calling all tidal experts

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ChrisGreaves
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calling all tidal experts

Post by ChrisGreaves »

I understand the table of moveable feasts, such as Easter, based on the first full moon after the spring equinox, or something like that.
So where can I find a table that allows me to calculate the tide times (high and low) for places around the world.

I know that the earth and moon's orbits have perturbations, that may not have been apparent to the folks who compiled the tables for Easter, but NASA et al by now must have a pretty good handle on the aberrations, enough to provide a table that could be incorporated into a human-form calendar for a period of, say, five years into the future?

When i look up "tide tables" I see pages of specific dates, but not a table that can be used to power a calculation.

(signed) "Curious" of Bonavista.
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HansV
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Re: calling all tidal experts

Post by HansV »

You might build a Tide-predicting machine

(From another Wikipedia page: "Tide prediction was long beset by the problem of laborious calculations. Before the use of digital computers tide tables were often generated by the use of a special-purpose calculating machine, the tide-predicting machine.")

Also see Is there a simple equation for calculating the time of high tides?
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Hans

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ChrisGreaves
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Re: calling all tidal experts

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OK, so I’ve read through the tidal papers (why am I doing this when the sun is shining for the second day in a row?), and I can see the idea behind the local influences (seabed variations, coast outline, and so on), but I am still confused.
The tidal energy comes from astronomical sources, simply put, the gravitational attraction of the sun and the moon. We have all seen those “bulging oceans” diagrams in our school-days atlas.
The astronomical forces must follow a regular pattern, perhaps several years in period (discounting the change in the moon’s orbit due to tidal friction ...). Our forbears could predict eclipses, which to me means that they could predict positions of these two major bodies.
Were the earth uniform, the prediction of tides (times and heights) should be straightforward. Local topographical conditions impose variations.
But then, those local topographical influences don’t change (over human lifespans). The cliffs are still there, although receding at perhaps a foot a year. The bay is just as deep, although perhaps silting up at the rate of two inches a year, and so on.

That is, one should be able to establish the “pure” tide prediction, and then impose observed variations on that pattern, adding in correctional factors every ten years, or so.
This would be accurate enough to allow sailing merchant vessels to be at the harbour (or be prepared to leave the harbour) to within five minutes or so.

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Chris
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Re: calling all tidal experts

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Have you had a read at https://oceanservice.noaa.gov/news/nov1 ... -2019.html" onclick="window.open(this.href);return false; ?

And check out the links provided at the above site
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ChrisGreaves
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Re: calling all tidal experts

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DaveA wrote:Have you had a read at https://oceanservice.noaa.gov/news/nov1 ... -2019.html" onclick="window.open(this.href);return false; ? And check out the links provided at the above site
Thanks Dave.
I have d/l this evening's reading material.
Most of the links seem to take me to a USA-only site, but I'll check again.
I suspect that this data is for people who want to know tides in advance, whereas I am interested in the mechanisms/algorithms behind predicting the tides.

Cheers and Thanks
Chris
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Re: calling all tidal experts

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ChrisGreaves wrote:
19 Jun 2019, 18:57
So where can I find a table that allows me to calculate the tide times (high and low) for places around the world.
Not a table, so I apologize to myself, but a fascinating video with some great animations of analogue and digital computers, integrators, and a call-out to Fourier analysis.

The Most Powerful Computers You've Never Heard Of

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Chris
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Re: calling all tidal experts

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While you're at it, you could let us know why the tide (at Sharpness Dock, as an example) comes in (about 4 hours) quicker than it goes out (about 8½ hours)...
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Re: calling all tidal experts

Post by HansV »

River tides, why the flood is faster than the ebb
But if I read Ebb rate vs flood? correctly, it's different in the Solent.
:shrug:
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Re: calling all tidal experts

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Since Sharpness Dock in on the Severn estuary, the explanation in the link you give seems quite plausible.
Chris should note this so that he does not follow me by asking silly questions!
Oh...
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Re: calling all tidal experts

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John Gray wrote:
26 Dec 2021, 15:01
Chris should note this so that he does not follow me by asking silly questions!
"Ignorance is not knowing; Stupidity is not asking."
Since Sharpness Dock in on the Severn estuary, the explanation in the link you give seems quite plausible.
"Sharpness is our watchword!".
Untitled.png
I tried to imagine that I was a tidal flow, ebb, etc.
Failed.
Then a map of the docks reminded me of pumping up a bicycle tyre, amongst other things. (Too I had been watching YouTube videos about water towers, two of which live quite close to me).
I can pump up a tyre quite quickly, enough force applied cyclically to force a fluid (air) through a narrow opening into a larger diameter inner tube.
Bonavista Town Council likewise can fill an enormous tank atop a tower by a small force applied to a (say) one-inch diameter pipe. Filling the tank 24/7 can provide enough reserve to permit peak outflows between 7am-9am and again 4pm-6pm.
At peak times (when I pump up the bike tyre) the rate of inflow is higher than the rate of outflow when the air escapes through a small puncture hole.

The force available to the incoming tide is significant, as it is relentless, almost infinite: thousands of miles of Atlantic Ocean being dragged by the moon.
The force available to the outflow is insignificant, being merely the water that flowed in tidally, plus any rivers outflow. Outflow has nothing like the reserves of momentum owned by The Atlantic Ocean.
That both in- and outflow are powered by gravity is irrelevant.

Now the opposite must also be true, under specific conditions.

There are places around the UK that experience four tides a day, two which arrive as water surges in a clockwise direction around the north of Scotland and then down into the North sea, and a second pair which arrives via the English Channel. I imagine that where these two surges meet the force might be well diminished, and that a mile or two north or south of the balance point would have a very weak tide (compared to the Severn) that might struggle to overpower the outflow of a local river or creek near Skegness.

A similar argument might apply to the mouth of the Fraser river in British Columbia; the river may well be a significant flow of water compared to the inflow of a tide.

The Severn Bore (and the bores at Moncton etc), are renown for the wave that flows up river with the incoming tide, but no-one raves about an outgoing wave, flowing downstream and out to sea.
(later) We might note too that both the Bay of Fundy and the Severn Estuary are similar in the sense that they both run SW/NE and both are open at the SW end, like giant funnels, poised to harvest the tidal surge as it travels around the globe from west to east. Clearly what we need is a table of tidal bores with the compass orientation of the open mouth. Perhaps 80% of the top bores face the south-west to maximize the utility of the approaching tide.

Cheers
Chris
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Re: calling all tidal experts

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John Gray wrote:
26 Dec 2021, 11:35
While you're at it, you could let us know why the tide (at Sharpness Dock, as an example) comes in (about 4 hours) quicker than it goes out (about 8½ hours)...
Hello John.

Please see below my "bicycle pump vs. small puncture" parallel. I can pump up the tyre in 4 hours, but it takes eight hours for the puncture to flatten the tyre.
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Re: calling all tidal experts

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I hope you are not being misled by the simplicity of Google maps in non-satellite mode!
There are lock gates just to the west of the Tidal Basin, and another set a little further east. From there onwards there are the 16-odd miles of the (necessarily land-locked!) Gloucester and Sharpness Canal, with no further lock until you get to Gloucester. The River Severn at Sharpness flows from north to south, just outside the interestingly-shaped breakwaters which provide a degree of shelter for ships waiting for high tide to enter the Tidal Basin and subsequent Docks...
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Re: calling all tidal experts

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John Gray wrote:
26 Dec 2021, 11:35
While you're at it, you could let us know why the tide (at Sharpness Dock, as an example) comes in (about 4 hours) quicker than it goes out (about 8½ hours)...
There is, too, a strictly mathematical/logic explanation, which is hinted at with your phrase "(about 4 hours) quicker than it goes out (about 8½ hours"

If we assume a simple model of two tides a day (I know it is more complicated on account of the earth's revolution, the moon's orbit etc, but we can imagine a twelve-hour interval between high tides), then for whatever reason, if it takes four hours for tidal forces to pump an estuary full of water, it must, by definition, take 12-4=8 hours for the estuary to empty.

Now the estuary doesn't empty at all, at all, but when we think "twelve hours between peaks", then we must think of the complement of twelve hours for the "goes out" given the "comes in", and of course, the complement of twelve hours for the "comes in" given the "goes out".

I suspect that this mathematical/logic argument must apply to all natural complementary activities, but it is time for my nap.

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Chris
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Re: calling all tidal experts

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John Gray wrote:
26 Dec 2021, 16:59
I hope you are not being misled by the simplicity of Google maps in non-satellite mode!
I have lost satellite mode since doing a Macrium Restore at lunchtime.

Hi John, no, not misled. I just used the first narrow-opening I could see. Basically the first "narrow opening" in any natural or man-made channel would satisfy my bike-pump analogy.
Untitled.png
The argument should apply equally to the configuration of Bonavista harbour and the creek from the pond/swamp.

(later) I suspect a parallel phenomenon might be found in a car exhaust's muffler/baffles, as in successive restriction of flow, but there the flow is always one-way, rather than cyclic.
Cheers
Chris
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