Re: Bulges

[Leigh Palmer <palmer@sfu.ca>]

> Well, no one "detects" the various harmonic forces directly.  However, note
> that, depending on the model, there may be over 100 harmonic terms in the
> calculation and according to the data before me the amplitude of the weakest
> harmonic considered is 5 or more orders lower than the principal lunar
> harmonic, M2.  A couple of years ago when this topic was rampaging on the
> list, I made a calculation of the principal Venus force, V2 say, and
> compared with the lowest order lunar and solar harmonics.  I found them to
> be comparable.  Of zzzzzzzzzzzz I am senile and may have made an error.   I
> will check.

You needn't check very hard. Venus has approximately 2x10^-6 of the Sun's
mass and its distance is approximately a factor of three nearer at its
nearest, so the amplitude of its term is about 6x10^-5 that of the Sun.
The Sun's term is smaller than the Moon's, and in any event tide tables
that are significant even at the 0.01% level do not exist. I realize that
resonances could *conceivably* increase the effectiveness of particular
components by a factor of ten or so, but I doubt that the effect is
detectable given that it would be so close to the solar term.

> > Tide tables are not generated by doing fourier transforms on tidal data.
> > They are generated by doing harmonic analysis, quite a different process.
>
> Well they *used to be* harmonic reductions, but I understand they now do a
> Fourier transformation -- the result should be the same -- the Fourier
> transform should yield the same frequencies as the astronomical force
> calculations.  The coefficients would need to be empirical in any case.  I
> will consult with my friend at NOAA and check.

My one question would be this: are tide tables constructed by this new
technique better than tide tables constructed in the old way? I realize
that performing a fourier transform is no more computationally difficult
than doing a harmonic synthesis, but is it in any way better? It does
seem that there is a certain rationality in discarding terms that arise
from noise (weather and seismic activity), and this can be done if one
has other prior knowledge such as astronomical information. What is the
advantage of discarding information?

Leigh