Moon Phase Calculator

The lunar cycle takes approximately 29.53 days to complete. It moves through 8 main phases.

Moon phases in order:

1. New Moon — not visible (day 0)
2. Waxing Crescent — sliver growing (days 1–7)
3. First Quarter — right half lit (day 7.4)
4. Waxing Gibbous — mostly lit, growing (days 8–14)
5. Full Moon — fully illuminated (day 14.8)
6. Waning Gibbous — mostly lit, shrinking (days 15–22)
7. Last Quarter — left half lit (day 22.1)
8. Waning Crescent — sliver shrinking (days 23–29)

The calculation uses a known new moon date as a reference point. January 6, 2000 was a new moon. The algorithm counts days since then and divides by 29.53 to find the current position in the cycle.

Illumination is estimated using a cosine function based on the phase angle.