from Hacker News

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

by imurray on 5/22/25, 6:58 PM with 90 comments

• by srean on 5/22/25, 7:51 PM

  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).

• by HPsquared on 5/22/25, 7:37 PM

  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.

• by antognini on 5/22/25, 8:49 PM

  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

• by mkl on 5/23/25, 10:42 AM

  That animation is great. I found the person who made it: 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/

• by MostlyStable on 5/23/25, 10:57 AM

  I 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!

• by tylervigen on 5/23/25, 10:36 AM

  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 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.

• by CommenterPerson on 5/23/25, 1:25 AM

  Six 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?

• by alejohausner on 5/22/25, 8:42 PM

  In the animations, New Zealand stood out: the high and low tide chase each other counterclockwise around the islands!

• by jxjnskkzxxhx on 5/23/25, 12:15 PM

  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? What am I missing?

• by chermi on 5/22/25, 7:31 PM

  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.

• by imurray on 5/22/25, 7:00 PM

  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 :-).

• by coolcase on 5/22/25, 9:13 PM

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

• by cwmoore on 5/24/25, 1:05 AM

  What's a similarity between Iceland and Southwestern Australia? TIL

  https://en.wikipedia.org/wiki/Amphidromic_point

• by why_at on 5/22/25, 10:57 PM

  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.

• by jxjnskkzxxhx on 5/23/25, 12:14 PM

  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?

• by joshmarinacci on 5/22/25, 8:21 PM

  I think of it not as Newton was wrong, but rather his explanation was incomplete.

• by umanwizard on 5/22/25, 8:08 PM

  So, yet another thing I learned at school was bullshit. Pretty interesting to know!