Moon Phase Calculator
Calculate the moon phase for any date. Find illumination percentage, phase name, and upcoming lunar events.
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About Lunar Phases
The lunar cycle lasts approximately 29.53 days. The moon goes through 8 distinct phases as it orbits Earth, affecting tides, wildlife behavior, and has been used for calendars throughout history.
Waxing Crescent
A sliver of the moon is visible, growing each night.
Moon Details
Upcoming Phases
What Is the Moon Phase Calculator?
The Moon Phase Calculator shows the current phase of the Moon for any date — from the new moon through all eight principal phases to the next new moon. It displays the phase name, illumination percentage (how much of the Moon's disk is lit), visual emoji representation, and a countdown to the next key phases. Understanding moon phases is valuable for astronomy, photography, gardening, fishing, religious observances, and anyone interested in the night sky.
The Moon doesn't produce its own light — it reflects sunlight. As the Moon orbits Earth, the angle between the Moon, Earth, and Sun changes, altering how much of the lit half we can see from Earth's surface. This visible portion changes smoothly throughout the approximately 29.53-day cycle, creating the distinctive sequence of phases.
This calculator uses a known reference new moon (January 6, 2000 at 18:14 UTC) as its anchor and computes the days elapsed to any target date, then determines the phase and illumination using trigonometric functions based on the synodic period.
Moon Phase Calculation
The Moon's phase is determined by its age within the current synodic cycle. Illumination is computed from the cosine of the phase angle.
Moon Illumination
Where:
- knownNewMoon= January 6, 2000 18:14:00 UTC (reference new moon)
- lunarCycle= 29.53058867 days — mean synodic period of the Moon
- currentCycleDay= ((daysSinceKnown % lunarCycle) + lunarCycle) % lunarCycle — days into current cycle (0–29.53)
- isWaxing= currentCycleDay < lunarCycle/2 (14.765) — Moon is growing brighter
- phasePercentage= currentCycleDay / lunarCycle × 100 — how far through the cycle (0–100%)
Phase Descriptions and What They Look Like
Each lunar phase has distinctive visual characteristics and astronomical significance:
- New Moon: The Moon is between Earth and the Sun; virtually no illumination visible. The sky is darkest for stargazing. Religious observances (Islamic month start, Hebrew Rosh Chodesh) begin with the new moon.
- Waxing Crescent: A thin, curved sliver of light on the right side (Northern Hemisphere). Visible in the western sky after sunset.
- First Quarter: The right half of the Moon is illuminated. The Moon is 90° ahead of the Sun and visible from midday to midnight.
- Waxing Gibbous: More than half illuminated, growing toward full. Visible from afternoon through most of the night.
- Full Moon: The entire facing hemisphere is illuminated. Rises at sunset and sets at sunrise. Brightness makes deep-sky observation difficult but illuminates outdoor scenes beautifully.
- Waning Gibbous through Waning Crescent: Mirror of the waxing phases — the lit portion shifts to the left side and shrinks toward the next new moon.
How to Use This Calculator
- Select a Date: Enter any date to see the Moon phase on that day. The calculator defaults to today.
- Read the Phase: The result shows the phase name, visual emoji, illumination percentage, and position in the cycle.
- Check Upcoming Phases: Days until the next new moon, first quarter, full moon, and last quarter are displayed for planning purposes.
- Photography Planning: The best portrait and landscape photography windows are listed relative to the moon phase.
Applications of Moon Phase Data
Astrophotographers plan their deep-sky imaging sessions around new moon when the sky is darkest, and schedule lunar photography around full moon for maximum surface brightness and dramatic crater shadows at first or last quarter. Knowing the phase 2–4 weeks in advance allows time to book observatory time, travel to dark-sky sites, and plan multi-night imaging campaigns.
Coastal fishers in many cultures worldwide time their fishing trips around lunar phases. Feeding activity in both saltwater and freshwater species correlates with tidal movement, which peaks around new and full moons (spring tides). Solunar tables — which combine lunar phase, moonrise/moonset times, and sun position — are standard tools for sport fishing guides and tournament anglers.
The Jewish calendar's month of Nisan (Passover month) begins with the new moon closest to the spring equinox. The Islamic month of Ramadan begins with the sighting of the crescent moon after the new moon. The Chinese New Year falls on the second new moon after the winter solstice. Moon phase data enables conversion between these traditional calendar systems and the Gregorian calendar.
Worked Examples
Full Moon Illumination
Problem:
What is the illumination percentage at exactly full moon (lunar age 14.77 days)?
Solution Steps:
- 1currentCycleDay = 14.77 days (past the halfway point, just entering waning)
- 2isWaxing = 14.77 < 14.765? No, just past the crossover
- 3illumination = (1 + cos(14.77/29.53 × 2π)) / 2 × 100 = (1 + cos(π)) / 2 × 100 = (1 − 1) / 2 × 100 = 0?
- 4Actually at full moon, cos(π) = −1: (1 + (−1)) / 2 = 0; the waxing formula gives: (1 − cos(π)) / 2 = (1 − (−1)) / 2 = 1 = 100%
- 5At cycle day = 14.77 (full moon), waxing formula: illumination = 100%
Result:
At full moon (lunar age ~14.77 days), illumination = 100% of the visible face.
Days Between Full Moons
Problem:
How many days are between two consecutive full moons?
Solution Steps:
- 1Full moon occurs at cycle day ~14.77 (once per synodic cycle)
- 2The synodic cycle = 29.53058867 days
- 3Days between consecutive full moons = 29.53058867 days
- 4This means full moons can fall on the same calendar month OR shift by 1–2 days each month
Result:
Consecutive full moons are separated by 29.53 days. A calendar month is 28–31 days, so two full moons can occasionally occur in the same month (a 'blue moon').
Moon Phase for a Historical Date
Problem:
Was there a full moon on July 20, 1969 (Apollo 11 moon landing)?
Solution Steps:
- 1Reference new moon: January 6, 2000
- 2Days from July 20, 1969 to Jan 6, 2000 = approximately 11127 days
- 3daysSinceKnown for July 20, 1969 = −11127 days
- 4lunarAge = ((−11127 % 29.53) + 29.53) % 29.53 ≈ 24.6 days
- 5At 24.6 days: Waning Crescent phase, illumination about 25%
Result:
Apollo 11 landed on the Moon on July 20, 1969 during a waning crescent phase — not a full moon. The Moon was about 25% illuminated.
Tips & Best Practices
- ✓New moon = darkest skies for stargazing and deep-sky astrophotography — plan your imaging sessions around new moon.
- ✓First and last quarter moons create dramatic crater shadows along the terminator — ideal for lunar photography and telescopic observation.
- ✓Full moon rises at sunset and is visible all night — it rises about 50 minutes later each subsequent night.
- ✓Spring tides (highest range) coincide with new and full moon; neap tides (lowest range) coincide with quarter moons.
- ✓For Southern Hemisphere observers: waxing crescent appears on the LEFT side, opposite to Northern Hemisphere.
- ✓The Moon moves about 12° per day relative to the stars — it passes near a different constellation each night through the zodiac.
Frequently Asked Questions
Sources & References
- Moon Phase — Wikipedia (2024)
- Moon Phases — NASA (2024)
- US Naval Observatory Moon Phases (2024)
Last updated: 2026-06-06
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Editorial Note
MyCalcBuddy Editorial Team
This page is maintained as an educational calculator reference.
Formula Source: Standard Mathematical References
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