Methodology

Every phase, illumination percentage, full-moon date, and moonrise/moonset time on MoonCadence is computed using astronomy-engine (Don Cross, MIT license), a well-established, high-precision astronomical library built on the same class of VSOP87/ELP2000-derived algorithms used by professional ephemeris software. Nothing is looked up from a static table or approximated by hand.

The Blue Moon rule we use

We use the modern, popular definition: the second full moon to occur within a single calendar month. This is technically a 20th-century misreading of an older definition (the third full moon in an astronomical season that contains four), but it's what nearly every reader means by "blue moon" today, so it's what drives our Blue Moon labeling — with the older definition explained here too, not hidden.

The Harvest Moon override

The Harvest Moon isn't tied to September on this site — it's computed as whichever full moon falls closest to that year's real September equinox, which occasionally lands in October instead (most recently 2020). We compute this fresh every year rather than hardcoding "September = Harvest Moon."

Moonrise, moonset, and your location

Phase and illumination are the same everywhere on Earth at a given instant, but moonrise and moonset genuinely depend on where you're standing. Those figures are computed client-side from the coordinates your browser's Geolocation API provides (or a location you type in), run through the same astronomy-engine rise/set solver rather than a simplified sunrise-style approximation — the moon's orbit is tilted and eccentric enough that a naive formula drifts by minutes over a lunar month.

Time zones and calendar dates

A full moon happens at one precise global instant, but the calendar date it falls on shifts depending on which time zone you're in — a full moon at 2 a.m. UTC on the 1st is still the 31st across most of the Americas. Our full-moon and calendar pages state the instant in UTC alongside the date, specifically so this doesn't silently produce a "wrong" date for readers outside UTC.

Zodiac sign placement

Each full moon's zodiac sign card states a computed datum, not an interpretation: astronomy-engine returns the Moon's ecliptic longitude at the exact full-moon instant, and that longitude is mapped onto the standard 30-degree tropical-zodiac bands to give the sign shown. It's the same real number behind every full-moon page's date and time, just expressed as a sign name instead of degrees — no separate astrological calculation and no manual lookup table involved.

Folklore vs. fact

Traditional full-moon names, ritual practices, and meaning content are always framed as tradition or reflection — never presented as scientific fact, and never given medical, financial, or psychological advice framing.

When we're wrong

Astronomical computation is precise but not infallible in how it's displayed — a mislabeled time zone or a stale cache is far more likely to cause an error than the underlying library. If a date, time, or illumination figure looks off, the contact page is the fastest way to get it checked against the raw calculation and corrected.