The Doodson-Légé Tide Predicting Machine
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Re: The Doodson-Légé Tide Predicting Machine
#2Video: 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-...
Re: The Doodson-Légé Tide Predicting Machine
#3There'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
Re: The Doodson-Légé Tide Predicting Machine
#4“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?
Re: The Doodson-Légé Tide Predicting Machine
#5There'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 sy…
I think the first "accurate" prediction machines used about 6 harmonics (cosines). Modern ones I believe use 37 with some look ups.
Re: The Doodson-Légé Tide Predicting Machine
#6“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).
Re: The Doodson-Légé Tide Predicting Machine
#7There'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 sy…
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!
Re: The Doodson-Légé Tide Predicting Machine
#8There'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 sy…
> science museum London
I cannot recommend this museum enough. So many great exhibits.
Re: The Doodson-Légé Tide Predicting Machine
#9Oh how wonderful! My late Aunt Valerie Doodson coordinated the restoration and exhibition of this. I am very happy to see it on HN. <3
Re: The Doodson-Légé Tide Predicting Machine
#10> 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?