A tide clock is a specially designed clock that keeps track of the Moon's apparent motion around the Earth. Along many coastlines the Moon contributes the major part (67 percent) of the combined lunar and solar tides.
Tide clocks keep an approximation of the average time between high lunar tides: 12 hours 25 minutes per revolution. As the precise average time between high lunar tides is actually half of a lunar day, or 12 hours 25 minutes 14 seconds, tide clocks gain about 15 minutes per month and must be reset periodically.
The bottom of the tide clock dial is marked "low tide" and the top of the tide clock dial is marked "high tide." The left side of the dial is marked "hours until high tide" and has a count-down of hours from 5 to 1. There is one hand on the clock face, and along the left side it points to the number of hours "until" the (lunar) high tide. The right hand side of the clock is marked "hours until low tide" and has a count-down of hours from 5 to 1. The number pointed to by the hand gives the time "until" the (lunar) low tide.
Some tide clocks count down the number of hours from high or low tide, as in "one hour past high or low tide". When the clock reaches the half way point ("half-tide"), it then counts the hours up to high tide or low tide, as in "one hour until high or low tide".
Tides have an inherent lead or lag, known as the lunitidal interval, that is different at every location, so tidal clocks are set for the time when the local lunar high tide occurs. This is often complicated because the lead or lag varies during the course of the lunar month, as the lunar and solar tides fall into and out of synchronization.
The lunar tide and solar tide are synchronized (ebb and flow at the same time) near the full moon and the new moon. The two tides are unsynchronized near the first and last quarter moon (or "half moon"). The best time to set the clock is at the new moon or the full moon, which is also when the clock can most reliably indicate the actual combined tide. Along shorelines where both components are important, a simple tide clock will always be least reliable near the quarter moon.
Tide range is the vertical distance between the highest high tide and lowest low tide. The size of the lunar tide compared to the solar tide (which comes once every 12 hours) is generally about 2 to 1, but the actual proportion along any particular shore depends on the location, orientation, and shape of the local bay or estuary. Along some shorelines, the solar tide is the only important tide, and ordinary 12 hour clocks suffice since the high and low tides come at nearly the same time every day. Because ordinary tidal clocks only track a part of the tidal effect, and because the relative size of the combined effects is different in different places, they are in general only partially accurate for tracking the tides. Consequently, all navigators use tide tables either in a booklet or on a computer.
- How Time & Tide Clocks Work, TrinTec