Moon calendar methodology
Calendar calculations use UTC internally and convert to an IANA time zone only for display. The UK default is Europe/London, including its daylight-saving transitions.
Daily calendar values
Each daily row is sampled at 12:00 local time. Local noon is documented because a continuously changing phase has no single value for an entire civil day. Phase names come from eight 45-degree sections of the geocentric Moon–Sun ecliptic longitude cycle.
Principal phase events
New Moon, first quarter, full Moon and last quarter use calculated event instants rather than the daily noon sample. The event keeps its UTC instant, then receives a local date and time for the selected zone.
Illumination, lunar age and rounding
Illumination is the calculated fraction of the lunar disc lit from the observer's geocentric perspective. Lunar age is elapsed time since the previous calculated new Moon. Internal values retain full precision; rounding happens only in displayed text.
Validation
Automated fixtures compare representative 2026 principal phases with the U.S. Naval Observatory table within a two-minute tolerance. Tests also cover UK daylight-saving boundaries, leap years, local-date event markers and the supported 1900–2100 input range.