Moon phase compatibility
Put in two birthdays. We work out the moon that hung over each night, set the two side by side, and tell you how close they come to making one full moon.
Enter both dates to see your result.
Which moon phases are compatible?
Four pairs complete each other exactly. In each one the two moons are lit on opposite sides and their illuminations add up to 100%, so setting them side by side fills the disc with nothing missing and nothing doubled.
How the score is worked out
Most versions of this test show you two pictures and leave the judging to your eyes. We measure it instead, because it is a real geometric question.
The boundary between the lit and unlit parts of a moon, the terminator, is a half ellipse. Its width depends on how much of the disc is lit, and which side it bulges toward depends on whether the moon is waxing or waning. That gives two facts about each of your moons: how much is lit, and which limb it is lit on.
- Lit on opposite sides. The two bright regions meet with no gap and no overlap exactly when the two illuminations add up to 100%. Below that they leave a dark gap, above it they overlap.
- Lit on the same side. The larger bright region simply contains the smaller one, so the pair covers no more than the brighter moon alone and doubles up on everything else.
Your score is how much of one full moon the pair covers, minus the part where the two of you cover the same ground. 100% means you form exactly one full moon.
Where the trend came from
The moon phase soulmate test spread on TikTok, where couples pull up the moon on each of their birthdays and hold the two screens together to see whether they close into a circle. It caught on because it is instant, visual, and, unlike most compatibility quizzes, built on something you can actually verify: the moon on a given date is a computable fact, the same for everyone who checks it.
Is any of this real?
The astronomy is. The moon's phase on any date in the past century can be computed to within a fraction of a percent, and this page does exactly that rather than looking anything up. The shapes really do complete each other in the four pairs listed above.
The romance is not. There is no evidence that the moon phase on the day you were born shapes your personality or predicts how you get along with anyone. We present it the way we present the astrology layer in the app: as something enjoyable that makes no claim to be a fact.
Common questions
What is the moon phase soulmate trend?
It is a social media trend where two people look up the moon phase on each of their birthdays and place the two moons side by side. If the lit parts add up to one complete full moon, the pair is said to be a match. The astronomy behind it is real: any date's moon phase can be computed exactly. The romantic reading is for fun.
How is moon phase compatibility calculated?
We compute the moon's illuminated fraction and which limb is lit for each birth date, then measure how much of one full moon the two discs cover together, minus any part where they overlap. A score of 100% means the two moons form exactly one full moon, with nothing missing and nothing doubled.
Which moon phases are most compatible?
The four complementary pairs are new moon with full moon, waxing crescent with waning gibbous, first quarter with last quarter, and waxing gibbous with waning crescent. In each pair the moons are lit on opposite sides and their illuminations add up to 100%, so together they complete a full disc.
What does it mean if two people have the same moon phase?
Two identical phases are lit on the same side by the same amount, so they sit on top of each other rather than completing each other, and the score is low. Some readings call this a twin flame pairing rather than a mismatch. It simply means the trend's geometry does not apply to you.
Is moon phase compatibility real?
There is no scientific evidence that the moon phase on your birthday affects your personality or your relationships. The moon phase itself is real and precisely computable, and the shapes really do complete each other. Treat the result as entertainment, which is how we present it.
Does the time of day I was born change the result?
Barely. The moon's illumination changes by only about three percent over a full day, so the phase on your birth date is effectively the same whatever hour you were born. We compute midday for your date, which keeps the answer stable no matter which time zone you were born in.
Curious about just your own? The birthday moon page gives you the phase, illumination, and zodiac sign for the night you were born. Or read what each of the eight moon phases means.