Moon Phase Calculator — Live Lunar Illumination & Age in Days

A Brief Astronomical History

In 2026, an amateur astronomer in Auckland planning a public sky-watching night needs to know the moon's age and illumination on May 27 to decide whether deep-sky targets are washed out. This calculator answers exactly that question — 76% lit, 10 days into the cycle, Waxing Gibbous — and shows the next full moon (May 31, 2026) and next new moon (June 15, 2026) so the event can be moved if necessary.

The synodic period of 29.530588 days — the average time between consecutive new moons as seen from Earth — was known to Babylonian astronomers before 700 BCE. They identified the Saros cycle of 6585.32 days (18 years 11 days 8 hours), after which the relative geometry of Sun, Earth and Moon repeats almost exactly. The Saros is the reason eclipses cluster in families that march across Earth over centuries.

The Julian calendar (45 BCE) was tied loosely to seasons but ignored the moon. The Hebrew lunisolar calendar locked Passover to the spring equinox via the 19-year Metonic cycle: 235 lunations equal 19 tropical years to within 2 hours. The Islamic Hijri calendar (since 622 CE) is purely lunar — 12 lunations per year, drifting 11 days back through the solar year each cycle. The Chinese, Hindu Vikram Samvat and Tibetan calendars are lunisolar with periodic intercalary months.

The Gregorian reform of October 1582 (Pope Gregory XIII) dropped the moon entirely from the civil calendar and made it purely solar, but Easter still uses the Paschal full moon. The First Council of Nicaea (325 CE) fixed Easter as the first Sunday after the ecclesiastical full moon falling on or after March 21. That rule drives the Computus — the algorithm that maps a Gregorian year to an Easter date.

Modern lunar theory traces to Ernest W. Brown (1919), whose tables held the standard for half a century. In 1991 Jean Meeus published Astronomical Algorithms, a single-volume textbook that translated the ELP-2000/82 series into code that any programmer could run. Today JPL DE441 ephemerides and the IAU 2009 system give sub-arcsecond accuracy for the Moon's position; the synodic-period approximation used here is a degree of approximation simpler than Meeus and roughly five orders of magnitude simpler than DE441.

Almanac names for full moons — Wolf, Snow, Worm, Pink, Flower, Strawberry, Buck, Sturgeon, Harvest, Hunter, Beaver, Cold — come from the Farmer's Almanac and Algonquin nation tradition. They are aesthetic rather than astronomical; the moon does not actually change colour. A blue moon — the second full moon in a calendar month — happens roughly every 2.7 years because the synodic month is 29.5 days, just slightly shorter than most calendar months.

For dates around an event use Days Until or pair this with the Sunrise & Sunset calculator to plan moonrise/moonset overlap.