Moon Compatibility

A compatibility reading built from the one thing two birthdays share with the sky: the phase of the moon. Each date's lunar age is placed on the 29.53-day synodic month, both moons are drawn and laid over each other, and the distance between them becomes the score.

• Overlaid phases — each moon drawn on its own canvas from its illumination fraction, terminator curve and all, then stacked so the difference is visible at a glance.
• Per-date readout — phase name, moon age in days, illumination, and the new moon that came before.
• Cycle chart — the full illumination curve from new to full to new, both dates plotted on it, with a bracket measuring the gap.
• The reading — written for the pair of phases you land on: twins, neighbours, complements or opposites.
• Date picker — two dates, optional names, a swap button, and a copy-link button that saves the pair in the URL. Phase ages come from the mean synodic month anchored to a known new moon, so they are within about a day.

Vanilla JS and the canvas API — no charting library, no astrology API. Lunar ages come from the synodic cycle, with the footer citing NASA RP 1349.

The astronomy is real — phase ages on the synodic cycle, illumination from the moon's age. The compatibility is poetry, and it says so by being beautiful rather than by pretending to be a test. It was made for one particular pair, whose dates are still the default. Now it takes any two dates, and the link remembers them.