What is your moon sign, and how is it worked out?

Your moon sign is the zodiac sign the Moon was passing through at the moment you were born. The Moon crosses a sign in about two and a half days, so a birth date alone often leaves two candidates. Astrology traditionally reads the Moon as the private, inward side of a person, beside the Sun's public face.

On your card: page 1 Your Moon

What it is

The Moon goes round the Earth in about 27 days, and on the way it passes in front of all twelve signs of the zodiac, spending two to three days in each. Whichever sign it was in when you were born is your moon sign. ¹

It is measured on the same zodiac as the Sun: the ecliptic cut into twelve 30° signs, counted from the March equinox. So a moon sign and a sun sign are directly comparable. Around a new moon they are the same sign; around a full moon they are opposite signs. ²

The old books call the Sun and Moon the two lights of the chart. Modern astrologers usually describe the Sun as the self you show and steer by, and the Moon as the self you come home to: moods, needs, habits, what settles you. That is the general idea, the same for everyone; what a Moon in a given sign means inside a given chart is the reading itself.

How it is calculated

Fix the birth instant. The date, clock time and time zone become one moment in Universal Time.
Find the Moon's longitude. The machine computes the Moon's position along the ecliptic for that moment with astronomy-engine, an open-source library with a full lunar theory, in the same tropical zodiac Western astrology's standard tables use.
Read off the sign. Divide the longitude by 30. The whole number is the sign: 0 for Aries, 1 for Taurus, on round to 11 for Pisces.
No time? Check both ends of the day. The machine checks the Moon at the earliest and latest moments the birth could have been, anywhere on Earth on that date. If both fall in one sign, that is the moon sign. If not, the card names both and says a birth time would settle it.

Why the birth time matters so much here

The Moon is the fastest thing in the chart. It covers about 13° a day, a little over half a degree an hour, roughly its own width every hour. The Sun, by comparison, takes a day to move one degree. ¹

Without a birth time, the machine has to allow for every time zone on Earth, a window of about 50 hours. Counted over every date from 1950 to 2029, 91.1% of birthdays could belong to either of two Moon signs on that basis. A birth time settles almost all of them.

Ptolemy's own method for an uncertain birth hour started from the Moon: find the new or full moon just before the birth, take its exact degree, and work forward from there to the rising point. ³

Moon sign and moon phase are different things

The sign is where the Moon was along the zodiac. The phase is how much of it was lit, which depends on its angle from the Sun. From Earth the Moon seems to go round us every 29 days or so, new to full to new, slightly longer than its true orbit because the Earth has moved on around the Sun in the meantime. ¹

A full moon is the Moon opposite the Sun, with the side facing us fully lit; a new moon sits on the Sun's side, dark to us. The card notes the phase you were born under as well as the sign. ¹

The Moon in astrology, era by era

Era by era, with sources. More in the Reading Room.

Babylonearly 2nd millennium BCEIn the Old Babylonian period, royal diviners begin to read eclipses and other sky events as omens for the king. ⁴
Babylon1st millennium BCEBabylonian observers track the Moon and planets along a fixed belt of stars, the Path of the Moon. The twelve zodiac signs are equal divisions of that belt. ⁴
Babylon410 BCE onwardBabylonian horoscope tablets give the Moon's sign along with the Sun's and the five planets'. A few from Uruk add short predictions looked up from the Moon's position in an omen list. ⁴
Babylon235 BCEA horoscope from Uruk for a child born in early June records that the Moon was "in Lion": a moon sign, written down over 2,200 years ago. ⁵
Roman Egypt2nd century CEPtolemy's chapter on marriage judges a man's prospects from the Moon in his chart and a woman's from the Sun. His fix for an uncertain birth hour begins from the last new or full moon. ^6,3
Psychological turn1936Dane Rudhyar's The Astrology of Personality reads the planets through Jung's psychology, as parts of a personality rather than signs of fate. The modern habit of calling the Moon the emotional self belongs to this tradition. ⁷
Today1996 onwardOnline chart services make the moon sign a two-second lookup for anyone who knows their birth time. ⁸

Asked at the booth

Can I find my moon sign without a birth time?

Often. If the Moon stayed in one sign all day, the date is enough. If it changed sign that day, you need the time, or at least to know whether you were born before or after the change.

How often does the Moon change sign?

About every two and a quarter days on average: a full lap of about 27.3 days divided by twelve signs. The Moon moves faster at some points in its orbit than others, so a stay in one sign runs from a little under two days to a little over two and a half.

Can my moon sign be the same as my sun sign?

Yes. It happens when you were born within a day or two of a new moon, when the Sun and Moon stand together. Roughly one birthday in twelve has the two in the same sign.

What's the difference between a moon sign and a moon phase?

The moon sign is where the Moon was on the zodiac. The phase is how much of the Moon was lit, which comes from its angle to the Sun. The two are related: a full moon is always in the sign opposite the Sun.

Is the moon sign more important than the sun sign?

Astrologers disagree, and the tradition has no single answer. Most treat the Sun, Moon and rising sign as the three main points of a chart and read them together.

Go deeper

That's how it works for everyone. Your card works it out for you.

This page explains the method and where it comes from. What it says about you is sealed on your card, worked out from the day, the minute and the place you were born.