The Moon
The Moon's phase on the day you were born is your birth moon phase.
Your birth moon phase is the illumination and shape of the Moon as seen from Earth on the day you were born, computed from the Moon's position along its orbit.
The Moon completes a lunation — one full cycle of phases — in about 29.53 days. By calculating the Moon's age (days since the last new moon) on your birth date, we determine which phase was visible.
We use the well-known astronomical method of computing the Julian Day, then deriving the Moon's mean elongation and age to get an illumination percentage accurate to within a few percent.
Moon age = (JD − referenceNewMoon) mod 29.53A moon age of 14.8 days is near full moon, with illumination close to 100%.
Our calculation is accurate to within about 1–2% illumination, enough to reliably name the phase but not to the minute.