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Old 26-01-2016, 12:42   #31
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Re: Tidal Cycle? Help me out here!

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Originally Posted by Mike1956 View Post
Having noticed a statement in a training booklet "Moon orbits the Earth over a 28 day cycle creating Spring & Neap tides" I emailed the publisher advising that a Tidal Cycle in fact averages 29 days, 12 hours, 44 mins & 3 seconds approximately. It is known as a Lunar month and does not vary much from this average and is never as little as 28 days. A google determined between the years 1760 and 2200 the shortest Lunar month is 29d 6h 34m and the longest 29d 19h 58m. Casual analysis of my tide table seems to confirm the above.

I was surprised when their reply strongly disagreed with my understanding and they went on to explain why....

The Tidal Cycle is due to an average of 3 cycles of the Moon.

Synodic Month 29.5 days
Tropical Month 27.5 days
Anomalistic Month 27.3 days

Average 28.1 days

I still feel my understanding is correct and have no comprehension of how the Tropical & Anomalistic would affect the Tidal Cycle.

I am sure someone on here will have a thorough knowledge of this subject and I look forward to reading replies.

Thanks

Mike
Mike, it is total academic, and probably has many variables. What is the practicality of it. Lat. is a given as to the effect. The Mayan's may have had it right.
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Old 26-01-2016, 13:13   #32
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Re: Tidal Cycle? Help me out here!

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Originally Posted by deblen View Post
But I also make allowance for strong winds which can affect tide time & height substantially also.

Cheers/ Len
Another thing that may effect tide and tidal currents in rivers and river fed estuaries is river currents, especially during periods of especially high (or low) runoff. I live on the Sacramento-San Juaquin river delta and there are periods where the high runoff eliminates all flood tidal currents. My boat has been the Race Committee boat in races in the Sacramento River and it has laid to the anchor in the same direction all day, bow facing upstream.

And in extreme drought conditions the tidal flood current has reversed the flow in the Sacramento and American rivers enough that in the years before treatment of the sewage, sewage that had been discharged in the Sacramento River downstream from the city of Sacramento would be found in the drinking water intake in the American River upstream of the city of Sacramento. This occurred in the 1930s.
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Old 26-01-2016, 13:31   #33
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Re: Tidal Cycle? Help me out here!

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Originally Posted by ThresherMan View Post
The thing that has me baffled, and perhaps someone here knows, is how the tides have switched times. Here in SF, when I was a lad, the the extreme low tides in the early morning were followed by the 'king tides', around noon or so. Now that I have returned to the bay area, after 25 years, the king tides are early and followed by the extreme lows.

When did this happen?

Ed
Ed, the answer is: every day. Really.

Since the moon's cycle is NOT exactly the same as any given 24 hour day, it changes.

Take any tide&current book, the little ones you get at chandleries and marinas, and read it.

It changes daily, then week to week, month to month and year to year.

Latitude 38 has weekend tides& currents in the front of each issues, or you could use noaa on the web.
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Old 26-01-2016, 14:42   #34
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Re: Tidal Cycle? Help me out here!

Tides are very predictable but the magnitudes can be distorted by storms or even sustained winds. The cycle of about 22 hours has 2 highs and 2 lows and the heights vary is a regular pattern. There must be 20 or more internet sites that will let you look up the times and amplitudes. You need to become familiar with one including the calcs for estimating height and flow at any point in the cycle.
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Old 26-01-2016, 14:47   #35
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Re: Tidal Cycle? Help me out here!

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A very good subject indeed. Local tables for areas of swift current in areas ...
We left on time and the error was 6 inches....low. Whew!!!!That was close. Science is cool huh? Perhaps the moon was a little early? Perhaps the slosh was a little slow?
Tide tables can't take weather into consideration. If the tide was higher than predicted, it is likely that atmospheric pressure was low.

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Old 26-01-2016, 16:08   #36
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Re: Tidal Cycle? Help me out here!

Ed-
The "lunar day", the time it takes for the moon to appear in the "same" place it did yesterday, is about 50 minutes longer than a 24-hour day. That's because the earth is moving around the sun, the moon isn't just moving by itself, so the motions are more complex.
So fifty minutes at a time, the tides will be advancing every day. Ignoring the effects of how the sun lines up. If you remember the tides always being neat and orderly and on a routine daily schedule when you were younger...that's just a little memory failure, nothing to lose sleep over.
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Old 26-01-2016, 17:04   #37
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Re: Tidal Cycle? Help me out here!

from simonV: "Moon shmoon. It's simple if the tide is out, it will be back in about six hours. Simple."
Er.... not that simple. In many parts of the world (e.g. Gulf of Mexico, and here in the south west of Australia), the tide takes about 12 hours to go out and 12 hours to come back in (diurnal tides). 6 hour tides (semi-diurnal) are more common, but not universal.
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Old 26-01-2016, 17:14   #38
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Re: Tidal Cycle? Help me out here!

As I sail mostly on Sundays, it is helpful to remember that you get more or less the same high tide time every other weekend, and the inverse once a week.
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Old 26-01-2016, 19:47   #39
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Re: Tidal Cycle? Help me out here!

Tides --- an interesting outline from the Navy, which includes (but doesn't name) the Barycentre diagram.

OC2910 - Tides: Basic Concepts and Terminology

Time and tide waits for no man!
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Old 27-01-2016, 01:08   #40
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Re: Tidal Cycle? Help me out here!

This one is simple enough to solve. We know springs and neaps are relative to the moon phase. Simply time how long the moon takes to go from full back to full again and you have your definitive answer.
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Old 27-01-2016, 08:09   #41
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Re: Tidal Cycle? Help me out here!

huh. been sailing for half a century and just learned more about tides than I might have ever imagined. now if I can just figure what to do with my newfound, "information."
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Old 27-01-2016, 08:30   #42
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Re: Tidal Cycle? Help me out here!

Thanks for replies. I am the OP.

Interesting reading posts from people who are at different stages in their comprehension of tides. Most of will have learned something from this thread.

Just recently got the internet so things are a bit new to me.

Before I post again could someone advise how I can bring an earlier post forward and include it in my reply so I can reply specifically?

Thanks

Mike
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Old 27-01-2016, 09:32   #43
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Re: Tidal Cycle? Help me out here!

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Originally Posted by Mike1956 View Post
Thanks for replies. I am the OP.

Interesting reading posts from people who are at different stages in their comprehension of tides. Most of will have learned something from this thread.

Just recently got the internet so things are a bit new to me.

Before I post again could someone advise how I can bring an earlier post forward and include it in my reply so I can reply specifically?

Thanks

Mike
Use the quote button to include a comment. You can delete portions that are not relevant.
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Old 27-01-2016, 12:23   #44
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Re: Tidal Cycle? Help me out here!

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This one is simple enough to solve. We know springs and neaps are relative to the moon phase. Simply time how long the moon takes to go from full back to full again and you have your definitive answer.
Maybe I am misunderstanding you.
In my area,there are two springs each "month" & two neaps.
One set of springs & neaps is "stronger" (full moon spring) than the other(new moon spring).
The 4 week sequence is as follows starting with full moon week:

higher high tide & lower low tides-3 days either side of full moon

a week of neaps

Lesser high tides & lesser low tides-3 days either side of new moon-but still higher highs & lower lows than in neap weeks.

another week of neaps

Probably sounds confusing-don't know how to explain verbally-will post a diagram later.

Another point: It is easy to tell spring weeks from neap weeks.
If high tide occurs at noonish & midnightish= spring tide week
If high occurs at 6am ish & 6pm ish = neap tide week

You can see I've wasted a lot of time in my life watching Fundy tides

Seriously though-there are channels around here that have 25ft depth at high tide & dry bottom 6 hrs later(or sooner). Tides & tidal current are as important as wind locally & it is just part of life.

There are many places in the world that are nearly as extreme as well.

Cheers/ Len
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Old 27-01-2016, 12:45   #45
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Re: Tidal Cycle? Help me out here!

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Here are tide graphs for Mar 2016 lunar cycle-from full moon to full moon.
Note the max. heights & max. lows vary each day. The highest high is about 3 days after full moon.Then there is a week of neaps,then a week of lower springs peaking about 3 days after new moon & then another week of neaps.

Hope this makes sense

Cheers/ Len

I just noticed this graphical display does not denote hour & minute of high & low-bang head!
I always use the tabular format. If you want to check the link in my earlier post,you can better see the "neap at 6am/pm" & "spring at noon & midnight" .
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