How to use it
- Pick a date. Today is the default.
- Read the phase, the illuminated fraction and the age of the moon.
- Read the coming quarters and the month of phases.
5 practical uses
- Plan a night of stargazing around the new moon.
- Pick a full-moon night for a hike or a beach walk.
- Check the phase for fishing, gardening or a festival date.
- Time a photo of the crescent or the full moon.
- Find the date of the next full moon.
The maths behind it
The phase angle i between the sun and the Earth seen from the moon comes from the sun's and the moon's ecliptic positions. The illuminated fraction is (1 + cos i) / 2 and the phase, from 0 at new moon to 0.5 at full, follows from i and whether the moon is waxing. The age is the phase times the synodic month, 29.530589 days. The quarter times are found by scanning the phase hour by hour and refining by bisection.
The cycle
The moon goes from new to new in 29.53 days on average, the synodic month. It is waxing from new to full, a crescent then gibbous, and waning from full back to new. The age is the days since the last new moon. The exact moments of new, quarter and full moon are the same everywhere on Earth; only the clock time differs by zone. The phases here are found by scanning the illumination day by day, which puts them within a few minutes of the almanac.