Prime Calculation

Moon Phase Soulmate Calculator

Enter two birthdays to see the real moon phase and illumination percentage on each date, then get a just-for-fun soulmate score based on whether your phases combine into a full moon.

Just for fun, no scientific basis. The moon phase and illumination percentage are real astronomy you can verify, but the soulmate score is an invented entertainment rule. It has no scientific or astrological validity and says nothing about any real relationship.

Your moon match result

Soulmate score (for fun) 6% Different Orbits

Your birth moon phase
Full Moon
Your moon illumination
97%
Partner's birth moon phase
Full Moon
Partner's moon illumination
97%
Combined illumination
194%

The phase and illumination are real astronomy (accurate to within about a day of the true phase). The soulmate score is entertainment only.

This tool does two separate things, and it is honest about which is real and which is just for fun. The moon phase and illumination percentage for each birthday are real astronomy you can check. The soulmate score is an invented rule for entertainment, with no scientific or astrological basis.

How to use this calculator

  1. Enter your birthday. The tool finds the exact moon phase and the illumination percentage on that date.
  2. Enter the other person’s birthday. This can be a partner, friend, family member, or coworker. You get their birth moon phase and illumination too.
  3. Pick who you are checking. This only changes the wording of the verdict, not the astronomy or the score.
  4. Read your results. You see each person’s named phase and illumination, the combined illumination, and a just-for-fun soulmate score and verdict.

How it works

The real part is the astronomy. For each birthday the tool works out the phase of the Moon and how much of the lunar disk was lit on that date. It does this offline, with no live data, using the standard mean-synodic-month method.

Here is the method in plain steps. First, it counts the days from a known new moon, the one at 2000-01-06 18:14 UTC, confirmed by the U.S. Naval Observatory (USNO). Then it divides that gap by the average length of one lunar cycle, the synodic month, which is 29.530588853 days (Lunar phase, Wikipedia). The result is where in the cycle the Moon sat on your birthday, written as a fraction from 0 to 1, where 0 is a new moon and 0.5 is a full moon.

The illumination follows from simple geometry. The lit fraction of the disk is (1 minus the cosine of the cycle angle) divided by two (Lunar phase, Wikipedia). So a new moon reads near 0 percent, a full moon near 100 percent, and the two quarters near 50 percent. The USNO sets the same anchors: new moon 0 percent, quarters 50 percent, full moon 100 percent (USNO moon phases FAQ). The tool then maps that fraction to one of the eight named phases.

This phase and illumination are accurate to within about a day of the true phase. The named phase matched USNO at every date tested. The illumination is within about 1 point at the new and full moon, and within about 4 to 8 points at the quarters, because the constant-cycle method does not capture the Moon’s slightly uneven speed.

The soulmate score is the second part, and it is not science. The viral idea is that two people are moon soulmates when their birth phases are complementary and together complete a full moon, for example one born near a new moon and the other near a full moon. To follow that idea, the tool adds the two illumination percentages, then rewards a combined total near 100 percent, a single completed full moon. It adds a small bonus of up to 10 points when one person was born on the waxing side and the other on the waning side, the two halves that fit together.

Every number in that scoring rule is arbitrary and chosen for fun. The “who are you checking?” selector only changes the wording of the top verdict, soulmates, kindred spirits, or a great team. It never changes the astronomy or the score. The score has no scientific or astrological meaning, and it says nothing about any real relationship.

Examples

Born near a new moon and near a full moon, a complementary pair. A birthday of 2000-06-02 reads New Moon at 0 percent illumination, and 2000-06-16 reads Full Moon at 99 percent. The phases come straight from the astronomy, verified against USNO. Their combined illumination is 99 percent, almost one full moon, so the just-for-fun score is 99 with the verdict Moon Soulmates.

Waxing crescent and waning gibbous, opposite halves. A birthday of 2024-01-15 reads Waxing Crescent at 18 percent, and 2024-01-29 reads Waning Gibbous at 87 percent. The combined illumination is 105 percent, close to a full moon, and the two people sit on opposite sides, so the for-fun rule adds the 10 point bonus and the score reaches 100.

Two full moons, the same phase. If both birthdays land on a full moon, each reads 100 percent and the combined total is 200 percent. The scoring rule rewards totals near 100 percent, so a combined 200 percent scores 0 with the verdict Different Orbits. A low score here only means the two illumination figures did not add up near one full moon. It says nothing about the people.

What this tool does that others don’t

Frequently asked questions

What is a moon phase soulmate?

It is a viral, for-fun idea that two people are a special match when the Moon’s phase on each of their birthdays combines into a complete full moon, for example one person born near a new moon and the other near a full moon. It is entertainment, not science or astrology.

How does the moon phase soulmate test work?

The tool first works out the real moon phase and illumination percentage for each birthday using astronomy. It then adds the two illumination figures and gives a higher soulmate score the closer they come to completing one full moon, 100 percent, with a small bonus for complementary waxing and waning phases. The score itself is just for fun.

How do you find your birth moon phase?

Count the days between your birth date and a known new moon, then divide by the average lunar cycle of 29.530588853 days. Where you land in that cycle tells you the phase. This calculator does it for you and also shows the exact illumination percentage.

What does the moon illumination percentage mean?

It is how much of the Moon’s visible disk was lit on that date, from about 0 percent at a new moon to about 100 percent at a full moon, with the quarters near 50 percent. Most other calculators hide this number. This one shows it so you can check it against the USNO.

Is the moon phase soulmate calculator accurate?

The astronomy half is accurate. The phase and illumination come from a standard formula and match authoritative sources like the U.S. Naval Observatory to within about a day. The soulmate score is an invented, just-for-fun rule with no accuracy claim at all.

Is the moon phase soulmate test real or scientific?

No. The moon phase and illumination are real astronomy, but the idea that they predict love or compatibility has no scientific or astrological basis. Treat the soulmate result purely as entertainment.

What does it mean if our moon phases don’t match?

Nothing about your relationship. A low soulmate score here only means your two illumination percentages did not add up close to a full moon. Real relationships are built on far more than a coincidence of birth dates.

How is the soulmate score calculated?

The tool adds your two illumination percentages and rewards results near 100 percent, a completed full moon, with a small bonus when one of you is on the waxing side and the other on the waning side. Every number in that rule is arbitrary and chosen for fun. It is fully transparent and has no scientific meaning.

Sources