How the forecast is calculated

Solunar theory in one paragraph

In 1926 John Alden Knight collected the folk wisdom that fish and game feed on a schedule tied to the moon and formalized it as “solunar” (sol + luna) theory. The core claim: animals are most active when the moon is at the meridian — directly overhead or directly underfoot — and, to a lesser degree, at moonrise and moonset.

The four periods, defined precisely

• Two major periods (~2 hours). Centered on the lunar transits — the moment the moon crosses the local meridian (upper transit, overhead) and the anti-meridian (lower transit, underfoot). Window = transit ± 1 hour.
• Two minor periods (~1 hour). Centered on moonrise and moonset. Window = event ± 30 minutes.

How we find the transits

We define the transit as the meridian passing — the instant the moon’s hour angle is zero — which is the same definition almanacs use. (Note this differs by a few minutes from the moment of maximum altitude, because the moon’s declination shifts measurably during the day.) For each location we scan the local day minute by minute to bracket the moon’s path, then refine to the exact hour-angle-zero crossing derived from the moon’s position. Moonrise and moonset are found where the moon’s altitude crosses the horizon. Everything is computed on the location’s own time zone, so daylight-saving transitions are handled correctly.

The day rating

Each day gets a 0–100 score and a 1–4-star bucket (Poor, Fair, Good, Excellent). The score is driven by the moon phase — strongest at the new and full moon (syzygy), weakest at the quarter moons — using 100 × (0.5 + 0.5 × |cos(2π × phase)|). We add a bonus when a feeding period falls within an hour of sunrise or sunset, since that dawn/dusk overlap is when activity tends to peak.

Data sources & accuracy

Positions, rise/set times, and illumination come from the open-source SunCalc library, which implements standard astronomical formulas. We validated the engine against fixed astronomical events — including the April 8, 2024 total solar eclipse (where the sun and moon must coincide in the sky) and known new/full/quarter-moon instants — and against meridian geometry. Event times land within a minute or two of published almanacs, comfortably inside the feeding windows.

The honest caveat

Solunar theory is a traditional aid, not a guarantee. Plenty of fish are caught on “poor” days and skunked on “excellent” ones. Weather fronts, water temperature and clarity, barometric pressure, the spawn, and the rut all matter as much or more. Use the forecast to decide when to go — then let local knowledge and conditions do the rest.