7 ms·
Does Earth have two high-tide bulges on opposite sides? (2014)
- joshmarinacci 1y agoI think of it not as Newton was wrong, but rather his explanation was incomplete.
- Calwestjobs 1y agoMost kind way of saying Newton was a simple man.
- imurray 1y agoI 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 1y agoIf 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 1y agoTL;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 1y agoSo 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 1y 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 1y agoThat 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.
- darkerside 1y agoBut it is actually a bit more accurate than saying, electricity goes in and information comes out
- dexwiz 1y agoInformation and electricity go in and information and heat go out, to be pedantic about a simplification.
- deleted 1y ago[deleted]
- srean 1y agoThe 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 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 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 1y agoVeritasium made a video on this topic a couple of years ago: https://www.youtube.com/watch?v=IgF3OX8nT0w https://www.youtube.com/watch?v=IgF3OX8nT0w
- CGMthrowaway 1y agoHave you seen the SF bay model? https://www.youtube.com/watch?v=i70wkxmumAw https://www.youtube.com/watch?v=i70wkxmumAw
- Clood5 1y ago[dead]
- School-Cotton 1y agoSo, yet another thing I learned at school was bullshit. Pretty interesting to know!
- daveguy 1y agoTurns out teachers are people and general understanding evolves over time and not all at once. Who would have guessed. Well, Laplace maybe.
- an0malous 1y agoWhat are the others? The Bernoulli principle is one.
- roelschroeven 1y agoThe 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.
- hinkley 1y agoBernoulli 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.
- School-Cotton 1y ago“Serious companies will require you to comment every line of code”
- 0xbadcafebee 1y ago[flagged]
- srean 1y agoHis 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 1y agoAnd to top it all off, that dang apple hit him on the head
- srean 1y agoIs 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.
- ahazred8ta 1y agoNewton - "the sole mortal who could grapple, Since Adam, with a fall or with an apple." — Byron A calculus fit to compute on, White light and a head to drop fruit on, A mind to absorb it, And soar into orbit — That's all that it takes to be Newton. — Gina Berkeley
- srean 1y agoMy favorite: Nature and Nature's laws lay hid in night: God said, Let Newton be! And all was light. -- Alexander Pope
- IAmBroom 1y agoWe ALL mostly make mistakes throughout our lives. Newton just happened to be much more brilliant than most others - and exhaustively documented his scientific thoughts.
- alejohausner 1y agoIn the animations, New Zealand stood out: the high and low tide chase each other counterclockwise around the islands!
- Calwestjobs 1y agoearth is 3D not 2D ;) "bulges" same. that is where confusion comes from. also tesseract is nonsense.
- fransje26 1y agoNicely spotted!
- antognini 1y agoWhen 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 https://en.wikipedia.org/wiki/Tidal_disruption_event
- zabzonk 1y agodestruction (or nearly) via tidal mechanics happens in several of larry niven's short sf stories
- ghaff 1y agoAs I recall there were issues with the math in Neutron Star though still a very good story.
- hinkley 1y agoThere’s been some backpedaling lately in the astrophysics community about whether a tidally locked planet could still maintain an atmosphere and potentially support life. More modeling on how such at atmosphere might work has turned from “no” to “maybe”.
- Sharlin 1y agoSee also: https://en.wikipedia.org/wiki/Roche_limit https://en.wikipedia.org/wiki/Roche_limit https://en.wikipedia.org/wiki/Roche_lobe https://en.wikipedia.org/wiki/Roche_lobe Indeed given that we now think most of the heavy elements in the universe were created in type 1a mass-transfer supernovae, we can ultimately thank tidal phenomena for the existence of things like rocky planets and humans.
- coolcase 1y agoTry to get your head around this while simultaneously not thinking of gravity as a force but curvature in spacetime.
- senderista 1y agoNo, don't! Use the simplest model that applies in your context!
- Calwestjobs 1y agoexactly, 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.
- inetknght 1y agoI like AlphaPhoenix's video on it. https://www.youtube.com/watch?v=C3gnNpYK3lo https://www.youtube.com/watch?v=C3gnNpYK3lo
- coolcase 1y agoI agree, I meant to add "as a mind bending exercise to realize how complex nature is!"
- verzali 1y agoIts all models in the end. Half of physics is just putting things in the easiest frame or model to solve the problem you want to solve. And the other half is often simplifying things down to the proverbial spherical cow in a vacuum.
- hinkley 1y agoWater 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.
- deleted 1y ago[deleted]
- why_at 1y agoDamn, 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.
- CommenterPerson 1y agoSix months ago, I spent a week at the shore. It happened to be full moon. We were out walking late at night while the moon was high up, and had to slog through ankle deep water on the way back. It was like clockwork roughly 12 hours apart. Did read through stackexchange. It is indeed complicated. But the top response feels like paralysis by analysis. If we analyzed turbulent flow too much we would be unable to build rockets. Remember frictionless planes and point masses in high school? Those results are not exact either but a great way to model and understand what is going on. Soooo .. could we make simplifying assumptions here? What if the earth was a smooth rigid sphere with a layer of water on the surface? The center of mass of Earth-Moon is at ~3/4ths of the earth's radius, from the earth's center. They are rotating about that center. The 12+ hour tides in many parts of the world start to make sense. Is there a mistake in this mental model?
- red369 1y agoFrom reading the accepted StackExchange answer, I think the answer to your last questions is that this model might still be too simplified. In your simplified model of the Earth, you would also need to make the ocean deep enough that the water could travel fast enough to keep up with the Earth's rotation (~22 km).
- dghlsakjg 1y agoYour 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 example, can have highs and lows just miles apart. Same in Panama, what happens on the pacific coast is wildly different to what happens on the Caribbean. In addition, the gravity of the sun comes to factor as well. Where I am, north of the 50th parallel, we simply don’t get very low tides during the day when we are near the winter solstice. The opposite happens in the summer. The timing of the tides for any given spot tend to be predictable (where it is semi diurnal anyway, other places are a mess). But heights are extremely variable.
- tylervigen 1y agoThe 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 concept, given the complexities of the system. Given it's the least intuitive part of the image, it invites additional consideration. But it's all the wrong consideration! The model would be more useful if it only showed the bulge on the moon side, and excluded the far side bulge. It would still be wildly imprecise, kind of like the orbital model of atoms is wildly imprecise, but at least it would be a slightly more accurate (and useful) initial mental model.
- srean 1y agoIt's an idealized model, accurate if Earth had only a single all encompassing deep ocean. Idealized models are good pedagogic tools to build corrections upon. It's similar to depiction of projectile motions as parabola s. The trajectories of artillery shells ar not like that, but helps get started.
- randallsquared 1y agoWell, a single all encompassing deep ocean of something in which waves could travel 1600 km/h, since that's one of the major constraints, too.
- Bjartr 1y agoI 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.
- pcrh 1y ago
- mkl 1y agoThat animation is great. I found the person who made it: https://ceoas.oregonstate.edu/directory/svetlana-erofeeva https://ceoas.oregonstate.edu/directory/svetlana-erofeeva That links to this website which has a similar animation for the current day: https://www.tpxo.net/ https://www.tpxo.net/
- MostlyStable 1y agoI took a graduate level physical oceanography course and never learned this and still believed the tidal bulge story. To be fair to the course, it was much more interested in currents than tides (I don't remember really discussing tides in any depth at all) This is a great answer!
- jxjnskkzxxhx 1y agoSo 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?
- throwaway173738 1y agoThe 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.
- jxjnskkzxxhx 1y agoSo 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? What am I missing?
- chermi 1y agoGood point. I'd be curious if anyone actually has the text showing he said this. It's in principia I guess. My bet was that he never gave a full description, but rather just said that it is moon/sun that *causes* the tides.-- I'd wager he acknowledged the incompleteness of it. Which would still be mostly accurate. It's hard to imagine him knowing about the complicated tides in England and saying definitively he had a full model of the tides.
- cwmoore 1y agoWhat's a similarity between Iceland and Southwestern Australia? TIL https://en.wikipedia.org/wiki/Amphidromic_point https://en.wikipedia.org/wiki/Amphidromic_point