Twenty-nine and a half days, on average
Shows the phase of the moon on any date with its age in days, the illuminated fraction and the next new and full moons.
The moon returns to the same phase every 29.53 days — the synodic month. Counting from a known new moon gives the age of the moon on any date, and from the age the illuminated fraction follows directly.
The word on average is doing real work there. The moon's orbit is elliptical, so a particular month runs shorter or longer than the mean, and a simple calculation can land a few hours away from the astronomical moment.
What this calculator shows
- The phase, its name and whether the moon is waxing or waning
- The age of the moon in days and the percentage lit
- The dates of the next new and full moons
What to keep in mind
- The mean synodic month of 29.530589 days is used, counted from the new moon of 6 January 2000.
- The illuminated fraction is a geometric figure; the moon looks brighter than the percentage suggests because the eye responds to light logarithmically.
FAQs
How can I tell waxing from waning?
In the northern hemisphere a waxing moon is lit on the right. In the southern hemisphere it is the other way round, which is worth remembering before trusting a mnemonic from the other side of the world.
How long is a lunar month?
29 days 12 hours 44 minutes on average. The sidereal month, the time to return to the same star, is shorter — 27.3 days — because the earth has moved along its own orbit meanwhile.
Why is the date sometimes a day off?
The mean-month model ignores the eccentricity of the orbit, which can shift a new moon by half a day either way. For a precise moment consult an ephemeris.
Why is a half-lit moon not half as bright?
Because the terminator falls on rough ground casting long shadows, and because the surface reflects most strongly straight back. A full moon is roughly ten times brighter than a half moon, not twice.
Worked example
The solstice
Input: 21 June 2026
Output: a waxing moon 6.2 days old, 37 percent lit
Note: First quarter comes at 7.4 days, when exactly half the disc is lit. From new to full is a fortnight, which is why full moons drift about two days later each month against the calendar.