Moon Phase Calculator
Moon phase name, age, and illumination for any date.
What it does
The moon wears eight names on a loop: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, then back to new. Pick any date and the tool above names the phase with the moon age and lit share beside it.
The default opens on Jun 1 2026, a Full Moon at age 15.8 days and 99% lit. Counting runs from a reference new moon at Jan 6 2000 18:14 UTC in steps of the mean synodic month, 29.530588 days, the NASA catalog value good to the second.
Age is the workhorse number. Near 0 or 29.5 days the night side faces us at new moon; near 7.4 the right half glows at first quarter; near 14.8 the full face shows; near 22.1 the left half remains at last quarter. Illumination follows a cosine curve, so it lingers near full and new while quarters swing fast.
Honesty first: mean month math drifts against the real moon because single lunations stretch from 29.26 to 29.80 days. Any date can sit about a day off the true phase time, which the limits state outright. Skywatch plans and photography deserve a check against NASA or USNO tables for the exact hour.
How to use
- Pick any calendar date.
- Read the phase name first.
- Read moon age to place it inside the cycle.
- Read illumination to judge viewing.
- Confirm exact phase times in NASA or USNO tables.
Formula
- age
- Days since the last mean new moon.
- 29.53058867
- Mean synodic month in days (NASA catalog).
- phase
- Nearest of the 8 traditional names by age eighth.
Mean month estimate, not an ephemeris. Real lunations vary 29.26 to 29.80 days under solar pull, so boundaries can shift about a day either way.
Worked example
Example: Jun 1 2026
Default: date 2026-06-01.
- Days since Jan 6 2000 18:14 UTC: 9642.74.
- Divide by 29.53058867: 326 full cycles plus 15.8 remainder.
- Age 15.8 falls in the full moon eighth.
- Lit: (1 minus cos(2 pi x 15.8 / 29.53)) / 2 = 99%.
- Tool shows Full Moon, 15.8 days, 99% lit.
Reference table
| Phase | Near age | Look |
|---|---|---|
| New Moon | 0 days | Dark face |
| Waxing Crescent | 3.7 days | Right sliver |
| First Quarter | 7.4 days | Right half |
| Waxing Gibbous | 11.1 days | Mostly lit |
| Full Moon | 14.8 days | Full face |
| Waning Gibbous | 18.4 days | Mostly lit |
| Last Quarter | 22.1 days | Left half |
| Waning Crescent | 25.8 days | Left sliver |
Limits
- Mean month estimate; exact times need ephemeris tables.
- Single dates can sit about a day off true phase.
- No eclipse prediction; nodes ignored.
- Northern hemisphere descriptions of halves.
Common mistakes
- Reading full as one night only; it glows for days.
- Confusing 27.3 day orbit with 29.5 day phases.
- Expecting exact rise times from phase names.
- Judging crescents by age alone near the boundary.
- Skipping the day error on event planning.
Try it yourself
Quick checks. Answers sit tucked under each question.
Jan 21 2000?
Full Moon, age 14.7 days, 100% lit.
Oct 11 2026?
New Moon, age 0.1 days, 0% lit.
Mar 15 2026?
Waning Crescent, age 26.4 days, 11% lit.
Questions
What phase is Jun 1 2026?
Full Moon, age 15.8 days, 99% lit.
How long is a moon cycle?
29.530588 days on average, varying 29.26 to 29.80.
What is moon age?
Days since the last new moon, 0 to 29.5.
Why is the estimate fuzzy?
Real lunations breathe with solar pull; means drift.
Orbit 27.3 versus phases 29.5?
Earth moves too, so the sun angle needs extra days.
Where are exact times?
NASA phase catalogs and USNO date tables.