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

http://physics.stackexchange.com/questions/121830/does-earth-really-have-two-high-tide-bulges-on-opposite-sides
100•imurray•4h ago

Comments

imurray•4h ago
I was asked why there are two tides a day in an interview for my undergraduate University place. I blundered through to the classic answer. This stackexchange discussion made me realize I was even more of an imposter than I thought :-).
Retric•3h ago
If it makes you feel better, the crust of the earth does bulge more in line with the classic answer due to the flow of the underlying magma being effectively uninterrupted by solid obstructions. Which then means the classic tidal answer is technically correct, except what we observe as tides is a delta between land and ocean.
chermi•3h ago
TL;DR newton basically got the FORCES right, but forces don't tell the whole story because of (mainly ) 1) insufficient propagation speed because ocean is deep 2) think of it kind of like a diff eq, the boundary conditions (largely from land masses) from the actual structure of the earth make the solutions much more interesting than F=ma might suggest.

Edit- I recommend actually reading it, especially the second answer.

HPsquared•3h ago
So it's a bunch of complicated splashy water that is excited by the moon moving past, and follows along at the same frequency - but it's not a simple wave travelling around the world, for various reasons.

The earth itself is squashed like that with two bulges, but the water on the surface exhibits a more complex motion.

tomxor•1h ago
> So it's a bunch of complicated splashy water that is excited by the moon moving past

This explanation is so much better.

If people want to use big words they can say fluid dynamics, but yeah, it's a complex system with a big orbiting body pulling on it regularly, that gives the complex system rhythm but not order.

II2II•14m ago
That would be akin to describing a computer as a complicated arrangement of switches that control each other through pulses of electricity to do useful stuff. While it may satisfy a bunch of people who aren't really interested in how computers work, and it may even inspire a few people who are intrigued by how such a simple notion could produce incredible results, it doesn't really explain how computers work.
srean•3h ago
The problem of predicting tides was so important that it attracted many Physics and Maths heavy weights. You can well imagine how important predicting tides would have been for D-day landing.

One related fascinating historical artifact is the special purpose analogue computer designed by Lord Kelvin in the 1860s based on Fourier series, harmonic analysis. Think difference engine in it's cogs and cams glory, but special purpose.

https://en.m.wikipedia.org/wiki/Tide-predicting_machine

Possibly one of the first examples of Machine learning, with Machine in capital 'M'. It incorporated recent tidal observations to update it's prediction.

Note that sinusoids are universal approximators for a large class of functions, an honour that is by no means restricted to deep neural nets.

George Darwin (Charles Darwin's son) was a significant contributor in the design and upgrade of the machine.

https://en.m.wikipedia.org/wiki/George_Darwin

Other recognizable names who worked on tide prediction problem were Thomas Young (of double slit experiment fame) and Sir George Airy (of Airy disk fame).

neilfrndes•3h ago
Veritasium made a video on this topic a couple of years ago: https://www.youtube.com/watch?v=IgF3OX8nT0w
CGMthrowaway•2h ago
Have you seen the SF bay model? https://www.youtube.com/watch?v=i70wkxmumAw
srean•2h ago
That was so fascinating. Thank you.
synalx•1h ago
If you're ever in SF, it's really worth going to see. Such a cool mixture of art and technology.
umanwizard•3h ago
So, yet another thing I learned at school was bullshit. Pretty interesting to know!
daveguy•2h ago
Turns out teachers are people and general understanding evolves over time and not all at once.

Who would have guessed. Well, Laplace maybe.

an0malous•53m ago
What are the others?

The Bernoulli principle is one.

roelschroeven•33m ago
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.

0xbadcafebee•3h ago
> one of Newton's few mistakes

fwiw, Newton was bipolar. High-strung, antisocial, egotistical, domineering, rage-filled. He fought with people often and refused to share his work out of fear of criticism. Most people really didn't like him and he was often severely depressed. Later in life, in part because of the torment of just being himself and having to work with peers, he refused to continue researching science, and instead became obsessed with God and alchemy.

The dude made mostly mistakes throughout his life, he just happened to be brilliant some of the time.

srean•2h ago
His childhood was quite emotionally traumatic. I can imagine severe abandonment wounds given his situation.

Not only would he have felt abandoned, when his mother quickly remarried after his father's death, he could actually see the distant steeple where her mother had to relocate after her marriage - source of affection and emotional connect just tantalizingly out of reach.

That might explain his behaviour.

parpfish•2h ago
And to top it all off, that dang apple hit him on the head
srean•1h ago
Is that so ?

I have to email hn@news.ycombinator.com about it. Those Apples are just too freaking expensive to throw around like that, Dang.

IAmBroom•2h ago
We ALL mostly make mistakes throughout our lives.

Newton just happened to be much more brilliant than most others - and exhaustively documented his scientific thoughts.

teruakohatu•2h ago
> High-strung, antisocial, egotistical, domineering, rage-filled.

I think you are doing the man a disservice summarising him in such a way.

His interest in unorthodox/heretical religion was at least since he was at university. He spent a significant amount of time on alchemy.

Newton was the President of the Royal Society for over two decades, an MP for a similar amount of time which I would think required a lot of interpersonal relationships and socialising.

He seemed to get along well with family who cared with and lived with him and described him as loving.

The traits of holding grudges and raging were probably as common in academia then as they are today (tech is benign in comparison), but are otherwise sociable and genuinely trying to be good, albeit flawed, people.

He made numerous statements of modesty, the most famous being "If I have seen further it is by standing on the shoulders of giants." This has, IMHO, been unfairly reinterpreted in recent times as being a insult to a rival rather than taken at face value.

If every comment, action, HN comment, tweet etc. of any person's entire life was interpreted in the least charitable light we would all be recorded in history as being as vile as you describe him.

I think at the end of the day he was just a gifted flawed human.

srean•1h ago
> I think at the end of the day he was just a gifted flawed human.

And what gifts !

Imagine anyone doing Principia at an age of 24 (the book was published much later, but he had the results by then).

He would have been notable even if he had borrowed an established discipline of calculus to elaborate it's Physical consequences. No he had to develop it himself first and double check the results by translating that into geometry, into power series to be sure they are correct.

Einstein and Newton are often spoken of in the same breath, but by sheer body of work it seems a no-contest to me. Einstein had the luxury of being able to borrow tensor calculus, by then well formed. Perhaps the person who comes closest to Newton would be Archimedes, considering the time that Archimedes was doing his thing.

btilly•2h ago
Can we at this distance tell the difference between bipolar, mercury poisoning, and repressed homosexual?

He was also responsible for the execution of a couple of dozen people. These executions were connected to his position as master of the mint.

senderista•16m ago
Executing counterfeiters is certainly distasteful to us today, but I find it less distasteful than ideologically motivated crimes by modern scientists such as Lakatos (one of my intellectual heroes who I find it difficult not to despise as a person).
hollerith•1h ago
Perhaps the person that did the most to raise our standard of living (by basically inventing modern science). I basically don't care about how miserable it would be to sit next to him on a long airplane ride (or carriage ride).
shakna•1h ago
This reads like you think no one with bipolar can live without ruining the people around them.
joshmarinacci•3h ago
I think of it not as Newton was wrong, but rather his explanation was incomplete.
Calwestjobs•26m ago
Most kind way of saying Newton was a simple man.
alejohausner•2h ago
In the animations, New Zealand stood out: the high and low tide chase each other counterclockwise around the islands!
Calwestjobs•19m ago
earth is 3D not 2D ;) "bulges" same. that is where confusion comes from. also tesseract is nonsense.
antognini•2h ago
When I was in grad school in astronomy, one of my professors told me "many a promising young researcher has run their career aground on the rocky shores of tides."

The mathematics involved in the theory of tides are formidable. Even in homogeneous, tidally locked systems things can get complicated very quickly.

But tides are nevertheless very important. One two objects pass very close to each other, tidal effects are substantial and can actual destroy one of the objects: https://en.wikipedia.org/wiki/Tidal_disruption_event

zabzonk•7m ago
destruction (or nearly) via tidal mechanics happens in several of larry niven's short sf stories
coolcase•2h ago
Try to get your head around this while simultaneously not thinking of gravity as a force but curvature in spacetime.
senderista•39m ago
No, don't! Use the simplest model that applies in your context!
Calwestjobs•23m ago
exactly, like water is excellent model for electricity, but youtubers want to be edgy, provocative so they intentionally drop something which needs 20+ years of intentional thinking / education on high schoolers.
why_at•27m ago
Damn, I just had one of those moments where you go from thinking you understand something to realizing it's really complicated and you don't understand it at all.

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