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Tides are weirder than you think

169 points by surprisetalk

by antognini

2 subcomments

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.

by geokon

3 subcomments

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...

by srean

0 subcomment

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).

by xg15

2 subcomments

> 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...

by johnea

1 subcomments

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...

by fhdkweig

0 subcomment

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.

by metalman

1 subcomments

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.

by o4c

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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

by esafak

0 subcomment

https://en.wikipedia.org/wiki/Tide-predicting_machine
Basically, a summation of sinusoids.

by teleforce

1 subcomments

Reading most of the technology articles would have you to believe that dark ages really existed and for more than thousands years, i.e from Greece to Renaissance time there is no marine technology advancement between them.
The truth of the matter is that the Arabic and muslim are the master of the sea during this so called "dark ages" time, and many advancement has been made from navigation gear of the Arabic astrolobe to the Idrisi map.
In tide prediction Al-Kindi for example, has notable work on tides in his seminal book Treatise on the Efficient Cause of the Flow and Ebb back in 9th CE [1],[2].
[1]al-Kindi:
https://en.wikipedia.org/wiki/Al-Kindi
[2] Al-Kindi:
https://muslimheritage.com/al-kindi/

by relwin

0 subcomment

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...

by cortesoft

4 subcomments

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.

by mycall

0 subcomment

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.

by globular-toast

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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.

by vax425

0 subcomment

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

by bmitch3020

0 subcomment

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.

by wasting_time

3 subcomments

Relevant xkcd: https://xkcd.com/3135

by ljgfxxxxxth

0 subcomment

All of these explanations are talking about “pulling” from gravity.
What they should really be saying is that spacetime is being affected as the moon orbits.

by fmjrey

1 subcomments

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...

by themafia

1 subcomments

> 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

by nrhrjrjrjtntbt

1 subcomments

Tides. The OG gravitational wave detectors.