Clock types · Time systems and signals

Tide clock

A one-handed clock that tells you not the hour but the state of the sea: how long until high water, and how long since the last low.

By David Ray · clocks.fyi

What is a tide clock?

A tide clock shows where you are in the cycle of the tides rather than the time of day. It usually has a single hand. The top of the dial marks high tide, the bottom marks low tide, and the marks in between show how many hours have passed since the last high or low and how many remain until the next.

Boaters, anglers, beachcombers and anyone living by the sea use them as a quick visual guide. The UK's National Tidal and Sea Level Facility stresses that they supplement local tide tables rather than replace them.

How it works

Most coasts see two high tides and two low tides in a lunar day, the time it takes for a spot on Earth to come back under the Moon. NOAA puts the lunar day at 24 hours and 50 minutes, about 50 minutes longer than a solar day, because the Moon moves along its orbit in the same direction that Earth spins. High tides therefore arrive about 12 hours and 25 minutes apart.

A tide clock simply runs its hand around the dial once every 12 hours 25 minutes or so, twice per lunar day, instead of once every 12 hours like an ordinary clock. NOAA puts the time for water to go from high to low, or low to high, at about 6 hours and 12.5 minutes, which is why the bottom of the dial sits halfway round. Chelsea Clock stresses that the high and low marks at 12 and 6 o'clock are reference points, not clock times.

NOAA adds an important caveat: most coastal areas see two highs and two lows a day, but there are exceptions. Those exceptions are exactly where a simple tide clock struggles, as the next section explains.

See also: where the Moon is in its monthly cycle right now on OnlineClock.app.

Want the exact time right now? See it on a big, clean clock at OnlineClock.app, free, with no sign-up.

Setting one and its limits

To set a tide clock, find the time of the next high tide from a tide table, such as NOAA's tide predictions, and turn the adjusting knob on the movement so the hand points to high tide at that moment. Chelsea Clock describes doing this by removing or opening the bezel to reach the knob on the back.

Real tides do not keep perfect time. The National Tidal and Sea Level Facility says tide clocks in the UK and Ireland are generally accurate to within about 30 minutes over 30 days and should be reset against tide tables roughly monthly, while Chelsea suggests about every four months depending on location.

They also assume two roughly equal tides a day. That suits the U.S. East Coast and Canada, according to Chelsea, but the U.S. West Coast has mixed tides of unequal height, and places like the Gulf of Mexico can have just one high and one low a day. Chelsea sells a separate Pacific version for western waters.

Tide watches face the same problem. The National Tidal and Sea Level Facility explains that watches with a tide mode need a local value called the lunitidal interval, the average delay between the Moon crossing the local meridian and the next high water, which differs from port to port. Its table gives, for example, 11 hours 27 minutes for Dover and 10 hours 4 minutes for Holyhead.

History: tidal dials on early clocks

Showing the tide on a clock is far older than the battery tide clock. Some 17th- and 18th-century English longcase clocks included tidal indications, often alongside a moon dial, since the tides follow the Moon. The Metropolitan Museum of Art holds a longcase clock by Thomas Tompion of about 1677-80 with calendar, lunar and tidal indications.

The Tompion clock is a reminder of how prestigious these complications were: its oak case, veneered with walnut and attributed to the case maker Jasper Braem, has oyster-cut olive wood panels and marquetry of stained bone, ivory and various woods around a gilded and silvered brass dial.

In the Science Museum Group collection, a London longcase clock by Christopher Pinchbeck, made around 1720, has a dial in its arch showing the Moon's phase and age along with the time of high tide at various English and Continental ports.

At St Michael's Mount in Cornwall, a tidal island, a longcase clock made around 1785 by Roger Wearne of nearby St Erth shows the time of high water in Mount's Bay alongside the time and date. The house's owners had good reason to watch it: the tides decide when people can travel to and from the island. The clock stands almost two meters tall, with a mahogany case, a painted arch and a brass face.

How to recognize one

A modern tide clock is easy to spot: one hand, a dial labeled High Tide at the top and Low Tide at the bottom, and hour markers counting down to each. Chelsea Clock, for example, sells its tide clocks as companion pieces to its clocks and barometers.

On an antique longcase clock, look at the arch above the main dial. On the Pinchbeck clock, for instance, the tide information shares the arch with the Moon's phase and age, and gives high water for named ports.

See also: clocks made for porches, docks and patios on OnlineClock.app.

Questions people ask

Why does a tide clock run on 12 hours 25 minutes?

Because the tides follow the Moon. A lunar day lasts about 24 hours 50 minutes, so on most coasts high tides come about 12 hours 25 minutes apart, and the clock's hand matches that rhythm.

How often should I reset a tide clock?

It depends on where you are. UK tidal scientists suggest checking roughly monthly, while Chelsea Clock suggests about every four months. Compare it with a local tide table and adjust when it drifts.

Will a tide clock work on the West Coast or the Gulf of Mexico?

A standard tide clock assumes two similar tides a day, which fits the U.S. East Coast. The West Coast has unequal mixed tides and parts of the Gulf have only one tide a day, so a standard tide clock will be less reliable there.

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