Moon phase compatibility is a test you can do with two birth dates. You look up the shape of the Moon on each birthday. If the two shapes fit together into one round full Moon, the trend calls you soulmates. If the two shapes are the same, it calls you twin flames.
We checked the odds of each result. We used the real phase of the Moon for 62.2 million US births, from 2000 to 2014. With the usual rule of eight equal phases, one couple in eight makes a full Moon. One more couple in eight shares the same phase. So one couple in four gets a special label by pure chance.
This guide gives you a way to find your own phase without an app. It explains the eight phases, which ones fit together, and what the numbers say. It also tests an old belief about births at the full Moon. Astrology is not a science, and no study has shown that a birth phase predicts love. Read this as a map of a fun idea, not as a forecast.
What is moon phase compatibility?
The idea spread on TikTok in 2023. A filter asked for two birth dates and showed two small Moon shapes side by side. People then checked if the lit parts filled one whole circle.
The trend uses three simple labels. Two shapes that make a full circle are called soulmates. Two shapes that match are called twin flames, or mirror souls. Any other pair is called a contrast, and each site gives it its own meaning.
The labels are new. Old books of astrology do not use them. But the phase itself is real astronomy. It is the angle between the Sun and the Moon, seen from Earth. When that angle is zero, the Moon sits next to the Sun and we see a dark new Moon. When it reaches 180 degrees, the Moon faces the Sun from across the sky and shines full.
That angle is why the full circle trick works. Two phases that make a whole Moon are always half a cycle apart. A thin crescent lit on the right fits a fat gibbous Moon that is dark on the right. So the soulmate pairs are simply the opposite phases on the wheel.
The eight moon phases, and why they are not equal
The Moon goes from new to new in 29.53 days on average. We call that a lunar month. We timed every lunar month from 1600 to 2400. The shortest took 29.27 days and the longest took 29.83 days.
Most apps cut that month into eight equal slices of 45 degrees each. Each main phase sits in the middle of its slice. So the full Moon phase runs from 22.5 degrees before the exact full Moon to 22.5 degrees after it.
Equal slices of angle do not give equal slices of time. The Moon runs a bit faster near new and full, and a bit slower near the quarters. Tycho Brahe found this speed change in the 1590s. Astronomers call it the variation. It makes the new and full phases the shortest of the eight.
| Phase | What you see | Typical length | Share of all time, 1600 to 2400 |
|---|---|---|---|
| New Moon | Dark, or a hair of light | 3.61 days | 12.31 percent of the time. |
| Waxing Crescent | Thin light on the right | 3.68 days | 12.50 percent of the time. |
| First Quarter | Right half lit | 3.73 days | 12.70 percent of the time. |
| Waxing Gibbous | Most of it lit, dark edge on the left | 3.67 days | 12.49 percent of the time. |
| Full Moon | Round and bright | 3.61 days | 12.29 percent of the time. |
| Waning Gibbous | Most of it lit, dark edge on the right | 3.67 days | 12.49 percent of the time. |
| Last Quarter | Left half lit | 3.73 days | 12.70 percent of the time. |
| Waning Crescent | Thin light on the left | 3.67 days | 12.50 percent of the time. |
The typical length is the middle value of all the passes we timed. A single pass can be much shorter or longer. The full Moon phase took as little as 3.14 days and as much as 4.17 days. The shapes in the table are the view from the United States. South of the equator, left and right swap.
The gap is small, but it is real. Out of 1,000 people, about 123 are born under a full Moon phase and about 127 under a first quarter. That is the first thing no free calculator tells you.
What moon phase was I born in?
You do not need an app for this. You need the date of one new Moon in your birth year, and a little counting. The table below gives the first new Moon and the first full Moon of each year from 1970 to 2010. Times are for New York, with winter time.
| Year | First new Moon | First full Moon |
|---|---|---|
| 1970 | Jan 7, 3:36 pm | Jan 22, 7:56 am. |
| 1971 | Jan 26, 5:55 pm | Jan 11, 8:20 am. |
| 1972 | Jan 16, 5:53 am | Jan 30, 5:58 am. |
| 1973 | Jan 4, 10:43 am | Jan 18, 4:28 pm. |
| 1974 | Jan 23, 7:02 am | Jan 8, 8:36 am. |
| 1975 | Jan 12, 5:20 am | Jan 27, 10:10 am. |
| 1976 | Jan 1, 9:40 am | Jan 16, 11:47 pm. |
| 1977 | Jan 19, 9:11 am | Jan 5, 7:11 am. |
| 1978 | Jan 8, 11:00 pm | Jan 24, 2:56 am. |
| 1979 | Jan 28, 1:20 am | Jan 13, 2:09 am. |
| 1980 | Jan 17, 4:20 pm | Jan 2, 4:02 am. |
| 1981 | Jan 6, 2:24 am | Jan 20, 2:39 am. |
| 1982 | Jan 24, 11:56 pm | Jan 9, 2:53 pm. |
| 1983 | Jan 14, 12:08 am | Jan 28, 5:26 pm. |
| 1984 | Jan 3, 12:16 am | Jan 18, 9:05 am. |
| 1985 | Jan 20, 9:28 pm | Jan 6, 9:16 pm. |
| 1986 | Jan 10, 7:22 am | Jan 25, 7:31 pm. |
| 1987 | Jan 29, 8:45 am | Jan 14, 9:31 pm. |
| 1988 | Jan 19, 12:26 am | Jan 3, 8:41 pm. |
| 1989 | Jan 7, 2:22 pm | Jan 21, 4:34 pm. |
| 1990 | Jan 26, 2:20 pm | Jan 10, 11:57 pm. |
| 1991 | Jan 15, 6:50 pm | Jan 30, 1:10 am. |
| 1992 | Jan 4, 6:10 pm | Jan 19, 4:29 pm. |
| 1993 | Jan 22, 1:27 pm | Jan 8, 7:37 am. |
| 1994 | Jan 11, 6:11 pm | Jan 27, 8:23 am. |
| 1995 | Jan 1, 5:56 am | Jan 16, 3:27 pm. |
| 1996 | Jan 20, 7:50 am | Jan 5, 3:51 pm. |
| 1997 | Jan 8, 11:26 pm | Jan 23, 10:11 am. |
| 1998 | Jan 28, 1:01 am | Jan 12, 12:24 pm. |
| 1999 | Jan 17, 10:46 am | Jan 1, 9:50 pm. |
| 2000 | Jan 6, 1:14 pm | Jan 20, 11:41 pm. |
| 2001 | Jan 24, 8:07 am | Jan 9, 3:24 pm. |
| 2002 | Jan 13, 8:29 am | Jan 28, 5:51 pm. |
| 2003 | Jan 2, 3:23 pm | Jan 18, 5:48 am. |
| 2004 | Jan 21, 4:05 pm | Jan 7, 10:40 am. |
| 2005 | Jan 10, 7:03 am | Jan 25, 5:33 am. |
| 2006 | Jan 29, 9:15 am | Jan 14, 4:48 am. |
| 2007 | Jan 18, 11:01 pm | Jan 3, 8:58 am. |
| 2008 | Jan 8, 6:37 am | Jan 22, 8:35 am. |
| 2009 | Jan 26, 2:56 am | Jan 10, 10:27 pm. |
| 2010 | Jan 15, 2:12 am | Jan 30, 1:18 am. |
Now follow four steps.
- Find your birth year and note the first new Moon.
- Count the days from that new Moon to your birthday.
- Take away 29.5 days as many times as you can. What is left is the age of the Moon, in days.
- Read your phase from the next table.
| Age of the Moon | Your phase |
|---|---|
| Under 1.8 days, or over 27.7 days | New Moon. |
| 1.8 to 5.5 days | Waxing Crescent. |
| 5.5 to 9.2 days | First Quarter. |
| 9.2 to 12.9 days | Waxing Gibbous. |
| 12.9 to 16.6 days | Full Moon. |
| 16.6 to 20.3 days | Waning Gibbous. |
| 20.3 to 24.0 days | Last Quarter. |
| 24.0 to 27.7 days | Waning Crescent. |
Here is an example. Take a birth on July 4, 1990. The first new Moon of 1990 fell on January 26. From January 26 to July 4 there are 159 days. Take away 29.5 five times and 11.4 days are left. That is a Waxing Gibbous Moon, and the real sky agrees.
If your birthday comes before the first new Moon, count backward instead. Then take that number away from 29.5. A birth on January 10, 1990 sits 16 days before the new Moon. So the Moon was 13.5 days old, which is a Full Moon. The real full Moon came that same night.
How good is this shortcut? We tested it on every day from 1970 to 2010. It gave the right phase 88 times in 100. When it missed, it was off by one phase, never more. The misses sit near the edge of a phase. If your age lands within a day of an edge, check an exact source, such as the phase tables of the US Naval Observatory.
Your time zone matters too. From 1970 to 2010, one new or full Moon in five fell on a different date in world time than in New York. One in eight fell on a different date in New York than in Los Angeles. A tool that uses world time can give a US birth the wrong day.
Three rules, three answers
Not every tool cuts the month the same way. That is why two sites can give you two phases for one birthday. Here are the three rules you will meet.
The first rule is the one above. It uses eight equal slices, with each main phase in the middle of its slice. Most apps and the Moon house guides on this site use it.
The second rule is the calendar rule. A main phase gets only the one date on which it happens. So a full Moon birthday means the exact full Moon fell on your birth date. Every other day of the month goes to one of the four in-between phases. With this rule, the main phases are rare. Each one covers 3.4 percent of US births. Each in-between phase covers about 21.6 percent.
The third rule comes from astrology. Dane Rudhyar set it out in his 1967 book The Lunation Cycle. His eight phases start at the exact new Moon, not before it. So his full phase runs from the full Moon to 45 degrees after it. He also gave the slices their own names: New, Crescent, First Quarter, Gibbous, Full, Disseminating, Last Quarter and Balsamic.
Rudhyar’s slices sit half a slice later than the app slices. So they share only half of each phase. For half of all birthdays, an astrologer and an app will give two different phase names. Neither is wrong. They answer two different questions.
Which moon phases fit together?
The trend has one main rule. Two phases fit when their lit parts make one whole Moon. On the eight phase wheel, that means the two phases sit four steps apart.
| Your phase | Your full circle match | Why they fit |
|---|---|---|
| New Moon | Full Moon | A dark disk and a bright disk make one whole light. |
| Waxing Crescent | Waning Gibbous | A thin right edge fills the dark right edge of the gibbous. |
| First Quarter | Last Quarter | A right half and a left half make one circle. |
| Waxing Gibbous | Waning Crescent | A thin left edge fills the dark left edge of the gibbous. |
The match works both ways. A Waning Gibbous person fits a Waxing Crescent person, and so on around the wheel. Some sites add softer labels for the other pairs. Two phases side by side are said to flow. Two phases a quarter turn apart are said to build or stretch. These words come from the sites that sell the test. You will not find them in any old text.
How rare is a moon phase soulmate match?
Here is the core of our count. We took the daily birth counts for the United States from 2000 to 2014. That is 62.2 million births. For each day, we worked out which phase the Moon was in, hour by hour, in Central time. Then we added up the births in each phase.
| Phase | Share of US births, equal slices | Share of US births, calendar rule |
|---|---|---|
| New Moon | 12.33 percent | 3.40 percent. |
| Waxing Crescent | 12.51 percent | 21.59 percent. |
| First Quarter | 12.69 percent | 3.41 percent. |
| Waxing Gibbous | 12.48 percent | 21.54 percent. |
| Full Moon | 12.28 percent | 3.40 percent. |
| Waning Gibbous | 12.46 percent | 21.49 percent. |
| Last Quarter | 12.67 percent | 3.35 percent. |
| Waning Crescent | 12.57 percent | 21.83 percent. |
Next we paired two people at random. The odds that both land in the same phase are the sum of each share times itself. The odds of a full circle pair are the sum of each share times the share of its opposite.
| Result for a random couple | Equal slices | Calendar rule |
|---|---|---|
| Full circle, the soulmate label | 12.5 percent, one in eight | 19.1 percent, about one in five. |
| Same phase, the twin flame label | 12.5 percent, one in eight | 19.1 percent, about one in five. |
| Neighbors on the wheel | 25.0 percent, one in four | Not used by this rule. |
| A quarter turn apart | 25.0 percent, one in four | Not used by this rule. |
| Three steps apart | 25.0 percent, one in four | Not used by this rule. |
So the answer depends on the rule more than on the sky. With equal slices, one couple in four gets a special label. With the calendar rule, it is closer to two couples in five. If a tool calls you soulmates, you share that result with millions of pairs who met by chance.
We ran the same count on a second set of data. The US health statistics office published daily births from 1994 to 2003, 39.7 million in all. It gave the same result to the first decimal: 12.5 percent for a full circle pair and 12.5 percent for a same phase pair.
Are more babies born at the full Moon?
Many nurses say the ward fills up at the full Moon. We tested that with the same birth data. For each full Moon date, we compared the births that day with the same weekday one to four weeks before and after. That removes the big weekday effect. Fewer babies are born on weekends, because hospitals plan fewer births then.
At first, full Moon days showed 0.9 percent more births than the days around them. Then we looked closer. The small excess came from holidays. When Christmas or July 4 fell in the weeks around a full Moon, the other days looked quiet, and the full Moon looked busy.
We took the holidays out and ran the test again. Full Moon days then had 0.01 percent more births than their neighbors. The margin of error was 0.45 percent. The 1994 to 2003 data gave the same answer: 0.2 percent fewer births, well inside the noise. In plain words, the Moon does not change the day you are born. Your phase is a matter of pure chance.
What astrologers say about the birth phase
The phase is old in astrology, even if the soulmate test is new. Ptolemy wrote about it in the second century, in book one, chapter eight, of the Tetrabiblos. He split the month into four parts. From new Moon to first quarter, he said, the Moon brings moisture. From first quarter to full, it brings heat. From full to last quarter, it brings dryness. From last quarter to the next new Moon, it brings cold.
Ptolemy did not use the phase to match couples. He used it to judge the strength of the Moon. Hellenistic astrologers such as Vettius Valens also cared whether the Moon was waxing or waning at birth. A waxing Moon was said to grow in power, and a waning Moon to lose it.
The eight named phases came much later. Dane Rudhyar built them in the twentieth century, as a picture of growth. In his reading, a New Moon person acts on instinct. A Full Moon person sees things in the light of others. A Balsamic person closes old stories. Demetra George wrote a whole book on that last phase, Finding Our Way Through the Dark.
For couples, most astrologers look elsewhere. They compare the sign and house of each Moon, and the angles between the two charts. That method is called synastry. Our guide to synastry and composite charts shows how it works. The phase is one clue in that picture, not the whole of it.
Our guide to the synastry chart counts how many aspects any two strangers share.
Honesty matters here. In 2007, the social scientist David Voas tested sun signs on 10.3 million married couples in England and Wales. He found no sign that the stars shaped who married whom. No one has run such a test on moon phases. But the logic is the same. Nothing we know links a birth phase to a good match.
Moon phase compatibility calculator: what the free tools do
This page has no calculator, on purpose. Every free test works the same way inside. It takes your birth date, finds the age of the Moon on that date, and puts it in one of eight boxes. Then it reads the gap between two boxes.
That means you can do it by hand with the tables above. It also means three things can go wrong. The tool might use world time and move your birth to the wrong day. It might use the calendar rule when you expect equal slices. And it might ignore your birth hour, which matters on the day a phase changes.
Some tools show a percent score. The score is made up by each site. No two sites use the same scale, so do not compare a score from one site with a score from another. If you want a real test of your match, compare two full charts, with birth times.
What moon phase is good for love?
There is no best phase for love in the data. Every phase has about the same share of births, give or take half a point. So every phase has about the same share of couples, happy or not.
In the trend, the full circle pairs get the warmest words. In Rudhyar’s scheme, each phase has its own way of loving. A crescent person wants to grow something new. A gibbous person wants to fix and refine. A disseminating person wants to share what they know. Read these as moods, not as rules.
Your Moon, beyond its phase
The phase tells you how far the Moon stood from the Sun. It does not tell you where the Moon stood among the stars. That is your Moon sign. Many astrologers read it as your way of feeling safe. To compare two Moon signs, see our guide to moon sign compatibility. Here are our guides to the Moon in each sign.
To weigh the Moon with the Sun, Venus and Mars, see our guide to birth chart compatibility.
- Moon in Aries, the fire Moon that moves fast.
- Moon in Taurus, the sign where the Moon is exalted.
- Moon in Gemini, where feelings come out as words.
- Moon in Cancer, the Moon in its own sign.
- Moon in Leo, a Moon that needs a witness.
- Moon in Virgo, and the myth of the weak Moon.
- Moon in Libra, the sign of the scales.
- Moon in Scorpio, the sign of its fall.
- Moon in Sagittarius, where many birth dates are split.
- Moon in Capricorn, the sign of its exile.
- Moon in Aquarius, a sign many people get wrong.
- Moon in Pisces, the last sign of the wheel.
The phase also works with your birth hour. The Moon rises about 50 minutes later each day, so each phase puts the Moon in a different part of the sky at a given hour. Three of our house guides show how the phase and the clock work together: Moon in the 1st house, Moon in the 4th house and Moon in the 11th house. For the Moon’s role in general, read what the Moon represents in astrology. For the twelve signs as a whole, start with our guide to the zodiac signs. And to follow the Moon month by month, see the lunar return.
How we measured
We used two separate sets of star tables. The first was the astronomy engine library by Don Cross. The second was the Swiss Ephemeris, in its built-in Moshier mode. For each one we found the angle between the Sun and the Moon. We timed every phase from 1600 to 2400, about 9,900 lunar months. The two tables gave the same shares to the second decimal.
We also checked 14 new and full Moon times against JPL Horizons, the NASA star table. Every one matched within one minute. All 82 dates in the yearly table matched between the two libraries.
For births, we used the daily counts from the Social Security Administration for 2000 to 2014, as published by FiveThirtyEight. We checked them against the daily counts of the National Center for Health Statistics for 1994 to 2003. Each day was read in Central time, in five minute steps. The couple odds assume that two partners are drawn at random and on their own.
Frequently asked questions
Is moon phase compatibility real?
The phases are real, but the match is not proven. No study has shown that a birth phase predicts a good couple. Our count shows that one random couple in eight makes a full Moon by chance alone.
What moon phases fit together?
Two phases fit when they sit four steps apart on the wheel of eight. New goes with Full, Waxing Crescent with Waning Gibbous, First Quarter with Last Quarter, and Waxing Gibbous with Waning Crescent.
Should soulmates have matching moon phases?
No. In the trend, matching phases earn the twin flame label, not the soulmate label. Either way, one random couple in eight shares a phase, so a match says little about a bond.
How can I check my moon phase compatibility with my partner?
Find the age of the Moon on each birthday with our yearly table. Turn each age into a phase. Then count the steps between the two phases on the wheel of eight.
What moon phase was I born in?
Count the days from the first new Moon of your birth year to your birthday. Take away 29.5 days as many times as you can. The days left are the age of the Moon, and the age gives the phase.
Are more babies born during a full Moon?
No. In 62.2 million US births, full Moon days had the same number of births as nearby days, once holidays were taken out. The gap was 0.01 percent.