The idea
Before clocks, calendars or birthdays, there was one timepiece everybody shared: the moon, filling and emptying every 29 and a half days. Every person who has ever lived has been keeping lunar time, whether they noticed or not.
This instrument reads your account. It counts every full moon that has risen since the night you were born, tells you which phase hung in the sky that night, and — because a life should have things to look forward to — books the date of your next big round-numbered moon.
How it’s measured
The moon returns to the same phase every 29.530589 days on average — the synodic month. We anchor the calculation to a precisely recorded new moon (January 6, 2000 at 18:14 UTC) and count forward and backward from there. Full moons fall exactly halfway through each cycle, so counting the full moons between two dates is a matter of counting half-cycle crossings.
The real moon wobbles: individual cycles run a few hours long or short as the moon’s elliptical orbit speeds up and slows down. Using the mean cycle keeps every count correct and every phase within a few hours of a professional almanac — plenty for a birth certificate of the sky.
Questions, answered
Why might my birth phase differ slightly from an astronomy site?
If you were born within hours of a phase boundary, the moon’s orbital wobble (which we average out) can tip the label one way or the other. For the other ~95% of birthdays, the phase matches exactly.
What is a “bonus moon”?
A lunar cycle (29.53 days) is shorter than an average calendar month (30.44 days), so full moons slowly pull ahead of your month count. The bonus figure is how many extra moons you have collected beyond your months lived.
Does the time of day I was born matter?
We read the sky at midday on your birth date. Only if you were born very close to a phase changeover could the exact hour move you into the neighbouring phase.