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Just like any regular clock, tide clocks are designed to tell you a time.
Regular clocks are calibrated to roughly match the earth’s rotation, giving you the normal time that we are all familiar with.
Tide clocks are calibrated to match the lunar cycle, approximately one rotation every 12 hours, 25 minutes. Given that the lunar cycle powers the tides, tide clocks are a simple way of tracking the times of high and low water in a given location.
Tide clocks are a surprisingly common technology, with some of the biggest online stores carrying a large selection.
For a quick example, you can see this tide clock on Amazon, although there are plenty of others if you want to get one that will match your home.
The hand completes one rotation every 12 hours, 25 minutes, corresponding to the average time period between two consecutive high waters.
Analogue Tide Clocks
Analogue tide clocks look similar to a traditional analogue clock.
High tide is marked at the 12 o’clock position, and low tide is marked at 6 o’clock.
As time passes, the hand moves around the face in the same way that normal clock hands move around a regular analogue clock.
The only difference is that it takes 12 hours, 25 minutes for the hands to complete one revolution.
12 hours, 25 minutes is the average time of a lunar cycle, which means that most cycles are not precisely that length.
Although the clock will indicate high and low water, usually the true time of high water falls a couple of minutes on either side of what the clock indicates.
Over the course of a month, however, the clock does a reasonable job of estimating the times of high and low water for a given location.
Digital Tide Clocks
Digital tide clocks are far less common than traditional tide clocks because they are really just small computers that tell you the state of the tide.
The main advantage of a digital tide clock is that it is more precise than an analogue tide clock, but they don’t look as good.
For most people, if you want precision, you would use tide tables or computer software to tell you the times of high and low water so that you can relate it to the actual time.
A tide clock will tell you how many hours until the next high water, but it will not give you a useful time to plan your passage
You really want to know what time to arrive at your boat so that you can go sailing.
For that, you would use a tide table instead, and relate that to the actual time.
Tide Clock vs Tide Watch
Tide clocks and tide watches are the same things, just different sizes.
While tide clocks are usually a traditional analogue style clock with a moving arm, a tide watch is just as likely to be digital.
Built into a regular watch, the tide watch tells you the same information as a tide clock, just on the smaller watch face.
How Accurate Are Tide Clocks?
Tide clocks are accurate enough for recreational uses where you just want to know if the tide is in or out.
For commercial use, however, a tide clock is not accurate enough to give reliable data. You would always need to use tide tables and relate them to the actual time.
Tide clocks are calibrated to run on a 12-hour 25-minute cycle, giving an average interpretation of the time of high and low water.
We have already discussed how the average time can wander over the course of a lunar month, which is due to an issue called lunar lag.
An accurate tide clock should be set at high tide on a new moon or a full moon when the sun and the moon are in line.
Throughout the course of the month, lag will mean the time wanders around, giving its peak errors around the time of the quarter moons.
For those familiar with the tides, you will already realise the significance of this, and how it is not quite as bad as it looks.
At full moon and new moon, when the clock is likely to be most accurate, you get spring tides. Then, when the clock is at its least accurate, during the quarter moon, it will be neap tides.
In this article: “What Is The Difference Between Spring & Neap Tides?”, we discussed how neap tides are the least extreme, almost making them most tolerant for inaccuracies.
Do Tide Clocks Work Everywhere?
Tides clocks only work where the local tide is regular, with two high tides and two low tides each day.
Most commonly they work along the Atlantic coasts because the Atlantic tides are influenced to the greatest extent by the moon.
Across the world, in the Pacific Ocean, for example, tide clocks are much less accurate because the moon’s gravity pulls on different parts of the ocean at different times.
Similarly, in areas where high waters are influenced by local geography, tidal clocks will not work.
One of the most notable examples is in the Solent in the UK where they experience double high waters.