The Tidal Clock: Precision Understanding of the Ocean's Rhythm

The Observatory Almanac — Living Environment Series

Tides are the heartbeat of the coast. Learn to read them and you navigate safely; ignore them and the sea has its own opinion.

INTRODUCTION: WHY TIDES MATTER

Every boat captain, every surfer, every clam digger, every coastal engineer lives by the tide. The difference between a safe passage and a grounding can be a single foot of water. The difference between a productive beach harvest and an empty bucket is understanding when the fish and shellfish are most exposed. The difference between a peaceful walk on the flats and being cut off by a racing incoming tide is knowing the Rule of Twelfths.

Tides are not random. They are the most predictable natural phenomenon on Earth — calculated centuries in advance, accurate to within minutes. This guide gives you the tools to read them with precision.

PART ONE: WHAT CAUSES TIDES?

The Gravitational Ballet

Tides are caused by the differential gravitational attraction of the Moon (and to a lesser extent, the Sun) on Earth's water.

The Moon pulls the water on the side of Earth closest to it with greater force than it pulls Earth's center, and it pulls Earth's center with greater force than it pulls the water on the far side. This creates two tidal bulges: one on the Moon-facing side (direct tide) and one on the far side (inertial/indirect tide).

As Earth rotates under these bulges, most locations experience two high tides and two low tides per day (semidiurnal) — though geography, ocean basin shape, and local resonance can produce diurnal (one per day) or mixed patterns.

The Lunar Day

The Moon orbits Earth eastward, so it takes 24 hours and 50 minutes for the same point on Earth to return under the Moon. This is the "lunar day." Each subsequent tide is therefore ~50 minutes later than the one before — a key piece of practical knowledge for anyone planning coastal activities.

If high tide was at 8:00 AM today, expect the next high tide at approximately 8:50 PM today, and high tide tomorrow at approximately 8:00 AM + 50 min = 8:50 AM.

(In reality, it's closer to 12 hours 25 minutes between consecutive high tides in a semidiurnal regime — the 50-minute drift applies per full tidal cycle.)

The Sun's Contribution

The Sun's tidal effect is about 46% as strong as the Moon's (despite the Sun's enormous mass, its greater distance reduces the tidal differential). The Sun's influence modulates the Moon's tidal pattern, creating spring tides and neap tides.

PART TWO: SPRING AND NEAP TIDES

Spring Tides (Not a Season)

When the Sun, Moon, and Earth align — at new moon and full moon — the solar and lunar tidal forces reinforce each other. The result is spring tides: tides with greater-than-average range (higher highs, lower lows).

The name has nothing to do with the season spring; it comes from the Anglo-Saxon springan, meaning to "spring forth" or "leap."

Spring tide characteristics: - Occur approximately every 14 days (alternating new and full moon) - Tidal range 20–60% greater than average - More extreme exposure at low tide (good for mudflat foragers, but strong currents) - Higher high tides can flood areas that normally stay dry - Associated with stronger tidal currents

Perigean spring tides ("King Tides"): When spring tides coincide with the Moon's closest approach to Earth (perigee), tides are even more extreme — sometimes 20–30% above normal spring tides. These are the tides that flood coastal streets and make the news.

Neap Tides

When the Moon is at first quarter or last quarter (90° from Sun-Earth alignment), the solar and lunar tidal forces partially cancel each other. The result is neap tides: smaller-than-average tidal range (lower highs, higher lows).

Neap tide characteristics: - More moderate — the difference between high and low is compressed - Calmer tidal currents (useful for boaters and divers) - Less extreme exposure — flats don't dry out as much at low tide - Higher low tides can prevent access to some areas

The Tidal Range Spectrum

Location	Average Tidal Range	Example
Open ocean	< 0.5 m	Mid-Pacific
Microtidal	< 2 m	Mediterranean, Gulf of Mexico
Mesotidal	2–4 m	US East Coast (mid)
Macrotidal	4–8 m	Bay of Fundy, UK coast
Hypertidal	> 8 m	Upper Bay of Fundy (16 m!)

PART THREE: THE RULE OF TWELFTHS

What It Is

The Rule of Twelfths is a mariner's rule of thumb for estimating tidal height at any point during a tidal cycle, without tide tables. It assumes a tidal cycle of six hours from high to low (or low to high) and that the tide follows a roughly sinusoidal curve.

The Rule

Divide the tidal range (difference between high and low) into 12 parts. The tide moves through these twelfths as follows:

Hour	Twelfths of Range	Cumulative
Hour 1 (just after HW/LW)	1/12	1/12
Hour 2	2/12	3/12
Hour 3	3/12	6/12 (halfway)
Hour 4	3/12	9/12
Hour 5	2/12	11/12
Hour 6	1/12	12/12 (full range)

The tide moves slowly at the beginning and end of each cycle, and fastest in the middle. The peak tidal current occurs around hours 3 and 4 — when the tide is moving most rapidly.

Worked Example

Situation: High tide at Boston is 10.0 feet at 6:00 AM. Low tide is 1.0 feet at 12:00 PM. What is the approximate water depth at 9:00 AM?

Step 1: Tidal range = 10.0 - 1.0 = 9.0 feet

Step 2: Each twelfth = 9.0 ÷ 12 = 0.75 feet

Step 3: 9:00 AM is 3 hours after high tide (hour 3 of the falling tide)

After 3 hours, tide has fallen: 1/12 + 2/12 + 3/12 = 6/12 of range

Step 4: Tide has fallen 6 × 0.75 = 4.5 feet from high

Step 5: Water level = 10.0 - 4.5 = 5.5 feet above MLLW

Why This Matters for Safety

The danger in the Rule of Twelfths is the middle hours. In a 10-foot tidal range: - Hours 1 and 6: 0.75 feet of movement per hour — slow, manageable - Hours 3 and 4: 2.25 feet of movement per hour — fast; can strand the unwary

A hiker exploring tidal caves, a fisherman in a dinghy on a falling tide, a child building sandcastles: all can be caught off guard by the rapid mid-cycle movement. The Rule of Twelfths makes this explicit and predictable.

PART FOUR: TIDAL COEFFICIENTS

What Are Coefficients?

Tidal coefficients (used extensively in European tide prediction) are dimensionless numbers that express the relative magnitude of a tide compared to a standard reference. They range from approximately 20 to 120, with:

20–45: Neap tides (weak)
45–70: Average tides
70–95: Spring tides (strong)
95–120: Very high spring tides (exceptional)

A coefficient of 95 represents tides approximately 95% of the theoretical maximum astronomical tide at that location. Coefficients above 100 occur during perigean spring tides.

Using Coefficients

European tide tables (particularly French, Spanish, Portuguese) routinely list the daily coefficient. This single number tells you immediately: - Whether currents will be strong or weak - Whether a tidal flat will be exposed or remain covered - Whether the afternoon anchor spot will be high and dry by morning

For the English Channel, a coefficient of 70+ means significant currents; 95+ means strong currents that challenge all but the most experienced boaters.

Practical Coefficient Guide

Coefficient	What to Expect
<45	Weak tide; small range; calm currents
45–70	Moderate; typical conditions
70–95	Strong springs; good for tidal power, difficult for boating
>95	Maximum spring; exceptional exposure of flats; strongest currents

PART FIVE: READING TIDE TABLES

Anatomy of a Tide Table

A standard tide table lists, for each day: - Times of each high and low water (in local standard or daylight time — check which) - Heights above the datum (reference level) in feet or meters

Example (simplified):

Portland, Maine — January 2024
Date    Time    Height    H/L
Jan 1   00:12   10.3 ft    H
        06:24    0.8 ft    L
        12:35   10.1 ft    H
        18:48    0.9 ft    L
Jan 2   00:57   10.2 ft    H
        07:07    0.9 ft    L
...

Datums Explained

All tide heights are measured relative to a datum (reference level):

MLLW (Mean Lower Low Water): US standard; the average of the lowest daily tides over the 19-year National Tidal Datum Epoch. Essentially the lowest "average" tide. Most US charts use MLLW.
MLWS (Mean Low Water Springs): UK standard; average level of low water on spring tides
CD (Chart Datum): Varies by country; typically close to MLLW or MLWS
MSL (Mean Sea Level): Average sea level; approximately 0.9 m above MLLW at many US locations
MHW (Mean High Water): Average of all high tides
MHHW (Mean Higher High Water): Average of the highest daily tide

Practical rule: A tide height of 0.0 feet on a US tide table means the water is at the average level of the lowest tides. Water will almost never be lower than 0.0 feet at that location. Heights can be negative (below datum) in unusual conditions.

Time Zone Caution

NOAA tide tables are typically listed in local standard time. During daylight saving time, add one hour to all listed times. This is a common source of errors in planning.

Clearance Under Keel

For boats, tidal height determines whether there's enough water to float. The calculation:

Available depth = Charted depth + Tidal height above chart datum
Minimum depth needed = Vessel draft + Safety margin (0.5–1.0 m recommended)

Never assume the tide will be at the charted depth. Always check the predicted tide and factor it in. Grounding on a falling tide can leave a vessel stranded for hours; in some areas (Bay of Fundy, Brittany), the boat may be left 15 meters above sea level.

Tidal Current vs. Tidal Height

Tidal height and tidal current (stream) are related but not identical:

In open channels: max current typically occurs near mid-tide (hours 3–4)
At harbor entrances: may be delayed relative to open water
In constricted channels: current may be very strong even with modest tidal range
Slack water (zero current) occurs near high and low tide, but timing varies

Strong tidal currents in channels can exceed 8–10 knots in extreme locations (Old Sow whirlpool, Reversing Falls in New Brunswick, the Pentland Firth in Scotland). Such currents can overwhelm underpowered vessels entirely.

Bar Crossings

Estuary bars and harbor entrances become dangerous when the swell interacts with a strong ebbing tidal current: - Ebb current opposes incoming swell → waves steepen and break unpredictably - Best crossing: Slack water or early flood (current and swell direction aligned) - Many coastal fatalities occur at bar crossings on ebbing tides

PART SEVEN: TIDAL BORES — THE DRAMATIC EXCEPTIONS

What Is a Tidal Bore?

A tidal bore is a surge of tidal water that travels up a river or narrow bay against the direction of the current. It forms when a very large tidal range meets a funnel-shaped estuary that amplifies the incoming water. The leading edge becomes a visible wave (bore) that moves upstream.

Famous Tidal Bores of the World

Qiantang River, China (Silver Dragon)
The most spectacular bore in the world. During extreme spring tides, a wall of water up to 9 meters (30 feet) high charges up the Qiantang River at 25–40 km/h. The event draws massive crowds every autumn around the Mid-Autumn Festival. The bore has claimed lives — spectators must stay well back from river banks.

Bay of Fundy, Canada (Petitcodiac River)
The Bay of Fundy has the world's largest tidal range (up to 16.3 meters / 53 feet). The Petitcodiac River bore is modest (0.5–1.5 m) but reliable, occurring twice daily. The bay itself produces dramatic tidal phenomena: high speed tidal currents, exposed mudflats the size of small countries, and boats left sitting on mudflats at low tide in the upper bay.

Severn Bore, England
One of the world's top six tidal bores, reaching heights of 2 meters (6.5 feet). Surfers ride it for kilometers upstream — it's one of the only places in the world where you can surf the same wave for miles. Occurs when very large tidal ranges coincide with the funnel shape of the Severn Estuary.

Turnagain Arm, Alaska
The second-largest tidal range in North America (after the Bay of Fundy). The bore regularly reaches 2–3 meters. The mudflats of Turnagain Arm are particularly dangerous — they contain quicksand-like silt that has trapped people (and moose) fatally as the bore rushes in.

Amazon River, Brazil (Pororoca)
An equatorial bore that travels up to 800 km inland. The roar can be heard 30 minutes before it arrives. Local surfers ride it; it destroys everything in its path, including riverbank trees and structures.

Mont Saint-Michel, France
The tidal range around this famous island monastery is among the largest in Europe (~13 m). The incoming tide is famously said to move "at the speed of a galloping horse" — actually about 6–7 km/h, but on perfectly flat sand, this is fast enough to outpace a runner in certain approaches. A tidal bore effect occurs in some channels.

Further reading: David Burch — Navigation Rules; Reeds Nautical Almanac; NOAA Tides and Currents (tidesandcurrents.noaa.gov); The Tides of History (François Bellec).