The moon remembers your birthday
Pick your birth date and see the moon that hung over that night: its phase, how lit it was, and the sign it was passing through. Computed right here in your browser; your date never leaves this page.
Your birth moon comes back
The phase you were born under was never a one-off. The moon returns to it about every 29.5 days, and to the same place among the stars about every 27.3 days, for as long as you are around to look up. The Gibbous app keeps both dates for you: set your birthday once and your birth moon sits on the Today screen, free. With Premium it tells you each time the moon comes back to that phase, roughly once a month, and keeps your birth moon in a Home Screen widget.
Do your moons match?
Once you know your birth moon, the obvious next question is whether it fits someone else's. The moon phase compatibility calculator takes two birthdays and works out how close the pair comes to making one full moon, which is the whole idea behind the moon phase soulmate trend.
How this works
The moon's phase repeats every 29.5 days, the synodic month, and its position is pure celestial mechanics, so any date's moon can be computed rather than looked up. This page runs a compact version of the same astronomy the Gibbous app uses. Because the moon's phase changes slowly through a day, we compute it for midday of your birth date, which keeps the answer stable no matter the time zone you were born in.
The zodiac sign here is the astronomical position rounded to the traditional twelve 30 degree signs, the same convention astrologers use for a "moon sign." Whether that means anything is up to you; the astronomy is real either way.