[HN Gopher] Tides are weirder than you think
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Tides are weirder than you think
Author : surprisetalk
Score : 154 points
Date : 2025-12-01 20:14 UTC (5 days ago)
(HTM) web link (signoregalilei.com)
(TXT) w3m dump (signoregalilei.com)
| fhdkweig wrote:
| https://www.youtube.com/watch?v=AxC770lpSLw shows one of the old
| tide prediction mechanical computers briefly mentioned in this
| article. It is effectively doing an inverse Fourier Transform,
| summing all the various 30+ sinusoids that affect the tides.
| wasting_time wrote:
| Relevant xkcd: https://xkcd.com/3135
| santoriv wrote:
| Reminds me of the time that Bill O'Reilly argued on national
| television that tides were inexplicable.
|
| https://www.youtube.com/watch?v=NUeybwTMeWo
| boothby wrote:
| I live near a river that flows backwards for some 50km or so
| with tides. The first time I saw it, I thought I'd gotten
| turned around because _surely water can 't flow uphill, right?_
| It's fun to find out that one is so wrong about basic physics.
| xg15 wrote:
| > _Kelvin's machine automates the second half of that task. As
| the pulleys spin, they pull on a common chain by the correct
| amount for each of the calculated components. A year's worth of
| tidal tables can be put together in half an hour, if you know the
| components. Even better, you can "guess and check" the components
| by comparing your machine's output to past records, and adjusting
| the pulleys until you get something that works correctly._
|
| I didn't know they already had machine learning and model fitting
| algorithms in the 1800s, but here we are...
| LoganDark wrote:
| https://xkcd.com/1838
| B1FF_PSUVM wrote:
| Sometimes we need dates attached - by the numbers (1838 of
| current 3176) that's probably some 5 or 6 years before
| ChatGTP. Or 3 or 4 years after the Netflix challenge.
| thunderbong wrote:
| Mobile view
|
| https://m.xkcd.com/1838/
| srean wrote:
| https://news.ycombinator.com/item?id=44066091
| metalman wrote:
| out the back door of my house, down through the field and woods
| to the water is a 12 min walk at high tide, and another 12 min +
| walk to the water at low tide, 53 vertical feet between the two.
| I know ,not the slightest thing about tides, other than what my
| senses tell me when walking the shore, and I suppose the
| practicalities of beach walking along cliffs are similar to
| planning and predicting arival times for ships loaded to the
| maximum draft, the money to be made there bieng the motivation
| behind the push to understand tides.
| ttoinou wrote:
| That seems huge. Are you in Fundy's bay in Canada?
| metalman wrote:
| huge, and where else? there are some places here where the
| slope is so gradual, that the low tide is over the horison
| growing up we had a camp right at the end of Burnt Coat Head,
| in Noel, and never did make it to the low tide line between
| there and Cape Split there is the full range of tidal
| phenominon on display, resonant standing waves, whirlpools,
| tidal bores(with surfing black ducks),whole bays emptied, and
| varios dikes and berms, dams, locks, to hold it back
|
| there are one ot two other places on the planet that are
| close, and from the charts in the article, spots with NO
| tide, which when I think about,is very wierd indeed
| antognini wrote:
| You may have seen diagrams of the tidal force of the Moon on the
| earth (like this one:
| https://www.oc.nps.edu/nom/day1/tide_force_diagram.gif).
|
| Intuitively you would think that the tide is being formed because
| the Moon is "lifting up" the water at the point closest to the
| Moon. But this contribution is actually very miniscule to the
| tidal effect. Instead the bulk of the tides are produced about 45
| degrees away where the tidal force is parallel to the Earth's
| surface. This has the effect of dragging the water closer to the
| tidal bulge.
| fsckboy wrote:
| but to develop better intuition, think of the sun's gravity as
| a field in space and nothing is being dragged anywhere, it's
| just that wherever you are feels appropriate to where you are
| and where you are going is the path of least resistance, and
| the places around you feel the same way, and where you all are
| in relation to each other (in this field) changes _its_
| relative position to everything else.
|
| the water of an incoming tide doesn't feel "i'm being dragged
| uphill", it feels "hey, the earth is moving underneath me".
| it's all in freefall all the time.
|
| you don't feel like you are rotating at 1000 mph (1600 kph) but
| you do feel your weight against the surface of the earth. same
| with the water, except it feels itself being squeezed by
| everything around it like you only feel that in the entrance to
| a crowded venue.
|
| so, the water on the side toward the moon and the water on the
| side away from the moon would mostly perceive the earth as
| dropping away or coming closer (if they could perceive anything
| at all) where they are is always their point of reference
| mcswell wrote:
| Thank you--the diagram you link is a better explanation of whey
| the tide "bulges up on the sides of the Earth closest to and
| farthest from the Moon"--the article left this entirely
| unclear.
|
| In particular, I could understand how two satellites connected
| by a cable would result in the cable being stretched. But I
| still find it hard to wrap my mind around the fact that we get
| a high tide where the Earth's gravity and the Moon's add (the
| far side of the Earth from the Moon), but we also get a high
| tide on the opposite side, where the Moon's gravitational pull
| is subtracted from the Earth's. The centrifugal force is (I
| think) a much better explanation. (I realize physicists don't
| consider that a _force_ , but...)
|
| So yes, tides really are weirder than I think.
|
| (The other facts in the article were actually familiar, e.g.
| the fact that the tides in Hawaii are quite small, because it's
| not far from an amphidromic point.)
| ianburrell wrote:
| Tides aren't caused by centrifugal force but by differing
| gravity. You would be ripped apart by tides falling straight
| into black hole.
|
| The near part of Earth experiences more gravity from the
| Moon, the far part less. The Earth moves in the center so the
| water bulges on the ends. Important part is that the Earth
| pulls things out their natural orbits.
|
| With circular orbits, gravity and centrifugal force are
| balanced so could be considered difference on centrifugal
| force. But that isn't true for all orbits.
| empiko wrote:
| Simple illustration of what you are explaining in 1D.
| Imagine a simplified Earth:
|
| Water -- Ground -- Water
|
| Now let's add a Moon with gravitational pull. The pull
| stretches the system, because gravity is stronger the
| closer you are to the Moon.
|
| Water ---- Ground ---- Water -//- Moon
|
| The water is farther away from the ground on both sides
| now, since both sides stretched.
| heavenlyblue wrote:
| so why is left water further away from ground? this
| example make 0 sense
| nicwolff wrote:
| The Earth is being pulled away from that water, just as
| the water on the right is being pulled away from the
| Earth.
| esafak wrote:
| https://en.wikipedia.org/wiki/Tide-predicting_machine
|
| Basically, a summation of sinusoids.
| cortesoft wrote:
| I know this is unrelated to the actual content, but I have such a
| visceral reaction to any headline in the "x is different than you
| think"... how do you know what I think? I am sure many people who
| read this will be experts on tides, and the title is completely
| wrong.
|
| For me, the worst are posts about scale and things I won't need,
| like "You don't need kafka" or "your data isn't actually big
| data" or "don't horizontally scale, just get a bigger server"
|
| I get that I am not the target audience and there are people for
| whom those statements are true, but I am running Kafka clusters
| with data from 10s of thousands of servers, I absolutely can't
| move that to a single machine.
|
| I wish they would phrase it as "Tides are weirder than most
| people think", although that probably doesn't drive as much
| engagement.
| renewiltord wrote:
| It is property of English language where "you" is used in place
| of "one" very often. This is particularly common in American
| English which has less formality to it; less Frenchiness. No
| 'on' etc. Everyone is 'tu'.
|
| It is language quirk, but you can probably use LLM to replace
| HN headlines like this. Pretty cheap and one will no longer
| have visceral reaction that one does not wish.
| panxyh wrote:
| Most people might think that someone doesn't need LLM for
| that.
| renewiltord wrote:
| Most people don't have visceral reaction. The world is full
| of tools. You can bend your experience of it to your will.
| nephihaha wrote:
| Because if someone says "one" it sounds affected.
| cwmoore wrote:
| If one says "someone" it may seem too targeted.
| foundart wrote:
| Methinks thou shouldst tame thine viscera lest they lead thee
| unto distraction
| relwin wrote:
| We have a "King Tide" today in SoCal where the local swing is
| +7/-1.5. Get to see normally hidden tidepools and a bunch of
| cobbles...
| johnea wrote:
| Nice article!
|
| Any talk of tide prediction should always mention xtide:
|
| https://flaterco.com/xtide/
|
| I've used it with great accuracy in a number of locations around
| the world.
|
| Another one of those free software packages that's been
| meticulously maintained by one person for decades...
| 2b3a51 wrote:
| Here we go (quote from the xtide Web page)
|
| _" Q: I live outside of the U.S. and my location is no longer
| supported. What happened?
|
| A: Many data were purged after a legal threat from the U.K.
| Hydrographic Office (UKHO) in January 2001. I ended maintenance
| of the non-U.S. data that replaced them for different reasons
| in early 2012."_
|
| I wonder how much money the Hydrographic Office saved by not
| allowing this data to be distributed?
| o4c wrote:
| Veritasium made a great video on Kelvin's Tide-predicting machine
| in 2021.
|
| Title: The Most Powerful Computers You've Never Heard Of
|
| URL: https://www.youtube.com/watch?v=IgF3OX8nT0w
| themafia wrote:
| > When they're least lined up (First and Last Quarter Moon),
| tides are more smoothed out. This is a "Neap Tide".
|
| And when they're the "most lined up" you get the "King Tide."
|
| https://en.wikipedia.org/wiki/King_tide
| pea wrote:
| We had one of these today in Sausalito!
| srean wrote:
| Relevant
|
| https://news.ycombinator.com/item?id=44065458
|
| ( _92 comments_ )
|
| Also
| https://content.teachastronomy.com/taweb/images/textbook/hrt...
|
| Inlining my comment that I had posted there:
|
| 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).
| globular-toast wrote:
| I see I'm not the only geek fascinated by the tide. I knew all
| the stuff in the article already, of course. It seems it's like
| railways or something, seems to trigger something in geeky
| brains. I live in the UK so see some of the largest "swings" in
| the world. It never ceases to amaze me that it all happens in the
| space of 6 hours. I've also gone to see spring tides just to see
| what it looks like. It's cool, and scary.
| nrhrjrjrjtntbt wrote:
| Tides. The OG gravitational wave detectors.
| layer8 wrote:
| Tides _are_ gravitational waves, as it were.
| geokon wrote:
| This explanation is a lot more thorough and detailed (and there
| are no tidal bulges.. Common myth)
|
| https://physics.stackexchange.com/questions/121830/does-eart...
| hyttioaoa wrote:
| I went down a rabbit hole a few weeks back to discover this. I
| was so upset with so many wrong explanations including Neil de
| Grasse Tyson saying we're moving into the bulges.
|
| it's very magical though discovering the beauty of it.
| B1FF_PSUVM wrote:
| What really gets me is the "Map of the strength of the 12-hr
| lunar tide" in the Layer 4 section of the article posted, which
| I haven't seen discussed.
|
| All the rest is swept away, and you have stretches of coastline
| with tides over one meter, and others with nearly zero, just
| because of landmass distribution ...
|
| If you moved Miami Beach to the French Atlantic coast, things
| would get interesting.
| mycall wrote:
| Odd weather wasn't mentioned. While weather does not create
| tides, but it can significantly modify the actual water level
| compared with the purely astronomical tide that the article
| describes.
| fmjrey wrote:
| Jeanne Rousseau, who passed away in 2012, asked in an interview
| [1]:
|
| Why are the syzygy tidal coefficients equal when the quadrature
| tidal coefficients are at opposite extremes? Why are the syzygy
| coefficients at opposite extremes when the others are equal? Who
| can explain this using the laws of universal gravitation?
|
| Her point is that you can't.
|
| Earlier in that interview she says: I was put in touch with the
| Institut de Physique du Globe [2]. In April 1953, I met with
| Professor Coulomb [3], who was the director at the time, and
| asked him about the ionic variations that might occur during the
| lunar phases. His formal response was that there were none.
| However, I had already been observing them for some time. I must
| say that in 1953 I had already begun to observe the phenomenon of
| the tides. Being told that, apart from a minimum of atmospheric
| ionization at 4 a.m., there is nothing else that can have an
| impact on the biological, human, or other levels, I said: but
| there is the phenomenon of the tides! And that's when I got this
| response, which marked a break with the scientific community for
| me:
|
| The phenomenon of the tides is a phenomenon that is beyond us. We
| waste our time when we take an interest in phenomena that are
| beyond us. If you don't want to waste yours, focus on other
| things.
|
| Jeanne Rousseau demonstrated through observation that tidal
| phenomena are not solely gravitational but primarily
| electromagnetic. One can read more about this in English in this
| paper [4].
|
| [1] https://youtu.be/ytWerrYTBLs
|
| [2]
| https://en.wikipedia.org/wiki/Institut_de_Physique_du_Globe_...
|
| [3] https://en.wikipedia.org/wiki/Jean_Coulomb
|
| [4]
| https://www.researchgate.net/publication/384443419_Cosmic_Re...
| typicalset wrote:
| This is, not to put too fine a point on it, crank science.
| Opening up the linked document, the DOI fails to resolve, the
| paper investigates pH variations in urine during tide cycles,
| and also skimming over it doesn't appear to make a single
| prediction. This is at best unfalsifiable, and a "break with
| the scientific community" is an accurate assessment.
|
| The physics of tides is largely well-understood, and the moon
| and sun provide the primary forcing. Accurate tide tables are
| regularly computed the world over, with measurements regularly
| made. Without even looking at measurements, the shipping
| industry demands accurate tide forecasts for navigating
| efficiently. The claim that "tidal phenomena are... primarily
| electromagnetic" requires some serious evidence to back it up,
| with calculates to boot, rather than invoking mysticism that
| tides are "beyond us". Many things are beyond our current
| scientific understanding, and that is humbling, but tides are
| quite well understood.
| fmjrey wrote:
| I would not dismiss this as crank science so quickly, first
| and foremost because it's not claiming to be science, not
| theoretically at least. It's observation of likely
| correlation. Also I would not say tides are "quite well
| understood" because it's misleading. I'd say sufficiently
| understood so that models are precise enough to be useful,
| but they're still an approximation to a few cm. For now water
| levels, costal geography, terrain, and weather seem to be the
| main parameters they attribute to the small variations
| observed, but who can say there aren't more parameters to
| take into account? Also what doesn't help in having a "well
| understood" feeling is that you can find different
| explanation of the bulges on either side of the earth.
|
| I don't have the scientific knowledge to assess all this. I'm
| not even sure how to understand properly the questions Jeanne
| Rousseau asks saying newtonian physics can't answer. What I
| hear however are competent people observing small variation
| in the properties of water and living systems that seems to
| be related to cosmic phenomenon, including moon phases.
| Variations we can also find in the atmosphere/ionosphere with
| more recent measurements of their ionic polarities. Adding to
| that are all the new discoveries that link weather phenomenon
| to electromagnetic influences from the sun, with water
| significantly influencing the electromagnetic properties of
| the atmosphere. Finally more people question the true
| molecular structure of water, as H2O seems to be a crude
| simplification over a dynamic mixture of isotopes and ions.
|
| Overall the tidal theory is not a done deal, we only have
| approximate models, and this topic can be discussed for years
| to come. That's probably why she was told the tides is a
| phenomenon that is beyond us.
| bmitch3020 wrote:
| A detail not included here, but important for people interested
| in the sea level, is the effects of the wind. Lots of places
| along the east coast of the US experience mild tides, but a
| steady wind in the right direction will push sea levels up in
| various bays and inlets. The storm surge of a hurricane turns it
| up to 11.
| vax425 wrote:
| I know!
|
| It took most of this summer to get my tide clock to predict
| accurately, using NOAA harmonic constituents data crammed into an
| Arduino, a GPS module, and published formulas.
|
| Take a look:
|
| https://digitalhorology.etsy.com/listing/4345582508
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