The Doodson-Légé Tide Predicting Machine | Hacker News Reader

The Doodson-Légé Tide Predicting Machine

ntslf.org

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There's a couple of other designs in the science museum London.

Iirc one uses inclined cones as a variable mechanism. It's described as a harmonic analyser but I am pretty sure the primary harmony here is the earth-moon-sun tidal system.

Or maybe I'm wrong .. I am. There is a harmonic analyser but its lord Thompson of kelvin doing atmospheric research for the met office. He also did a tide predictor, it's a pulley system . They're both in London on display

I think the first "accurate" prediction machines used about 6 harmonics (cosines). Modern ones I believe use 37 with some look ups.

Lord Kelvin's looks to be about 6-8 sets of coupled pulleys top and bottom, the one in Liverpool is a baeowulf cluster of pulleys!

> science museum London

I cannot recommend this museum enough. So many great exhibits.

Sadly, it used to be better. Dense cases of science trash and a true discovery for young minds. Now it's better contextualised and sanitised but less exciting. I mean, who won't want to play with a 1950s neutron source? Or stumble upon something instead of being led there? Damn those OSHA and pedagogy theorists.

If you like a hunk of amazing stuff that's not well-presented, then I suggest checking out the Cradle of Aviation Museum. They have a 707 cockpit with a periscopic sextant in it.

Video: https://www.youtube.com/watch?v=ANOboDEFr7M

The Doodson-Lege tidal prediction machine being put back together: https://www.youtube.com/watch?v=DI9JoDxaEMU

The exhibition – Roberts-Légé and Doodson-Légé machines: https://tide-and-time.uk/exhibition

Tide predicting machines restored and re-displayed: https://www.liverpoolmuseums.org.uk/stories/tide-predicting-...

Oh how wonderful! My late Aunt Valerie Doodson coordinated the restoration and exhibition of this. I am very happy to see it on HN. <3

> Before tides for a particular location can be predicted, a series of observed hourly heights, preferably of twelve months duration, are mathematically analysed. This analysis separates the observed tide into its astronomically forced components, known as harmonic constituents or harmonic constants, ... An analysis for this machine resolved up to 42 constituents. A modern analysis frequently resolves 120 or more constituents.

I imagine that tides could also be computed with CFD and a 3D model of the sea floor, without having to observe for 12 months at each new location. Is this done, and if so, how does the accuracy compare?

In theory you should be able to do this, but I expect in practice it is such a complex system that you couldn't faithfully model the tides by looking at the bathymetry.

It's one of those problems where everything affects everything. It's a gigantic pool of liquid, so you'd have to model the entire ocean. For many areas you'd need to be taking into account how the shape of the coastline itself changes with the tide as well. Computationally extremely complex to do it this way.

If you’re interested in tides, I recommend Jonathan White’s book “Tides: The Science and Spirit of the Ocean”. It’s lower on science and higher on personal experience, but it all worked well for me.

It was the first place I heard the term amphidromic point:

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

“The Doodson-Légé Tide Predicting Machine is used for occasional demonstration purposes only now, the results comparing very well with those produced by the modern computer.”

Surely they mean that it tracks the actual tides?

> Surely they mean that it tracks the actual tides?

No, they mean it computes the value (as a function of time) of a harmonic series with 42 parameters which ideally will correspond to the tides at some specific location.

It's not in any way "tracking" or connected to the actual tides.

If you think of some idealised tidal bulge (pairs! one either side of the earth) tracking about the globe in response to the motion of the moon, that's the baseline "tide" .. now imagine that tidal bulge being funnelled between the narrow gap between Australia and PNG .. that's a complication .. now imagine that being slowed and channelled by various islands and headlands before being squeezed into a horizontal waterfall (something to look up ).

That specific geographic location experiences a unique pattern of tides that are driven by the primary earth|moon dynamics but transposed, amplified, and delayed in a complex manner that can be described(?) by a set of parameters that take some effort to derive.

The Doodson-Légé Tide Predicting Machine has a go at that task using 42 parameters, modern computing tables might require 120 or so, and both systems would routinely require parameters to be updated in response to computational drift and changes in landform (erosion, silt and mud build up, new channels being carved out, etc).

Right I was confused when the article mentions the other parameters other than the moon and sun ones track other astronomical variables, surely they are modeling geological and hydrological variables also (or potentially primarily)

There are two intertwined "sets" of effects to be modeled:

Primarily Earth + Moon, with a secondary twist of Sun, and a layered decline of precession | orbit woblle, lessor effects (the astro forces),

And then the ground effects; shaping around headlands, sloping of seafloors, funnelling through channels, etc. with a rinse and repeat cycle for sea areas that are "chained" backwards from the main flux via multiple bays and estuaries (internal bodies of water large enough to have their own tides via mood gravity while also connected with a delay to an outer ocean via a long channel, etc.

Fun stuff - I primarily worked with exploration geophysics but dabbled a little in tides and ocean levels across Australia.

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