformations are logically much, much smaller. And water tends to go back to a spherical shape.
Then, depending of each case, quite interesting phenomena (as what mentioned by you) can occur. But next day, when when Moon reaches the eastern edge of the basin, high tide and the bulge "chase" of the Moon occur again.

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #62 on: 06/10/2016 11:23:42 »
#58 (continuation)
We should also keep in mind that we say "bulge", but this term is just a massively exaggerated analogy ... Sea surface deformation is relatively very, very tiny. Even strongest tides are just in the order of app. a couple of tens of meters. Locally it could be consider something big, but that hight decreases very, very slowly from high tide place to low tide places, app. 90 degrees eastward and westward the Moon, along 10,000 km ... (at the equator) (!!!)

#### rmolnav

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##### Re: Why do we have two high tides a day?
« Reply #63 on: 01/12/2016 12:08:44 »
Yesterday I sent some comments to a Scientist I am discussing with, relative to cause of sea tides, and centrifugal forces.
As it could interest other people, I am going to put them here.
Previously I have to remember something additional, necessary to understand my arguments.
Many people don´t realize that "the Moon rotates around the Earth" is not 100% exact … Cosmic rotating objects actually rotate in pairs, around their barycenter, kind of center of gravity of whole masses of the pair.
If one of the objects is much more massive, then they are still orbiting about their barycenter, but the “center of mass” is located inside the larger object, at some distance from its C.G. That is the case of Moon "around" the Earth:
"It is quite clear, for me too, that the addition of all forces of attraction from the Moon, what produces is the required centripetal acceleration for the whole of the Earth.
But, as far as I can understand, that is not the whole picture.
I see you yourself have been very close to the "edges" of my stand when saying:
"If you're on the side of the Earth facing away from the Moon, you are being pulled slightly less than the Earth as a whole, so your centripetal acceleration due to the Moon is a bit LOWER".
Please kindly note those places further from the Moon are also further from the axis of rotation. And required centripetal acceleration, for same angular speed as the whole Earth, must be actually HIGHER, proportionally to the radius.
The bulk of our planet kind of obliges both closer and further parts to rotate at same angular speed.
Any portion of Earth, either liquid or solid, if only with the rotation movement, has to satisfy the law that the addition of all force vectors acting ON it, divided by its mass, has to be the required centripetal acceleration.
That only can be reached through additional forces, resulting from internal stresses.
Let us imagine a narrow cylinder connecting closest to and furthest from the Moon Earth surface points (I suppose isotropic material and perfect spherical Earth, for the sake of simplicity), and let us divided it into e.g. 1 m pieces.
Forces acting laterally on those pieces would be equilibrated.
And longitudinally acting forces on each of them would be own weight (attraction from the whole Earth), attraction from the Moon, and tensile or compressive stresses/forces exerted by contiguous pieces. Shear stresses would also be present, but for simplification I won´t take them into consideration.
Let us consider four contiguous pieces A, B, C and D at furthest half of the Earth, A the closest to Moon and axis of rotation.
Between contiguous pieces stresses/forces are opposite but equal (3rd Newton´s Principle).
Required centripetal acceleration of B is slighty bigger than A´s, and slightly smaller than C´s. Considering only changes in comparison with when no rotation, one can deduce that the addition of forces on B (exerted by A and C) has to be centripetal, but smaller than what exerted on C by B and D, because the further from rotation axis, the bigger Moon´s attraction "deficit".
That means that every pair of contiguos pieces are pulling each other, half of those forces inwards (centripetal), and the other half outwards, for me CENTRIFUGAL forces (because they are in the sense of fleeing from a center).
I know that word is considered "politically incorrect" among many Physicists … But, whatever we call them, they are REAL forces, which apparently have been obliterated by many.
And I can´t really understand why! Those forces are present across the universe: planet/satellites, star/planets, twin stars … For me, Inner bulges are due to the higher attraction from the other massive object, but at outer bulges those centrifugal forces are paramount ...

#### Janus

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##### Re: Why do we have two high tides a day?
« Reply #64 on: 02/12/2016 18:36:14 »
In pdf linked in reply #23, I can see that Schutz also mentions only the different gravitatory pulls from the Moon …
I repeat the question on my post #22:
"If those three zones of the Earth experience only those gravitatory accelerations from the Moon, why ALL of them don´t cause a movement of the Earth towards the Moon? “
Tomorrow I´ll try to explain my “theory” of the centrifugal force used in my first post, which I consider necessary to have a correct explanation of the issue. I´m slow when typing in English, and have no time now. But just an analogy that I consider useful:
Imaging an athlete of hammer trow speciality. He or she can´t keep verticality when throwing the hammer. It is necessary to lean a little backwards. Otherwise the hammer could not be rotated. Both the athlete and the hammer will rotate around an axis situated near the forward part of the athlete´s body.
If the hair of the athlete is long and not  fixed by some device, instead of keeping its normal downward direction due to its weight, it will move back and upwards …
That cannot be due to anything similar to what stated by Schutz relative to tides: CENTRIFUGAL force is the cause.

Both the Earth and Moon are in free fall.  So would be an Earth-Moon pair falling directly towards each other.  IOW, if you were to stop both the Earth and Moon in their orbital motion, so that they suddenly started to fall directly towards each, the tidal bulges would not change in shape at that moment.  If the Moon had been falling in towards the Earth from some far distance, at the moment it reached the present Earth-Moon distance, the tides would be the same as they would be for the Moon orbiting at that distance. The relative motion of the Earth and Moon with respect to each other doesn't matter.

#### evan_au

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##### Re: Why do we have two high tides a day?
« Reply #65 on: Today at 06:33:44 »
Quote from: Janus
If the Moon had been falling in towards the Earth from some far distance, at the moment it reached the present Earth-Moon distance, the tides would be the same as they would be for the Moon orbiting at that distance.
Resonance is one factor in the height of tides - the most extreme tides in the world (eg near Anchorage in Alaska) occur because the resonant frequency of the ocean basin is close to the driving frequency of the Earth's rotation (slightly over 12 hours between high tides).

If the Earth & Moon were falling directly towards each other (without the Earth rotating), these resonances would not occur, and these instances of extreme tide heights would not occur.

Similarly, there are regions in the middle of Earth's oceans which have almost zero tidal range (amphidromic points), because the oscillations cancel out at that point.  If the Earth & Moon were falling directly towards each other, these oscillations would not occur, and I expect that the distribution of amphidromic points would be quite different.

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##### Re: Why do we have two high tides a day?
« Reply #65 on: Today at 06:33:44 »

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