[HN Gopher] Does Earth have two high-tide bulges on opposite sid...
___________________________________________________________________
Does Earth have two high-tide bulges on opposite sides? (2014)
Author : imurray
Score : 94 points
Date : 2025-05-22 18:58 UTC (4 hours ago)
(HTM) web link (physics.stackexchange.com)
(TXT) w3m dump (physics.stackexchange.com)
| imurray wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| > 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.
| srean wrote:
| 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 wrote:
| Veritasium made a video on this topic a couple of years ago:
| https://www.youtube.com/watch?v=IgF3OX8nT0w
| CGMthrowaway wrote:
| Have you seen the SF bay model?
| https://www.youtube.com/watch?v=i70wkxmumAw
| srean wrote:
| That was so fascinating. Thank you.
| synalx wrote:
| If you're ever in SF, it's really worth going to see. Such
| a cool mixture of art and technology.
| umanwizard wrote:
| So, yet another thing I learned at school was bullshit. Pretty
| interesting to know!
| daveguy wrote:
| Turns out teachers are people and general understanding evolves
| over time and not all at once.
|
| Who would have guessed. Well, Laplace maybe.
| an0malous wrote:
| What are the others?
|
| The Bernoulli principle is one.
| roelschroeven wrote:
| 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 wrote:
| > 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 wrote:
| 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 wrote:
| And to top it all off, that dang apple hit him on the head
| srean wrote:
| 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 wrote:
| 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 wrote:
| > 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 wrote:
| > 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 wrote:
| 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.
| hollerith wrote:
| 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 wrote:
| This reads like you think no one with bipolar can live without
| ruining the people around them.
| joshmarinacci wrote:
| I think of it not as Newton was wrong, but rather his explanation
| was incomplete.
| alejohausner wrote:
| In the animations, New Zealand stood out: the high and low tide
| chase each other counterclockwise around the islands!
| antognini wrote:
| 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
| coolcase wrote:
| Try to get your head around this while simultaneously not
| thinking of gravity as a force but curvature in spacetime.
| senderista wrote:
| No, don't! Use the simplest model that applies in your context!
| why_at wrote:
| 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.
___________________________________________________________________
(page generated 2025-05-22 23:00 UTC)