Today at the Harbour

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NOAA CO-OPS harmonic constants

Pick a Day

Units

Pick a Station

The Tide Today

Loading the constants for 61 stations…

The line takes the colour of the sky at each hour Night The hairline: drag it and the board follows the Sun

Water Below m

Type a height and the chart shades the hours the water is under it: tide pools, a launch ramp, the walk round the point.

Height Now

See the Sum

Next Turn

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Turns This Week

Range Today

The Week Ahead

Sun and Moon

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Tide Cycle

Harmonic Fingerprint

This stationNearest station

The Week Ahead

Higher high waterLower low water

Turns This Week

Water	Day	Time	Height

The Sum at the Hairline

Each constituent's share of the height at the hairline
Constituent	H	f	Share

Nearby Stations

Your Favourites

Tap the star beside a station's name and it waits here, on this device.

The Light Today

The board takes its colours from these hours. Drag the hairline across them and it follows.

Twilight, sunrise, golden hour and sunset at the station
Light	Sun	Morning	Evening

The Month

Every high and low of the month, with sunrise, sunset and the Moon. The calendar file holds the same turns, in the harbour's own time.

How It Is Computed

The tide at a harbour is a sum of cosines. Each one is a constituent: a steady rhythm set by the Moon or the Sun, with a speed known from astronomy and an amplitude and lag measured at that harbour over years of gauge records. M2, the principal lunar tide, turns once every 12 hours 25 minutes; K1 and O1 carry the daily rhythm; S2 beats against M2 to make springs and neaps.

h(t) = Z₀ + Σ fᵢ Hᵢ cos(ωᵢ t + (V₀ + u)ᵢ − gᵢ)

NOAA publishes H and g for 37 constituents at each station. The page computes the rest as you look: V₀ from the mean longitudes of the Moon, the Sun and their perigees (Meeus); f and u, the 18.6-year nodal corrections, from Schureman's formulas, held for mid-year as NOAA holds them. Z₀ lifts the sum onto mean lower low water, so heights read as NOAA prints them.

Highs and lows come from the rate: sampled every 6 minutes, then bisected to the second where the water stops. The Sun and the Moon come from Meeus as well, and the sky behind the curve is the Sun's altitude at the station, minute by minute.

The Receipts

The test suite checks this page against NOAA's own published highs and lows for 5 to 11 October 2026. NOAA's tolerance is 0.06 m and 12 minutes, and it was not loosened for anyone.

Station	Events	Worst height	Worst time	Verdict
Boston 8443970	27	0.006 m	0.7 min	pass
San Francisco 9414290	27	0.013 m	1.6 min	pass
Honolulu 1612340	27	0.006 m	2.3 min	pass
Pensacola 8729840	14	0.004 m	14.4 min	miss, pinned
Anchorage 9455920	27	0.305 m	28.5 min	miss, pinned

Pensacola misses on one low over a 5 cm swing, where a millimetre moves the turn by minutes. Anchorage is a shallow-water tide that NOAA predicts with 120 constituents; this page carries 37.

What It Does Not Know

Weather. A storm surge or a week of onshore wind moves the water more than every rounding on this page combined.
River stage. Astoria, Philadelphia and Baltimore follow their rivers as well as the Moon.
New constants. NOAA revises its constituents; this page carries the snapshot fetched on —.
More than 37 constituents. Anchorage misses NOAA by 0.31 m for exactly this reason.
Other places. Only the 61 stations listed. A harbour between two of them is not an average of the two.

Time

Every time is the station's own clock, named: Honolulu in HST, Anchorage in AKDT, wherever you read it from.

On Your Home Screen

Add the page to your home screen and it opens like an app, offline, with your favourites where you left them.