Sri Lanka Moon Phase & Poya Day Calculator

This tool computes the Moon's phase from deterministic astronomy — the same periodic-term method used in professional almanacs — rather than looking dates up in a fixed table. Sri Lanka uses a single time zone (Sri Lanka Standard Time, UTC+5:30), so every computed instant is shifted by +5½ hours before its calendar date is taken, giving the correct local day island-wide.

1. Julian Day. The chosen date is converted to a Julian Day (JD) using the algorithm in Meeus Astronomical Algorithms, Chapter 7 — a continuous day count that makes astronomical arithmetic exact.
2. New & Full Moon instants. Chapter 49 gives the mean phase JDE = 2451550.09766 + 29.530588861·k for lunation number k (full moons use k+ ½), then adds the periodic corrections for the Sun's and Moon's mean anomalies and the Moon's argument of latitude. The result is accurate to under a minute.
3. Moon age & phase fraction.The Moon's age is the days since the previous New Moon; dividing by the length of the current lunation gives the phase fraction (0 = New, 0.5 = Full, 1 = next New).
4. Illuminated fraction. The lit fraction of the disc is (1 − cos 2πφ) / 2, where φ is the phase fraction — 0% at New Moon, 50% at the quarters, 100% at Full Moon.
5. Phase name. The fraction is binned into the eight principal phases, each a ⅛-cycle window centred on New, First Quarter, Full and Last Quarter.
6. Poya identification. Every Full Moon is a Poya. Names follow the traditional sequence anchored to the Gregorian month (Duruthu in January through Unduvap in December). When a month holds two full moons — an adhi (intercalary) month — the earlier is flagged “Adhi”. The legal public-holiday date always defers to the official Sri Lanka Holidays Gazette.

As a credibility check, the tool also runs the textbook “quick” formula — Moon age = (JD − 2451550.1) mod 29.53058867 — alongside the precise Meeus engine and reports the difference, which stays well under a day. Computed New and Full Moon instants for 2026 were cross-checked against NASA/Espenak phase tables and agree to within ±1 day after the SLST shift.

• NASA Goddard / Fred Espenak — Six Millennium Catalog of Phases of the Moon
• Jean Meeus, Astronomical Algorithms (2nd ed.) — Ch. 7, 47, 49
• Ministry of Public Administration — Sri Lanka Holidays Gazette (Poya holidays)
• Holidays Act No. 29 of 1971 — statutory basis for Poya public holidays

Astronomy engine and the 2026 Poya dates were last cross-checked against the NASA/Espenak phase catalogue on 2026-06-12. Legal holiday dates are reviewed against each year's official gazette as it is published.