Moon Phase Calculator
Check the moon phase for any date, along with the moon's age and an approximate illumination percentage.
You can use this for stargazing, fishing plans, photography, gardening, or simple lunar curiosity.
Calculated Result
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Run the calculation to see the moon phase.
Quick Answer
The calculator estimates the moon's age in days since the last new moon, then maps that age to a named moon phase.
How to Calculate Moon Phase
- Pick the date you want to check.
- Convert the date to a day count and compare it with a known new moon reference.
- Wrap the age across the 29.53058867-day synodic month.
- Match the lunar age to a phase name and illumination estimate.
Formula
Lunar age = (days since reference new moon) mod 29.53058867
Illumination = (1 - cos(2 pi x age / synodic month)) / 2
Worked Examples
Example 1: A lunar age near 0 days is a new moon.
Example 2: A lunar age near 7.4 days is around first quarter.
Example 3: A lunar age near 14.8 days is a full moon.
How to Interpret Your Results
| Age range | Phase meaning | Typical look |
|---|---|---|
| 0 to 1.5 days | New moon | Moon is dark or nearly invisible |
| 1.5 to 14.8 days | Waxing phase | Illumination grows toward full moon |
| 14.8 to 29.5 days | Waning phase | Illumination shrinks back to new moon |
Frequently Asked Questions
It is a practical estimate. Small differences can appear because real lunar motion is slightly irregular.
Lunar age is the number of days that have passed since the previous new moon.
Yes. The phase and illumination estimate are useful for planning moon shots and night sky sessions.
Related Calculators
References
- OmniCalculator reference page
- Synodic month length: 29.53058867 days.
- Last reviewed: March 2026.