Moon phase calculator

How moon phases are calculated

The moon phase is calculated astronomically using the SunCalc library, which models the synodic lunar cycle — the time between two identical moon phases — at approximately 29.53 days. For any given date, SunCalc computes the moon's illumination fraction and its position within the cycle (expressed as a 0–1 value from new moon back to new moon), placing it in one of eight named phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. All calculations happen entirely in your browser. For Moon Match, the phase distance between two birth dates is measured as the shorter arc on the circular phase scale, then converted to a 0–100 compatibility score. Learn more on our methodology page.

Frequently asked questions

What's a moon phase soulmate?

A "moon phase soulmate" is a concept from social media (popular on TikTok) that compares the lunar phases on two people's birthdays. When the two phases are complementary — for example, one person was born on a new moon and the other on a full moon — they are said to form a complete lunar cycle together. This calculator computes the actual astronomical moon phase for each birth date and measures the circular distance between the two phases on a 0–100% compatibility scale.

Why does the heart shape appear in the moon match result?

The heart overlay appears when two birth moon phases are highly complementary — specifically when they sit on nearly opposite sides of the lunar cycle (roughly 0.4–0.5 apart on the 0–1 phase scale). This reflects the "two halves of a whole" idea behind the soulmate concept: one person's lunar phase completes what the other is missing. The visual is purely symbolic and is intended to make sharing the result more fun.

Is this scientific?

The moon phase calculation is astronomically accurate — it uses the SunCalc library, which models the synodic lunar cycle (≈29.53 days) using standard celestial mechanics. The calculated phase for any date will match published almanac data. The compatibility interpretation, however, is a numerological and cultural tradition, not a scientific claim. We present it as a tool for self-reflection and entertainment, not as a prediction about your relationships.