Moon Phase on Birthday Calculator

The quantity computed here is the age of the Moon in days at a given date, measured from the most recent new moon, together with the phase name and illuminated fraction that follow from it. The cycle used is the synodic month, the interval between successive identical alignments of Sun, Earth and Moon, whose mean length is 29.530588 days. It is longer than the sidereal month of 27.321661 days because the Earth advances along its own orbit while the Moon completes one revolution, so the Moon must travel further to restore the alignment. The date is first converted to a Julian Day Number, a continuous day count that sidesteps calendar irregularities, then the days elapsed since a reference new moon at JD 2451550.1, corresponding to 6 January 2000, are divided by the synodic month. The fractional remainder is the Moon's age, and illumination is estimated as (1 - cos(2 pi times age divided by 29.53)) divided by 2. The eight named phases divide the cycle into octants of about 3.69 days each, with new, first quarter, full and last quarter given windows of roughly plus or minus 0.92 days around the exact instant. Accuracy is limited by design. Using a mean synodic month ignores the eccentricity of the lunar orbit and solar perturbation, so the true interval varies between about 29.27 and 29.83 days and a predicted new moon can be off by up to roughly fourteen hours. The calculation also takes a calendar date with no clock time or time zone, so a birth close to a phase boundary may be assigned the neighboring phase. Personality traits attributed to birth phase have no empirical support.

Predicting lunar phase is one of the oldest quantitative sciences. Babylonian astronomers had reduced the mean synodic month to a sexagesimal value of 29;31,50,8,20 days, equal to 29.530594 days, a figure preserved in cuneiform tablets and transmitted through Hipparchus in the second century BCE and Ptolemy's Almagest in the second century CE. It differs from the modern mean of 29.530588 days by well under a second. Meton of Athens established in 432 BCE that 19 solar years contain almost exactly 235 synodic months, the cycle still underlying the Hebrew calendar and the computation of Easter, while Babylonian records of the saros, 223 synodic months or about 18 years and 11 days, allowed the repetition of eclipses to be forecast. The bookkeeping device used here is far more recent. Joseph Justus Scaliger proposed the Julian Period in 1583 in De emendatione temporum, a 7980-year span whose plain day count avoids the discontinuities of calendar reform; John Herschel adapted it for astronomical use in 1849, and the Julian Day Number, counting from noon on 1 January 4713 BCE in the proleptic Julian calendar, became standard. Compact formulas of the kind implemented here were codified for programmers by Jean Meeus in Astronomical Algorithms, published in 1991, while authoritative phase times are published by the US Naval Observatory. The separate claim that lunar phase influences human behavior has been tested repeatedly; a 1985 meta-analysis by James Rotton and Ivan Kelly in Psychological Bulletin pooled dozens of studies and found no reliable effect.