Moon Phase Calculator
Enter a date to see the moon phase, age in days, and illumination — calculated from the lunar synodic cycle in your browser.
Waning Crescent
Moon Age
24.8
days
Illumination
24%
Cycle
29.53059d
⚠️ Educational approximation based on the ~29.53-day synodic cycle — not a precise astronomical ephemeris.
🌙 Moon Phase Reference Guide
How to use Moon Phase Calculator
The Moon Phase Calculator determines the moon's phase for any date you enter — from the dim New Moon through Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and back to Waning Crescent. It shows the moon's age in days within the current ~29.53-day synodic cycle and an approximate illumination percentage, all computed instantly in your browser. This is an educational astronomical approximation based on the lunar synodic cycle — not a precise ephemeris — and is suitable for learning, planning, and everyday curiosity.
- Enter a date using the date picker — it defaults to today.
- The phase name, a phase emoji, and the moon's age in days appear instantly.
- Check the illumination percentage and the progress bar to visualise how much of the moon's disk is lit.
- Expand the "Moon Phase Reference Guide" at the bottom to see all eight phases with their day ranges and typical illumination.
- Click "Copy result" to copy the phase summary for sharing.
Your data never leaves your device — 100% private processing.
How moon phase is calculated
The calculation anchors to a known new moon epoch — January 6, 2000 at 18:14 UTC — and measures how many days have elapsed since then. Dividing by the synodic period (29.53059 days) and taking the remainder gives the moon's age within the current cycle. The illumination fraction is derived from a cosine approximation: illumination = (1 − cos(2π × age / 29.53059)) / 2 × 100%. This gives 0% at new moon, 100% at full moon, and approximately 50% at the quarter phases. The approach is standard for educational tools and produces results within a day of the actual phase for most historical and near-future dates.
The eight lunar phases explained
The moon's appearance changes predictably over ~29.53 days. New Moon (age 0–1.8 days): the moon is not visible, lit side faces away from Earth. Waxing Crescent (1.8–7.4 days): a thin crescent grows on the right side in the Northern Hemisphere. First Quarter (7.4–9.2 days): exactly half the moon is lit, rising around noon. Waxing Gibbous (9.2–14.8 days): more than half is lit, approaching full. Full Moon (14.8–16.6 days): the entire disk is illuminated, the moon rises at sunset. Waning Gibbous (16.6–22.2 days): begins to shrink from the right. Last Quarter (22.2–24.0 days): the left half is lit, rising around midnight. Waning Crescent (24.0–27.7 days): a thin crescent, visible before sunrise. Then the cycle repeats.
Accuracy and limitations
This calculator uses a simplified synodic-cycle approximation. The actual synodic period varies slightly due to the elliptical shape of the moon's orbit (the moon moves faster near perigee and slower near apogee), gravitational perturbations from the sun and planets, and the inclination of the lunar orbit. Precise phase times can differ by several hours from this approximation. For exact new-moon and full-moon times to the minute, consult a dedicated astronomical ephemeris such as the US Naval Observatory's data or NASA's HORIZONS system. For everyday planning — moon-lit hiking, photography, gardening by the moon, or understanding lunar symbolism — this tool's results are more than sufficient.
Glossary
- Synodic period
- The time for the moon to return to the same phase as seen from Earth — approximately 29.53059 days.
- Lunation
- One complete cycle from new moon to new moon, synonymous with one synodic month.
- Illumination fraction
- The percentage of the moon's visible disk that is lit by the sun as seen from Earth.
- Waxing
- Increasing — applied to the moon when the illuminated fraction is growing from new toward full.
- Waning
- Decreasing — applied to the moon when the illuminated fraction is shrinking from full toward new.
- Perigee
- The point in the moon's elliptical orbit closest to Earth; a full moon near perigee is sometimes called a "supermoon".
Related reading
Frequently Asked Questions
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Why use Moon Phase Calculator?
- Handles timezone conversions and DST automatically
- Supports date arithmetic across years and leap years
- ISO 8601 and human-readable output formats
- Works for historical and future dates with full accuracy
Common use cases
- Calculate how many days until a project deadline
- Find the day of the week for a date in the past
- Convert meeting times across different time zones
- Calculate age from a date of birth
- Add or subtract business days for contract terms
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