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.
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.)
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.
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
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.
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.
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).
> 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...
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.
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.
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.
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.
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.
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.
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.
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
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.
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.)
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.
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
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).
> 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...
https://news.ycombinator.com/item?id=44066091
https://xkcd.com/1838
Mobile view
https://m.xkcd.com/1838/
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.
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.
https://en.wikipedia.org/wiki/Tide-predicting_machine
Basically, a summation of sinusoids.
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
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...
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.
Because if someone says "one" it sounds affected.
If one says "someone" it may seem too targeted.
Methinks thou shouldst tame thine viscera lest they lead thee unto distraction
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.
Most people might think that someone doesn't need LLM for that.
Most people don’t have visceral reaction. The world is full of tools. You can bend your experience of it to your will.
> 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
We had one of these today in Sausalito!
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...
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.
Relevant xkcd: https://xkcd.com/3135
Reminds me of the time that Bill O'Reilly argued on national television that tides were inexplicable.
https://www.youtube.com/watch?v=NUeybwTMeWo
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.
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.
That seems huge. Are you in Fundy’s bay in Canada?
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