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

http://physics.stackexchange.com/questions/121830/does-earth-really-have-two-high-tide-bulges-on-opposite-sides

by imurray • 1 year ago

294 75 1 year ago

physics.stackexchange.com

imurray• 1 year ago

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 :-).

Retric• 1 year ago

If 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.

chermi2• 1 year ago

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.

HPsquared• 1 year ago

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.

tomxor2• 1 year 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• 1 year ago

That 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.

srean3• 1 year ago

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).

neilfrndes• 1 year ago

Veritasium made a video on this topic a couple of years ago: https://www.youtube.com/watch?v=IgF3OX8nT0w

CGMthrowaway• 1 year ago

Have you seen the SF bay model? https://www.youtube.com/watch?v=i70wkxmumAw

srean3• 1 year ago

That was so fascinating. Thank you.

trillic• 1 year ago

Check out current lab

Hyperlocal ocean modeling for science, defense, and recreational applications.

https://www.current-lab.com

Anecdotally works very well in Tidal harbors with multiple rivers.

rhdjsjebshjffn• 1 year ago

You can well imagine how important predicting tides would have been for D-day landing.

Is this intended to communicate positivity or negativity?

Predicting tides was known to the ancients; it would be lovely to explore the hubris of the modern narrative.

Edit: fundamentally, if hacker news has taught me anything, it's that "downvote = makes me feel bad and doesn't want to answer questions". The entire concept of democratic news aggregation was a lie.

grues-dinner• 1 year ago

I think there are two ways to interpret that sentence: "it would have been important": one which implies tidal prediction was unavailable at D-day but would have been useful, and one that implies it was indeed available (subjunctive conditional or "the Anderson case", apparently, per Wikipedia)

I don't think anyone is claiming tide times were so unpredictable in 1945.

krisoft• 1 year ago

Is this intended to communicate positivity or negativity?

It just says it was important to predict the tides. There is no positivity or negativity to it. Your question doesn’t make sense, hence the downvotes.

Predicting tides was known to the ancients

Good. To which ancients? With what accuracy and how far into the future? What techniques did they use? Tell us more.

it would be lovely to explore the hubris of the modern narrative.

Explore it then! Would love to read it. It is not like there is some conspiracy holding you back.

arghwhat• 1 year ago

Edit: fundamentally, if hacker news has taught me anything, it's that "downvote = makes me feel bad and doesn't want to answer questions". The entire concept of democratic news aggregation was a lie.

Guidelines:

Please don't comment about the voting on comments. It never does any good, and it makes boring reading.

That it feels bad to not win the popular vote does not make democracy a lie, and there's no surprise in not winning favor when blanket discarding the current topic and describing it as "hubris", while not adding any new or constructive information.

TomK322• 1 year ago

The Battle of Clontarf on April 23rd 1014 springs to mind. While the high tide was of favour for the invading Vikings (who had already founded and still ruled Dublin) at 5:30 in the morn, the battle lasted all day and the next high tide at 17:55 cut off their way to a nearby wood and many killed or drowned as their were pushed against the tide. The times were calculated in 1860 by Samuel Haughton.

There is of course an In Our Time episode https://www.bbc.co.uk/programmes/m0029qh3

umanwizard2• 1 year ago

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

daveguy• 1 year ago

Turns out teachers are people and general understanding evolves over time and not all at once.

Who would have guessed. Well, Laplace maybe.

an0malous• 1 year ago

What are the others?

The Bernoulli principle is one.

roelschroeven• 1 year ago

The 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.

umanwizard2• 1 year ago

“Serious companies will require you to comment every line of code”

joshmarinacci• 1 year ago

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

Calwestjobs3• 1 year ago

Most kind way of saying Newton was a simple man.

alejohausner• 1 year ago

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

Calwestjobs3• 1 year ago

earth is 3D not 2D ;) "bulges" same. that is where confusion comes from. also tesseract is nonsense.

fransje26• 1 year ago

Nicely spotted!

antognini• 1 year ago

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

zabzonk• 1 year ago

destruction (or nearly) via tidal mechanics happens in several of larry niven's short sf stories

ghaff• 1 year ago

As I recall there were issues with the math in Neutron Star though still a very good story.

hinkley4• 1 year ago

There’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• 1 year ago

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

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