Talk:Arthur Thomas Doodson

Tidal calculations - node factors and equilibrium arguments
Can't see that the calculations take account of node factors and equilibrium arguments. What I really want to know is how you can clauculate node factor and eq arg instead of looking em up in a table. Anyone got ideas? Mister Flash (talk) 18:19, 8 June 2009 (UTC)


 * After you get the frequencies from the Doodson numbers, you then do a Harmonic Analysis. The equalibrium arguments come out of the harmonic analysis with the amplitudes, and the nodal factors are from the terms fitting Beta_5. Drf5n (talk) 13:50, 18 May 2015 (UTC)

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Usage of Doodson numbers in tidal analysis
''[Note: the material below was extracted from the article; it is of very high quality, but not encyclopedic. I'm moving it here, hoping that it can find a new home in Wikibooks perhaps. fgnievinski (talk) 17:46, 27 March 2020 (UTC)]''

The usual analysis of a periodic function is in terms of Fourier series, that is, over a period of observation covering a time interval $$T$$, the behaviour is analysed in terms of sinusoidal cycles having zero, one, two, three, etc. cycles in that period; in other words, a collection of frequencies all being a multiple of a particular fundamental frequency. If for example, measurements are made at $$N$$ equally-spaced times (thus at times $$0$$, $$ h$$, $$ 2h$$, $$ 3h$$, $$ ...$$, $$ (N - 1)h = T$$) then there are $$N$$ observations, and the standard analysis provides an amplitude and phase figure for $N/2$ different frequencies having a period of $$0$$, $$ T$$, $$ T/2$$, $$ T/3$$, etc.

In the case of tidal height (or similarly, tidal current) analysis of the situation is more complex. The frequency (or period) and phase of the forcing cycle is known from astronomical observations, and, there is not just one such frequency. The most important periods are the time of Earth's revolution, the completion of the moon's orbit around the earth, and Earth's orbit around the sun. Notoriously, none of these cycles are convenient multiples of each other. So, rather than proceed with one frequency and its harmonics, multiple frequencies are used.

Further, at each frequency, the influence is not exactly sinusoidal. For each fundamental frequency, the tidal force has the form $$A\cos(\omega t + \phi)$$ - that is, an amplitude $$A$$, an angular frequency $$\omega$$, and a phase $$\phi$$ related to the choice of a zero time and the orientation of the astronomical attribute at that zero time. However, because the orbits are not circular, the magnitude of the force varies, and this variation is also modeled as a sinusoidal factor (or cosinusoidal), so that the amplitude is given by $$A[1 + A_a\cos(W_at + P_a)]$$ where $$A_a$$ represents the size of the variation around the average value of $$A$$, $$W_a$$ the angular speed of this variation and $$P_a$$ its phase with regard to the time $$t = 0$$.

Because $$\cos(a)\cos(b) = [\cos(a + b) + \cos(a - b)]/2$$, a product of cosine terms can be split into the more convenient addition of two simple cosine terms, but having frequencies that are the sum and difference of the frequencies of the two product terms. Thus, where there was one cosine term whose amplitude varied, there are now three terms, with frequencies $$\omega$$, $$\omega + W_a$$, and $$\omega - W_a$$. Further, although a variation is well represented by a cosine curve, it is not exactly represented by a cosine curve and so each spawns further terms that are multiples of its fundamental frequency just as in the simple Fourier analysis with one fundamental frequency where the variation being analysed is not exactly sinusoidal.

A determined analysis, such as Doodson excelled at, generates not just dozens of terms but hundreds (though many are tiny: tidal prediction might be performed with one or two dozen only) and the Doodson Number is a part of organising the collection. A particular component will be described with a name (M2, S2, etc.) and its angular frequency specified in terms of the Doodson Number, which specified what astronomical frequencies have been added and subtracted for that component. Thus, if $$f_1$$, $$ f_2$$, $$f_3$$, $$f_4$$, $$f_5$$, $$f_6$$ are the basic astronomical frequencies, and a particular component has the frequency  $$(f_2 + f_3 - 3.f_5)$$, its Doodson Number would be given as 0110-30, meaning $$0.f_1 + 1.f_2 + 1.f_3 + 0.f_4 - 3.f_5 + 0.f_6$$. To avoid the typographical inconvenience of negative signs, the digit string might be presented with five added to each component so that fanciful example would be presented as 566525, except that the first digit may not have five added.

Precise usage depends on the precise choice of the component frequency definitions, whether or not five is added (if not, the string might be called an Indicative Doodson Number), and also, as some forces vary only slowly with time, a calculation once a month (say) might suffice so certain components might not be separated into additive terms following that variation.

Code
This is adapted from a script for the MATLAB system, and its main merit is that it actually does generate a suitable curve. In more general work, times and phases are usually referenced to GMT, and the prediction would be annotated with actual dates and times. NOTE: This should say T + h = 15.041068639°/h And it's not with respect to the fixed stars, but with respect to the equinox. (EMS.s should be based on the tropical month, ca 27.3216 days, rather than on the sidereal month, ca 27.3217 days.) Eric Kvaalen (talk) 13:48, 16 September 2022 (UTC) NOTE: This should be w2 - w5, giving 0.55122091 degrees/hour and a draconic month of 27.2123 days And thanks to for inserting this code back in 2007! Eric Kvaalen (talk) 09:41, 22 January 2022 (UTC)

Results
This shows the common pattern of two tidal peaks in a day, though remember that the repeat time is not exactly twelve hours but 12.4206 hours. The two peaks are not equal: the twin tidal bulges beneath the moon and on the far side of the earth are aligned with the moon. Bridgeport is north of the equator, so when the moon is north of the equator also and shining upon Bridgeport, Bridgeport is closer to its maximum effect than approximately twelve hours later when Bridgeport is on the far side of the earth from the moon and the high tide bulge at Bridgeport's longitude has its maximum south of the equator. Thus the two high tides a day alternate in maximum heights: lower high (just under three feet), higher high (just over three feet), and again. Likewise for the low tides.

This shows the spring tide/ neap tide cycle in the amplitudes of the tides as the moon orbits the earth from being in line (Sun - Earth - Moon, or Sun - Moon - Earth) when the two main influences combine to give the spring tides, to when the two forces are opposing each other as when the angle Moon - Earth - Sun is close to ninety degrees producing the neap tides. Note also as the moon moves around its orbit it also changes from north of the equator to south of the equator. The alternation in the heights of the high tides becomes smaller, until they are the same (the moon is above the equator), then redevelops but with the other polarity, waxing to a maximum difference and then waning again.

This shows just over a year's worth of tidal height calculations. The sun also cycles between being north or south of the equator and as well the Earth - Sun and Earth - Moon distances change on their own cycles. None of the various cycle periods are commensurate, and the pattern does not repeat.

Remember always that calculated tidal heights take no account of weather effects, nor include any changes to conditions since the coefficients were determined, such as movement of sandbanks or dredging, etc.