Tidal Teaser

I think it's got to do with all that liquid sloshing about in the lower parts of the world's surface, and the surface not being quite as liquid as good old H2O.

Why must "the tidal bulges on the sides pointing towards and away from the Sun/Moon must lift up the Earth's surface and the waters of the oceans an equal amount"? Doesn't everything just want to bulge toward the sun/moon?

bhankiii, I'm not too sure I understand your question, but if it is "why is there also a bulge on the opposite side?", here's the modern answer.

As the Earth and the Moon falls towards each other under their mutual gravity, the nearer sides of both experience a slightly stronger gravitational force than their respective centres of mass. Likewise, the opposite sides of both experience a slightly weaker gravitational force than their centres of mass.

This causes a stretching effect on both the Earth and the Moon, which in turn causes 'bulges' on both the nearer and the farther sides. In the transverse (or tangential) directions of both bodies, there is a 'squeezing' effect caused by the tidal gravity.

Does this make sense to you?

It seems like the side closest to the sun / moon would bulge outward away from the earth and toward the sun/moon - that much is obvious. But, it seems like the opposite side would be pulled in the same direction - i.e. into the earth towards the sun/moon, not away from it.

In other words, if I was standing on the side facing the sun/moon, I would feel the earth's gravity minus the sun/moon gravity and grow a little taller. But on the opposite side, I would feel the earth's gravity plus the sun/moon gravity, and get a little shorter.

bhankiii, maybe I have not stated it precise enough: If I said: "Likewise, the opposite sides of both experience a slightly weaker gravitational acceleration towards the other body than their centres of mass", would it have been more comprehensible?

There is a bulge on the opposite side of Earth because the centre of Earth tries to accelerate towards the Moon a higher rate than the opposite side's surface. Why? Because Earth's center is closer to the Moon than the opposite side and "feels more gravity" from the Moon. This difference in acceleration (or force) subtracts from the normal 1g at Earth's surface, which in turn stretches Earth somewhat.

The two 'bulges' are the root cause of the 2 high tides per 24 hours at most places on Earth.

Simple. The effect is not static, but dynamic. The Earth rotates and the Moon also orbits the Earth and there is a lag as the water is "pulled" towards the Moon, which creates more of a tidal current than a simple swelling like that of the crust.

The crust may buldge, but it can't "flow" like water, which is fluid in its motion. If memory serves, the high tide of the water does not exactly synchronize with the position of the Moon above it.

Hero wrote: "If memory serves, the high tide of the water does not exactly synchronize with the position of the Moon above it."

I always wondered about that. It seems some places the high tide comes before the full moon and some places way after it. How's that happening?

A little reading leads me to believe that the whole thing is more complex than we might expect. Not only is there tidal lag, but the topography of the ocean floor is an important element. It seems that the southern hemisphere is where the tidal current starts and it propagates as a wave, so there will be nodes and anti-nodes for the tide as it sweeps north.

Additionally, the Sun is a component of the tide, although only about half as strong as the Moon. Since the two don't always line up the tide min and max vary depending on the positions of the Sun/Moon. I think that might be what you are talking about. Just an educated guess, although. Highest tide should be when the Moon is between the Sun and Earth. Oh, don't forget that the orbits of the Moon and Earth are both elliptical, so perigee and apogee impact tidal forces (cube of the distance rule).

There are tides in the solid of the earth, but the movement is quite small compared to the water motion, as it is not very fluid. Some say it contributes to the heating of the core. Others see a slight connection to triggering of quakes

I think you have hit on it Aurizon. The both experience similar gravitational effect but since the earth is not fluid an not extremely elastic the water exhibits greater movement therefrom.

rcapper, when you said: "They both experience similar gravitational effect but since the earth is not fluid an not extremely elastic the water exhibits greater movement there from.", you are correct if you mean that the Earth's crust flows horizontally much less than the water.

However, the vertical tidal 'stretching' done by the Moon (causing the 'bulges') is dominant in the bulk of Earth and absolutely negligible for the waters of the ocean! The reason is that the Earth has some 6380 Km of radius that can be stretched, while the ocean's depth is negligible in comparison.

This is why the teaser stated: "Since the tidal bulges on the sides pointing towards and away from the Sun/Moon must lift up the Earth's surface and the waters of the oceans an equal amount, . . .", meaning the water simply 'lifts' with the crust.

Let me make sure I understand your initial statement/question. We are trying to establish why there are tides. You note that the "raising up" of the gravitational pull is equal and so why should the water level rise. Am I understanding correctly so far? Assuming so, we also agree that water is not compressible and so we would expect not stretchable either. We also agree that in fact the tides do rise. So, is it not correct to assume that the additional depth of water is provided by flow from other areas? Assuming this is so, the question would then be why is this? I would conjecture that it is for the same reason the tides occur. Since the water is closest and furthest it experiences the greatest acceleration and deceleration, respectively. Additionally, since it is inelastic yet able to flow, water from other areas rushes in to fill the vacuum created by the greater forces nearest and farthest on the axis of attraction. What else could it be? So, in the words of Beavis "Uh, it's because the water like, huh-huh sucks, uh-huh-huh. Am I missing something?

Rcapper, your latest analysis is very close to the truth, but it is still missing one crucial element and there is a flaw in this statement you made: "Additionally, since it is inelastic yet able to flow, water from other areas rushes in to fill the vacuum created by the greater forces nearest and farthest on the axis of attraction."

I will refrain from giving it away yet, so as not to spoil the fun of other participants.

Ok. Nothing is totally inelastic so the term itself is only a relative term but clearly I should have qualified it to say something like "is inelastic and consequently not compressible in so far as the degree to which this effect occurs is insignificant to the observable effect." And your later comment would seem to corroborate that so I'm not sure what is the flaw in the statement unless it is the somewhat ambiguous reference to "greater forces nearest and farthest on the axis of attraction" which was meant to refer to the … hmm, well clearly the greater acceleration on the near side but I suppose really the lesser force on the far side. I suppose I was thinking in terms of greater tendency to not be accelerated but since I process visually I don't always make the most correct translation into words. All that said, I'm not sure that aside from the minor technical accuracies it fixes the solution. Especially since you stated "is still missing one crucial element and there is a flaw…" So, I am thinking that perhaps it has something to do with the change in the shape of the on axis areas further away from spherical than they would normally be. I really can't say why that creates the effect, if indeed it is the cause, because again it is a visual solution and this one I really can't come up with the verbal reasoning to support the picture in my head. It's just more pointy and that sounds kind of silly but that's all I've got.

Rcapper, your: "...so I'm not sure what is the flaw in the statement unless it is the somewhat ambiguous reference to "greater forces nearest and farthest on the axis of attraction" which was meant to refer to the …" is close to the 'flaw'.

These tidal forces do not "lift up" some water so that other water rush in to fill the space! The acceleration difference is in the sub-μg range and utterly incapable of lifting anything! It can stretch Earth a little bit purely because it decompresses slightly over such a huge diameter. This causes in the order of 0.3m 'bulge' in the crust at some places.

This brings us to what I called the "crucial missing element": what is the mechanism for getting the excess water at the two high tides a day? (Some places have only one high a day - a real 'missing element'!).

I think I've given it away now, but let's hear your comment.

Ok I see my chasing the gravitational thing was a result of being uninformed with regard to the scale of things. After all gravitational attraction is pretty weak. I am amazed that it is strong enough to keep bodies in orbit. So how about a big shift. It seems that we have pretty much eliminated any direct action on the water due to gravity so could in be that the bulge in the crust causes a local increase in the centrifugal force experienced by the water due to a slightly higher angular acceleration at the bulge as the Earth rotates?

Rcapper, with: "so could in be that the bulge in the crust causes a local increase in the centrifugal force experienced by the water due to a slightly higher angular acceleration at the bulge as the Earth rotates?" you are close, but not quite there.

Way back in post #5, Hero basically had it when he wrote: "Simple. The effect is not static, but dynamic. The Earth rotates and the Moon also orbits the Earth and there is a lag as the water is "pulled" towards the Moon, which creates more of a tidal current than a simple swelling like that of the crust."

I was bad and did not acknowledge his contribution because it was so early in the thread. Then… in post #8, he went and screwed the whole thing up, simply because he read some more… Shows you a lot of confusion can be spread by the written word! Your high school science or geography book probably also told you the wrong story about the tides.

Because Hero did not give the whole story, which is a long story, I decided to be bad again and post it in my CR4 Blog. So, at the risk of infuriating everyone, I ask you to click through to there and read it. You will see that your centrifugal force idea is not too way off, because NOAA also uses it, but in a slightly different sense.

"Then… in post #8, he went and screwed the whole thing up"

I resemble that remark! ;-)

If you ever take a voyage from a port in the Med, don't wait for the tide, you'll be there a long time!

I always assumed it was because water is easier to stretch than rock.

Guest wrote: "I always assumed it was because water is easier to stretch than rock"

Don't feel alone - I held the same view before getting my head around it!

Obviously, water is compressible; otherwise sound could not propagate in it. But the amount of compression or 'stretch' that the moon's tidal gravity can achieve over 3 km depth is absolutely negligible.