Instead of telling you what time of day it is, tide clocks tell you how long it is until either high or low tide.
Tide clocks are commonly used by seafarers and people who live on the coast, who rely on knowing when the tide will be up or down. Seafarers benefit from knowing whether the tide is going in or out for navigation and people who live on the coast often plan activities according to the tide.
The 12 o'clock position marks high tide on a tide clock, and the 6 o'clock position marks low tide. There is one hand that rotates clockwise and, as it moves between tides, numbers indicate how many hours are left until the next high or low tide.
How Does a Tide Clock Work?
In order to know how a tide clock works, you first need to know how tides work.
Tides are mostly controlled by the gravitational pull of the moon. As the earth rotates on its axis, whichever area of the earth is directly underneath the moon is most affected by its gravitational pull. This gravitational pull causes the water (oceans) on earth to move towards this area. This is what we know as high tide.
While the sun's gravity and other forces also control the tide, the majority (67%) of control of the tide is performed by the moon.
Tide clocks work according to the moon. It takes 24 hours and 50 minutes for a complete lunar tidal rotation to occur (for the earth to spin until the same spot is back underneath the moon). Tide clocks simply make a complete rotation every 24 hours and 50 minutes as well. This means that they stay in time with the lunar tide cycle.
The actual mechanism of a tide clock is no different from a standard clock. It is just that they rotate at the same speed as the lunar tide cycle.
Do Tide Clocks Always Work?
While tide clocks are handy, they are only really useful in certain parts of the world and, even in those regions, they are not always accurate.
Tide clocks only work in areas with a semidiurnal tide (a tide that takes place twice a day). While this is the case, many parts of the world do have this kind of tide and where tide clocks can be used. The Atlantic Ocean is a good example.
While they can be used in these areas, they are inaccurate at certain times. During certain phases of the lunar cycle, the tide is less controlled by the moon's gravity. At these times, other forces, such as wind and atmospheric pressure, also affect the timing of high or low tide and can cause it to change from its normal pattern. This causes tide clocks to be inaccurate at those times.
How to Set a Tide Clock
Tide clocks usually have a control knob on the rear, which allows you to move the hand to set them accurately. You should set a tide clock when the tide is most in sync with the moon (when it is most strongly controlled by the moon). This is either a new moon or a full moon.
Nowadays, it is best to find a tide timetable and set the clock to exactly high tide according to the time of a normal clock. Alternatively, the traditional way is to watch the ocean and to wait for high tide.