Moon Phase Calculator (Illumination, Age, Next Full/New)

The new and full moon timestamps are accurate to about a minute: they start from the mean lunation and add the periodic and planetary corrections from Meeus, Astronomical Algorithms ch. 49, then convert Terrestrial Time to UT with an approximate ΔT (so dates far in the past or future drift further, as ΔT itself is uncertain there). The phase name, lunar age and illumination for the chosen day still come from the simpler mean-synodic model, which ignores the Moon's elliptical orbit and solar perturbations and can lead or lag the true phase by up to about 14 hours — so near a new or full moon the phase name can differ by a day from the listed event.

From the Southern Hemisphere the same phase appears mirrored — what's a right-hand waxing crescent in Seoul looks like a left-hand waxing crescent in Sydney. The phase NAME, illumination percentage and age are identical worldwide, just the lit limb appears on the opposite side of the disk. Mentally flip the glyph if you're south of the equator.

Two definitions: the seasonal blue moon is the third of four full moons in an astronomical season — rare because synodic months (29.5 days) almost-but-don't-quite divide a season (~91 days) into three. The calendrical blue moon is the popular but technically erroneous definition: the second full moon in a Gregorian calendar month. Both happen roughly every 2.5–3 years; neither has anything to do with the moon's actual color.

Time since the last new moon, in days. 0 = new moon, ~7.4 = first quarter, ~14.77 = full moon, ~22.1 = last quarter, ~29.53 = next new moon. Lunar and lunisolar calendars track this number directly: the Islamic (Hijri) calendar is purely lunar, while the Hebrew and Chinese calendars are lunisolar.