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Does Earth have two high-tide bulges on opposite sides? (2014)

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Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#81

Post by pcrh »

Bjartr wrote: post #75
tylervigen wrote: post #67

The explanation is phenomenal. I particularly like the elevation heat map, which helps me intuitively grasp what is going on. This raises a question for me though: why do we show the tidal bulge graphic in any educational context? Like OP, the "far bulge" was always the most surprising and difficult-to-grasp part of the image. But this explanation would indicate that the far bulge is almost totally pointless as a con…

I expect because without the far bulge, 12 hour tides can't be explained. One bulge would mean 24 hour tides. Not that either explanation is actually correct, but the two bulge explanation matches the obseved periodicity, which is all most people would ever need or care to know about tides these days. I can't for the life of me understand why graduate level oceanography courses would be teaching it though.

If the bulges were caused by water being attracted to the moon, there should not be a "far bulge"?

So how was the existence of a far bulge justified?

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#82

Post by pfdietz »

grues-dinner wrote: post #59
rhdjsjebshjffn wrote: post #50

Earlier quoted context omitted.

> You can well imagine how important predicting tides would have been for D-day landing. Is this intended to communicate positivity or negativity? Predicting tides was known to the ancients; it would be lovely to explore the hubris of the modern narrative. Edit: fundamentally, if hacker news has taught me anything, it's that "downvote = makes me feel bad and doesn't want to answer questions". The entire concept of de…

I think there are two ways to interpret that sentence: "it would have been important": one which implies tidal prediction was unavailable at D-day but would have been useful, and one that implies it was indeed available (subjunctive conditional or "the Anderson case", apparently, per Wikipedia) I don't think anyone is claiming tide times were so unpredictable in 1945.

They were predictable. Interestingly, Rommel misunderstood how tides affected landings. He thought the landings would be done at high tide, so the invading troops wouldn't have to advance across wide expanses of beach. In reality, the allies wanted to invade on a rising tide, so the landing craft, grounded to let out troops, would refloat and be able to move back out. Also, invading at lower tide meant beach obstacles would be exposed and unable to damage the landing craft.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#83

Post by darkerside »

dexwiz wrote: post #58
darkerside wrote: post #46

Earlier quoted context omitted.

But it is actually a bit more accurate than saying, electricity goes in and information comes out

Information and electricity go in and information and heat go out, to be pedantic about a simplification.

If you're going to be pedantic, information and heat go into and come out of just about everything

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#84

Post by rootbear »

ghaff wrote: post #40
zabzonk wrote: post #39

Earlier quoted context omitted.

destruction (or nearly) via tidal mechanics happens in several of larry niven's short sf stories

As I recall there were issues with the math in Neutron Star though still a very good story.

I believe the issue is that the ship leaves the star in a spin, perhaps too fast to be survivable.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#85

Post by calfuris »

pcrh wrote: post #81
Bjartr wrote: post #75

Earlier quoted context omitted.

I expect because without the far bulge, 12 hour tides can't be explained. One bulge would mean 24 hour tides. Not that either explanation is actually correct, but the two bulge explanation matches the obseved periodicity, which is all most people would ever need or care to know about tides these days. I can't for the life of me understand why graduate level oceanography courses would be teaching it though.

If the bulges were caused by water being attracted to the moon, there should not be a "far bulge"? So how was the existence of a far bulge justified?

The Moon's gravity isn't just pulling on the water, it's pulling on the Earth as a whole. It's pulling more on the Earth as a whole than on the water on the far side. In the Earth's frame of reference, that looks like it is pushing the water on the far side away a little bit.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#86

Post by CommenterPerson »

kgwgk wrote: post #56
dghlsakjg wrote: post #48

Earlier quoted context omitted.

Your clock was off. Tides advance ~30 minutes per day. But not exactly 30 minutes. Sometimes more. Sometimes less. Sometimes it doesn’t follow a semi diurnal pattern. Water can’t pass through landmasses, and that is a huge factor. If the earth had no landmasses, the tides would be entirely as you expect. However, if you look at a global visualization of tidal heights, you will see that a small landmass, NZ is a great…

> Your clock was off. Tides advance ~30 minutes per day. “roughly 12 hours”

Thank you. I still feel the stackexchange post misses the forest for the trees (and weeds). Take a look at the NOAA global map showing coasts that have semidiurnal tides (that is, one in roughly ~12 hours) [1].

Most all of the coast of Africa is semidiurnal. So is east coast of North America, a lot of South America. Bay of Bengal, a lot of Europe. If you see the map on the RWU [2] site it shows Greenland and the north coast of Russia (although stretched due to the latitude) are also semidiurnal. This is a major part of the global coastline. The simple mental model explains this. I feel that going to partial differential equations, fourier series, etc. etc. is a little too complicated.

[1] https://oceanservice.noaa.gov/education/tutorial_tides/tides... [2] https://rwu.pressbooks.pub/webboceanography/chapter/11-3-tid... -

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#87

Post by hinkley »

coolcase wrote: post #19

Try to get your head around this while simultaneously not thinking of gravity as a force but curvature in spacetime.

Water has to flow. It doesn’t just appear where it “needs” to be.

There’s a brackish pond/lake in a park in Victoria BC, you go over a bridge from downtown to get to the main entrance, though the locals can cross a street.

If should actually be a bay, but under said bridge is a stone formation that forms the throat of this bay, which being so long and narrow, cannot fill up or drain as fast as the tides. So at high tide there is a waterfall flowing into the pond, and as the tides recede it’s a waterfall going the other direction.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#88

Post by hinkley »

roelschroeven wrote: post #32
an0malous wrote: post #30

Earlier quoted context omitted.

What are the others? The Bernoulli principle is one.

The Bernoulli principle is not bullshit -- it is very valid physics. You might be thinking the way it's often used to wrongly explain how airplane wings generate lift. Yeah, that's bullshit. I mean, the principle still applies, if applied correctly. The equal transit bullshit that it's often associated with, well yes, that's complete and utter bullshit.

Bernoulli apparently caused some serious problems during peak shipping on the Thames. Big boats navigating in opposite directions having to work not to bump into each other.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#89

Post by Bjartr »

pcrh wrote: post #81
Bjartr wrote: post #75

Earlier quoted context omitted.

I expect because without the far bulge, 12 hour tides can't be explained. One bulge would mean 24 hour tides. Not that either explanation is actually correct, but the two bulge explanation matches the obseved periodicity, which is all most people would ever need or care to know about tides these days. I can't for the life of me understand why graduate level oceanography courses would be teaching it though.

If the bulges were caused by water being attracted to the moon, there should not be a "far bulge"? So how was the existence of a far bulge justified?

It's not justified for any rigorous setting at all.

In a layperson setting, it's as justified as saying the speed of light slows down in non-vacuum. It doesn't, but it's a close enough explanation for most people most of the time, and if you squint it's sort of saying the right thing, but missing all of the details. In the same way as the observed speed of light is slower in air, the tides happen every 12 hours. But c doesn't change and there aren't two bulges.

It 100% does not, every single photon is moving at the full c speed of light at all times. It's not even that the photons are bouncing around and so they, on average do not make progress as fast. I believe it's a factor of how the moving EM field of the photon nudges particles like electrons a little, whose now moving field results in a lower net wave phase velocity such that observed propagation time is < c, but every photon still moves at exactly c.

Re: Does Earth have two high-tide bulges on opposite sides? (2014)

#90

Post by throwaway173738 »

jxjnskkzxxhx wrote: post #72

So it appears that the answer is that the bulges are a forcing function, not a displacement. Am I the only one skeptical that Newton would confuse a force with a displacement?

The term “newtonian solution” just implies that you’re using gravitational and kinematic approaches to the model. I think Newton probably would have done much better had he made an earnest study with the resources he had available. People in shipping towns knew when tides and currents would be favorable and ships would try to leave at those times.
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