Moon phase versus Moon sign

They get mixed up constantly, and they are different measurements. The phase is how much of the Moon's sunlit side we can see. The sign is where the Moon is along the zodiac. A full moon can be in any sign, and the Moon changes sign roughly every two and a half days while its phase takes a month to come round.

The phase: an angle between two bodies

The phase is set by the angle between the Sun and the Moon as seen from Earth: none at new moon, a right angle at the quarters, opposite at full. NASA's phase explainer (opens in a new tab) has the geometry. It is the same everywhere on Earth at once.

The sign: a place on a belt of sky

The zodiac is the band of sky along the Sun's yearly path, and astrology divides it into twelve signs of 30 degrees each, a division Britannica (opens in a new tab) dates to the Babylonians around 500 BCE. The Moon's sign is simply which slice it is in at that moment.

Which zodiac

There are two in wide use. Most Western astrologers use the tropical zodiac, which starts the signs at the March equinox. Vedic astrology uses the sidereal zodiac, anchored to the stars; a Jyotish teaching site puts the gap between them at around 24 degrees (opens in a new tab) today, and notes that Lahiri's value for it is the one recognised by the Indian government. So a Moon in roughly the first 24 degrees of a tropical sign, most of it, is in the previous sign in the sidereal zodiac.

Signs are not constellations

The signs are equal slices; the constellations behind them are not. The Sun passes through thirteen constellations, Ophiuchus among them, which NASA points out (opens in a new tab) the Babylonians left out, and because Earth's axis slowly wobbles, the signs and the constellations of the same names have drifted apart: by Britannica's (opens in a new tab) example, the Sun is in the constellation Aries from about 19 April, while the sign of Aries begins on 21 March.

What this site calculates, and what it does not

The Tonight page works out the Moon's phase for everyone, and in Historical beliefs and Astrology & myths it also gives the Moon's tropical sign, its decan and its sidereal nakshatra, all from the same computed position: real positions, with meanings that are tradition. The birthday explorer gives the phase for a date, not the Moon's sign, because the Moon crosses a whole sign in two or three days and a date without a birth time cannot settle it. Nothing here builds a birth chart.