How solunar theory works

Where the idea comes from

John Alden Knight published the solunar tables in 1926 after noticing that anglers’ best hours clustered around the Moon’s passage overhead. The word is his: sol for the Sun, lunar for the Moon, and the tables have been reprinted in fishing almanacs ever since.

The underlying observation — that many marine animals keep lunar and tidal rhythms — is well documented. The specific claim that four windows a day predict feeding is a rule of thumb, and we present it as one. Nothing on this site treats a solunar window as a forecast; it is a way of ranking the hours of a day you were going to fish anyway.

Major periods: transit and anti-transit

A major period is centred on lunar transit — the moment the Moon crosses your meridian and stands at its highest for the day — and on its opposite, the anti-transit, when the Moon is underfoot on the far side of the Earth. Each runs roughly two hours, an hour either side of the event.

There are two per lunar day, about 12 hours 25 minutes apart, because a lunar day is 24 hours 50 minutes long. The Moon does not have to be visible for a major to count: a new Moon transits at about the middle of the day, in daylight, and an anti-transit happens with the Moon below the horizon by definition. If you are waiting to see it before you start, you will miss half of them.

Minor periods: moonrise and moonset

Minor periods are centred on the Moon rising and setting, and run about an hour each. They are the weaker pair, and they fall between the majors: each minor follows its major by roughly six hours and a quarter, because rise and set sit a quarter of a lunar day either side of transit.

That is the whole day: four windows, which are not four independent numbers but one lunar cycle sampled at four points. And because the lunar day is 50 minutes longer than the solar one, the whole set slides later every day — a window that lands at first light this morning falls almost six hours later a week from now, and the four of them will have swapped places with the daylight entirely. It is the same 50 minutes that moves high water later each day.

What lifts a day’s rating

Days near new and full Moon score highest, for two reasons that arrive together. The tidal range is largest then — those are the spring tides, and the tidal coefficient on the day says how large — so there is more water moving through every channel and over every flat. And the same geometry puts the minor periods on the light: at new Moon the Moon rises and sets with the Sun, and at full Moon it rises as the Sun sets and sets as the Sun rises, so moonrise or moonset lands on dawn or dusk. The majors, meanwhile, sit near the middle of the day and the middle of the night.

That coincidence — not the Moon on its own — is the part most people actually notice. A window at three in the afternoon on a slack neap is a line on a chart. A window at first light on a spring tide is the morning you remember.

What the evidence says

Two claims are tangled together here, and they deserve different levels of confidence.

The first is that sea life runs on lunar and tidal clocks. That is not in doubt. Corals spawn on particular nights around a full Moon; grunion come ashore to spawn on the high tides that follow new and full; and many intertidal animals keep a rhythm close to the 12 hour 25 minute tidal period even when they are kept in a tank away from the sea. Animals that live by the tide have every reason to anticipate it.

The second claim is narrower: that those four windows tell you when the fish in front of you will feed. We know of no body of controlled evidence for it, and solunar theory says nothing at all about water temperature, bait, barometric pressure, clarity or season — the variables anglers spend most of their time arguing about. Treat the windows as one input among several, and be sceptical of anything that sells them as more.

The Moon is a good clock and a poor promise. It tells you when the water will move, not whether anything is hungry.

How we compute it

We calculate the Moon’s transit, anti-transit, rise and set for your station’s exact coordinates from the JPL DE421 ephemeris, then build the windows around those events: about two hours for a major, about one for a minor. The day’s rating comes from the Moon’s phase and from whether a window falls near sunrise or sunset.

Nothing is copied from a printed table, so the times are right for where you are rather than for some reference port a few hundred kilometres away — which matters, because moonrise at Newlyn and moonrise at Dover are not the same minute. How we compute sets out the ephemeris and the tide model behind the rest of the page.

Read it with the tide, not instead of it

The most useful signal on the Fish tab is where a solunar window overlaps a tide turn or a strong run of current. When the two coincide, that is the hour worth fishing; when they disagree, most experienced anglers follow the water, and on a coast with a real range they are right to.

How much the tide should outweigh the Moon depends on where you are. On a big-range coast the current is the event, and the windows are a tiebreaker between two otherwise similar afternoons. In a near-tideless sea — much of the Mediterranean moves less than half a metre — there is little current to follow, and the Moon’s clock is more of what you have left to go on.

One caveat holds everywhere: a tide prediction is astronomy, and the weather is not in it. Pressure and wind can move the water half a metre or more for a day at a time, and neither the tide table nor the solunar window knows that is coming. Check the forecast before you commit to a mark.

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