Summary
The moon phase calculator shows the current lunar phase, illumination percentage, upcoming phase dates, a monthly calendar of daily phases, and the moon’s distance from Earth. These data points are useful for astronomers, photographers planning moonlit shoots, gardeners following lunar planting calendars, anglers timing tidal activity, and anyone curious about what the moon is doing tonight.
The engine implements algorithms from Jean Meeus’s Astronomical Algorithms (2nd Edition) — the standard reference for positional astronomy. Chapter 47 covers the moon’s geocentric position, Chapter 48 covers illumination, and Chapter 49 covers the dates of lunar phases.
The eight named phases
The lunar cycle is divided into eight named phases based on how much of the moon’s visible face is illuminated by the sun:
| Phase | Illumination | Description |
|---|---|---|
| New Moon | 0% | The moon is between the Earth and the sun. The sunlit side faces away from us. |
| Waxing Crescent | 1% to 49% (growing) | A thin sliver of light appears on the right side (in the Northern Hemisphere). |
| First Quarter | 50% (right half lit) | Exactly half the moon is illuminated. The moon is 90 degrees from the sun. |
| Waxing Gibbous | 51% to 99% (growing) | More than half is lit and the illuminated area continues to grow. |
| Full Moon | 100% | The Earth is between the sun and the moon. The entire visible face is illuminated. |
| Waning Gibbous | 99% to 51% (shrinking) | The illuminated area begins to shrink from the left side. |
| Last Quarter | 50% (left half lit) | The opposite half from the First Quarter is lit. The moon is 270 degrees from the sun. |
| Waning Crescent | 49% to 1% (shrinking) | A thin sliver remains before the cycle resets to New Moon. |
The terms “waxing” (growing) and “waning” (shrinking) describe whether illumination is increasing or decreasing. “Crescent” means less than half lit; “gibbous” means more than half lit.
The synodic month
The average time from one New Moon to the next is 29.53059 days — this is the synodic month (also called a lunation). It is longer than the moon’s orbital period around the Earth (27.32 days, the sidereal month) because while the moon orbits the Earth, the Earth is also orbiting the sun. The moon must travel an extra ~2.2 days to catch up and return to the same sun-Earth-moon geometry.
The synodic month is not exactly constant. It varies between about 29.27 and 29.83 days because of the eccentricity of both the moon’s orbit and the Earth’s orbit around the sun.
How it works
Step 1: Moon phase angle
The phase angle (also called the elongation) is the angular separation between the sun and the moon as seen from Earth. At New Moon, the phase angle is 0 degrees. At Full Moon, it is 180 degrees. The calculator determines the phase angle using the difference between the moon’s ecliptic longitude (from Meeus Chapter 47) and the sun’s ecliptic longitude.
Step 2: Illumination fraction
The illuminated fraction of the moon’s disc is derived from the phase angle (i) using:
k = (1 - cos(i)) / 2
This formula gives 0 at New Moon (i = 0 degrees) and 1 at Full Moon (i = 180 degrees). The calculator converts this to a percentage for display.
The calculation also accounts for the moon’s distance from Earth and the sun’s distance from Earth, which create a small parallax correction. In practice this correction is tiny — less than 1% — but the Meeus algorithm includes it for completeness.
Step 3: Phase classification
The calculator classifies the current phase into one of the eight named phases based on two values:
- Phase angle — determines the illumination level (how much is lit)
- Whether the moon is waxing or waning — determined by whether the phase angle is increasing or decreasing
The four principal phases (New Moon, First Quarter, Full Moon, Last Quarter) occur at specific phase angles (0, 90, 180, 270 degrees). The four intermediate phases (crescents and gibbous) fill the intervals between them.
Step 4: Next phase dates (Meeus Chapter 49)
To find the dates of upcoming New Moons, First Quarters, Full Moons, and Last Quarters, the engine uses the algorithm from Meeus Chapter 49. This method:
- Estimates an approximate Julian Ephemeris Day (JDE) for each phase based on the lunation number (k) — the count of New Moons since a reference epoch.
- Applies a series of periodic correction terms involving the sun’s mean anomaly, the moon’s mean anomaly, the moon’s argument of latitude, and the longitude of the ascending node.
- The corrections account for orbital eccentricity, evection, variation, and other perturbations.
The result is accurate to within about one minute of the actual phase time.
Step 5: Moon distance
The moon’s distance from Earth varies because its orbit is elliptical:
- Perigee (closest approach): approximately 356,500 km
- Apogee (farthest point): approximately 406,700 km
- Mean distance: approximately 384,400 km
The calculator computes the instantaneous distance using the lunar parallax from the position algorithm (Meeus Chapter 47). The horizontal parallax is converted to distance in kilometres using:
distance = 6,378.14 / sin(parallax)
where 6,378.14 km is the Earth’s equatorial radius.
The calculator also reports whether the moon is near perigee or apogee and shows the proximity as a percentage between the two extremes.
Monthly calendar
The monthly calendar view shows the phase, illumination percentage, and a small phase icon for every day of the selected month. This makes it easy to plan around Full Moons, identify the darkest nights for stargazing, or spot the thin crescents that mark the beginning and end of each lunation.
Moon age
The moon age is the number of days since the most recent New Moon. It ranges from 0 (at New Moon) to approximately 29.5 (just before the next New Moon). The calculator displays this alongside the phase name and illumination to give a complete picture of where the moon is in its cycle.
Worked example
4 March 2026
Key outputs (approximate):
| Output | Value |
|---|---|
| Phase | Waxing Gibbous |
| Illumination | ~97% |
| Moon age | ~12.5 days |
| Next Full Moon | ~6 March 2026 |
| Distance | ~370,000 km |
Inputs explained
- Date — The calendar date for the calculation. The moon’s phase shifts noticeably from day to day, with illumination changing by roughly 3-4 percentage points per day near the quarters.
Outputs explained
- Phase — The named phase (one of eight) describing the current appearance of the moon.
- Illumination — The percentage of the moon’s visible disc that is lit by the sun, from 0% (New Moon) to 100% (Full Moon).
- Moon age — Days elapsed since the last New Moon.
- Upcoming phases — The dates and times of the next New Moon, First Quarter, Full Moon, and Last Quarter.
- Monthly calendar — A grid showing the phase and illumination for every day of the selected month.
- Distance — The moon’s current distance from Earth in kilometres, with perigee/apogee context.
Assumptions & limitations
- Accuracy: Phase dates are accurate to within approximately one minute. Illumination percentages are accurate to within about 1%. These figures are verified against US Naval Observatory data.
- Geocentric calculation: The phase and illumination are computed for an observer at the centre of the Earth. The actual appearance from a specific location on the Earth’s surface can differ very slightly due to parallax, but for lunar phases this effect is negligible.
- No libration modelling: The calculator does not model lunar libration (the slight wobble that lets us see slightly more than 50% of the moon’s surface over time). This does not affect phase or illumination calculations.
- Atmospheric effects not modelled: Near the horizon, the moon can appear dimmer or more orange due to atmospheric scattering. The calculator reports the geometric illumination, not the apparent brightness.
- Distance precision: The distance calculation is accurate to within approximately 100 km compared to full numerical ephemerides (such as JPL DE440).