Making an accurate tide clock

This is the first in a series of blog posts where I will record the progress with the construction of a long case clock which aims to accurately predict the tides for Fidra, North Berwick on the East coast of Scotland or indeed anywhere in Northern Europe.

There are many quartz tide clocks available on the market at reasonable prices and they are quite basic in their operation in that they have a movement which predicts tides based on the lunar day generating high tides every 12 hours, 25 minutes and 14 seconds.  Anyone who has owned one of these clocks and compared the reading with the actual tides will notice that they are disappointing in their accuracy, sometimes deviating up to an hour in the tide predictions.

As an experiment I made a crude one myself based on this simple pattern and confirmed that it was indeed not accurate.

The reason for the inaccuracy of these clocks is that they simply assume the tides are influenced purely by the gravitational effect of the moon rotating around the earth pulling the ocean water on the earth’s surface resulting in water bulges on opposite sides of the earth.  The reality of predicting tides is somewhat more complex and there are many factors influencing tides and much academic research has been carried out into this field.  

These tide clocks which only take into account the influence of the moon would be far more reliable if they also considered the gravitational pull of the Sun on the tides and added this to the equation.  The Sun, although much further away from the Earth than the moon, has a gravitational pull 46% as strong as that from the moon.  

The alignment of the sun and moon in different positions also influence the height of the tide.  

You get the highest tides (spring tides) when the Sun, Earth and the Moon are in line; this is where the solar and lunar gravitational pull acts together.  This happens 2 times each syndic month which lasts 29 days, 12 hours, 44 minutes and 3 seconds.

Between the spring tides, you have low tides which are known as neap tides.  This is where the Sun, Earth and moon are aligned at a right angle. The lunar tide is canceled out slightly by the solar tide at this time of the month.

This is of course a simplified view of tides known as equilibrium theory which is based on the movement of water on a planet which has a uniform depth of water on its surface and no land! The actual tide patterns are influenced by many other factors such as our continental shelves and resistance of water moving over the sea bed, but for the purpose of improving the accuracy of the unreliable tide clocks out there, this is a good starting point, the dominant influences of our tide patterns are from the moon and the sun combined.  It doesn’t apply to all parts of the world, but for Northern Europe which has a semidiurnal tide pattern (roughly 2 tides a day) things are easier to calculate.

What I’m trying to achieve here is merging 2 harmonic motions, that being of the sun and the moon.  The gravitational pull of the moon results in 57 peaks per lunar month and the gravitational pull of the sun results in 59 peaks per lunar month.

The graph below shows 2 waveforms.  The red wave is the basic moon influence alone which shows the regular 12 hours, 25 minutes and 14 seconds pattern.

The green waveform is where I’ve added the sum of the moon (57 peaks per lunar month) to the sum of the sun (59 peaks per lunar month).  What we can see is very interesting and forms the basis of my improved tide clock.  You can see at the start of the graph we have a spring tide, and as we move to the right you can see the peaks become lower simulating a move towards a neap tide, but you will also see the peaks of the tide shifting too. 

In the diagram below I’ve shown the waveform a little closer to show the shifting of the peaks; this is of particular interest to my design as it shows the way the lunar tides shift the times of the high tides over the lunar month.  As we move from a spring tide to a neap tide the combined waveform peaks are gradually delayed when compared to the moon only waveform.  Then as we move from neap tide back to spring tide the peaks start to catch up with the moon waveform again. 

This shifting of the tide peaks is the key to improving the tide clock design.  Mechanically my design goal here is to make a clock with a rotating tide hand which doesn’t advance in a linear pattern.  It needs to speed up and slow down over a 2 week period!  I have a design for achieving this but it’s taken a lot of research to come up with a design.

Talking about the design you may be thinking what style of clock will this tide dial be included in.  I’ve decided to make this as a longcase clock based on a regulator clock I made in 2008.  The design of the clock is very clean and simple and it has a number of features such as a temperature compensating pendulum which is about as accurate as you can make a mechanical clock.

In my next blog post I’ll introduce some of the basics for starting to make a long case clock movement and also start to show how the design of the complex tide predictor works. 

For pictures of work in progress take a look at my Instagram page.