What the tool shows
For the date, time and place of your birth, the tool gives the Moon’s phase, the share of its face that was lit, its age in days since the last new moon, the sign it was in, the dates of the previous new moon and of the next full and new moons, and a short note on the phase in modern astrology.
The eight phases
The Moon takes about 29.5 days to go from one new moon to the next. Astronomers describe the phase by the Moon’s elongation, its angle ahead of the Sun as seen from Earth. At 0 degrees it is new, at 90 degrees it is at first quarter, at 180 degrees it is full and at 270 degrees it is at last quarter. Between those four points the Moon is a crescent or gibbous: waxing crescent, waxing gibbous, waning gibbous and waning crescent. That makes eight phases in all.
The four principal phases are single moments, not whole days. If the exact moment of a new moon, quarter or full moon fell on your birth date, the tool names that phase and tells you the time. On other days it names the crescent or gibbous phase you were born under.
How much was lit
The lit share is worked out from the elongation: the Moon is lit by (1 minus the cosine of the elongation) divided by two. It was close to nothing around the new moon, half at the quarters and nearly all at the full moon. We checked the figure against NASA’s JPL Horizons service and it agrees to within about a fifth of a percentage point.
The age of the Moon
The age is the time since the last new moon. The tool finds that new moon by solving for the moment the Moon was exactly 0 degrees ahead of the Sun, using the Sun and Moon positions from Swiss Ephemeris data. The same method gives the next full and new moons, which match the Swiss Ephemeris to within a fraction of a minute.
If you do not know your birth time
The Moon changes phase slowly, so a time matters little for the phase name. Without a time the tool uses 12:00 local time at your birth place and, if the phase changes during your birth date, it tells you so. Add the place so that the right time zone is used; it can move the result by a day near a phase boundary.
What the phase is said to mean
Modern astrologers link each phase with a temperament: new moon births with beginnings, full moon births with openness and feeling. These are modern ideas that many people enjoy, and some think they fit and some do not. They are offered for reflection and are not a proven measure of character. If you want a chart to go with it, our Sun, Moon and Rising calculator gives the Moon’s sign and your full chart.
Frequently asked questions
↳ Do I need my birth time to find the Moon phase?
No. The phase changes slowly, so a date and place are usually enough. If the phase name changes during your birth date, the tool says so, and a birth time settles which one applies.
↳ Is the Moon phase the same everywhere on Earth?
Almost. Everyone sees the same phase at the same moment, but the date can differ by a day because of time zones. That is why the tool uses the time zone of your birth place.
↳ What is the age of the Moon?
It is the number of days since the last new moon. It runs from 0 at the new moon to about 14.8 days at the full moon and about 29.5 days before the next new moon.
↳ Does my Moon phase say anything about my personality?
Some modern astrologers read the birth phase as a temperament, for example new moon as a starter and full moon as expressive. It is an idea for reflection, not a proven link, and many people with the same phase are very different.
↳ Is the Moon phase the same as my Moon sign?
No. The phase is the Moon's angle from the Sun, and the Moon sign is the zodiac sign the Moon was in. The tool shows both so you can see the difference.