Moon Sign Calculator (Your Moon Sign, Degree & Moon Phase at Birth)

How it works

UT = local time − offset → Julian Day, + ΔT → Terrestrial Time · Moon longitude λ = L′ + Σ (60 periodic terms in D, M, M′, F) + nutation (Meeus ch. 47, accuracy ≈ 0.01°) · sign = ⌊λ ÷ 30°⌋ · phase from elongation λMoon − λSun; illuminated fraction = (1 − cos elongation) ÷ 2

Your Moon sign is the zodiac sign the Moon occupied when you were born. The Moon circles the zodiac every 27.3 days, spending about 2½ days in each sign, so the date alone is usually enough — but on the day it changes sign the birth time decides. The position comes from the standard lunar theory in Jean Meeus's Astronomical Algorithms, accurate to a fraction of an arc-minute. In astrology the Moon sign is said to describe emotions and instincts; that interpretation is entertainment, with no scientific support.

Worked example

Born May 15, 1990 at 8:30 a.m. in Mexico City (UTC−6, 14:30 UT), the Moon was at 298.45° ecliptic longitude — 28°27′ Capricorn, so the Moon sign is Capricorn, a cardinal earth sign. It was a Waning Gibbous Moon, 72% illuminated. The Moon entered Aquarius at 11:30 local time that day, so anyone born on May 15, 1990 after 11:30 in Mexico City has an Aquarius Moon — this is why the birth time matters on sign-change days.

Sources: Meeus, Astronomical Algorithms (2nd ed.), ch. 47 Position of the Moon · NASA — Moon in motion

Frequently asked questions