Moon Phase Calculator
Moon Phase Reference Chart
The eight phases of the lunar cycle with approximate timing and illumination.
| Phase | Moon Age (Days) | Illumination | Description |
|---|---|---|---|
| New Moon | 0 | 0% | Moon between Earth and Sun, invisible |
| Waxing Crescent | 1.8 – 7.4 | 1 – 49% | Growing sliver visible on the right |
| First Quarter | 7.4 | 50% | Right half illuminated |
| Waxing Gibbous | 7.4 – 14.8 | 51 – 99% | More than half lit, approaching full |
| Full Moon | 14.8 | 100% | Fully illuminated, opposite the sun |
| Waning Gibbous | 14.8 – 22.1 | 99 – 51% | Shrinking from the left side |
| Last Quarter | 22.1 | 50% | Left half illuminated |
| Waning Crescent | 22.1 – 29.5 | 49 – 1% | Thin sliver on the left before new moon |
How We Calculate This
This moon phase calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
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The Moon Phase Calculator uses the synodic period of the moon (29.53059 days) to determine the current phase based on a known new moon reference date.
The basic rule:
- The moon completes a full cycle every 29.53 days from new moon to new moon
- Illumination is calculated from the moon's position in its cycle using a cosine function
- Eight distinct phases are identified based on the moon's age within the cycle
This calculator provides accurate phase information for any date. The visual display shows the current illumination pattern. For precise astronomical observations, consult an ephemeris.
When Would You Use This Calculator?
This moon phase calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How accurate is this moon phase calculator?
This calculator uses the synodic period method based on a known new moon reference date. It is accurate to within a few hours for phase identification and illumination percentage. For precise astronomical timing, consult a detailed ephemeris.
What is the synodic period of the moon?
The synodic period is the time it takes for the moon to return to the same phase, approximately 29.53059 days. This is the time from one new moon to the next new moon, which differs from the sidereal period (27.32 days) because Earth is also orbiting the sun.
What are the eight moon phases?
The eight phases are: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. Each phase lasts roughly 3.69 days.
What does moon illumination percentage mean?
Moon illumination percentage indicates how much of the moon's visible surface is lit by the sun as seen from Earth. At new moon it is 0%, at full moon it is 100%, and at quarter phases it is approximately 50%.
What is moon age?
Moon age refers to the number of days since the last new moon. It ranges from 0 (new moon) to approximately 29.53 days. At around day 14.76, the moon is full.
Can I check moon phases for past or future dates?
Yes. Enter any date into the calculator to see the moon phase, illumination, and cycle information for that date. This works for both past and future dates.