Explain it like I'm 12
The calculator estimates lunar age inside the 29.5306-day synodic month, then labels the phase and illumination percentage.
Moon Phase Calculator
Estimate Moon phase, lunar age, illumination and days to the next full or new moon from a calendar date.
What-if check
Synodic month position
Moon phase is estimated by placing the date inside the roughly 29.53-day cycle from one new moon to the next.
| Check | Value | Note |
|---|---|---|
| lunar age | 1.21 days | position in synodic month |
| new moon gap | 1.21 days | nearest new phase point |
| full moon gap | 13.56 days | nearest full phase point |
| illumination | 1.6% lit | phase brightness estimate |
Visual proof
Result path
2026-07-16 approximate lunar age 1.21 days in a 29.5306-day synodic month. Next full moon phase point in 13.56 day(s); next new moon phase point in 28.32 day(s).
Source references
- NASA: Moon Phases — phase background
Formula
Lunar age ≈ days since known new moon modulo 29.530588853. Illumination fraction ≈ (1 − cos(2π × age ÷ synodic month)) ÷ 2.
Worked example
Find days since the reference new moon, divide by 29.530588853 and keep the remainder. The remainder is lunar age; illumination follows from the phase angle.
Professional note
Master’s Tip: phase and moonrise are different questions. A full moon can be below the horizon at a given local time.
Regional and unit assumptions
Standard or basis: approximate synodic-month phase arithmetic at UTC midnight.
Assumptions and limitations
- Uses a simple synodic-month approximation from a known new moon reference.
- Good for educational phase estimates, not official observation planning.
- Local viewing conditions, moonrise time and libration are not included.
- Dates are evaluated at UTC midnight.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Lunar age ≈ days since known new moon modulo 29.530588853. Illumination fraction ≈ (1 − cos(2π × age ÷ synodic month)) ÷ 2.
Standard or basis
Standard or basis: approximate synodic-month phase arithmetic at UTC midnight.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: phase and moonrise are different questions. A full moon can be below the horizon at a given local time.
Related calculators
Questions
What is lunar age?
Lunar age is the number of days since the last approximate new moon phase point.
Is illumination the same as visibility?
No. Illumination describes the lit fraction; weather, horizon, moonrise and local time affect visibility.
Is this an official ephemeris?
No. It is a transparent educational approximation.
Calculation note
Moon phases follow the synodic month, the cycle from one new moon to the next.