Moon Phase Calculator
How it works
The Moon completes a synodic cycle (new moon to new moon) in approximately 29.530589 days — the lunar month. Tracking moon phases has practical applications in fishing and hunting (animals adjust feeding patterns with light levels), gardening (some traditional and biodynamic farming practices use moon cycles for planting and harvesting), photography and astrophotography, and cultural/religious observance.
**The eight phases** 🌑 New Moon → 🌒 Waxing Crescent → 🌓 First Quarter → 🌔 Waxing Gibbous → 🌕 Full Moon → 🌖 Waning Gibbous → 🌗 Last Quarter → 🌘 Waning Crescent → New Moon again.
"Waxing" means the illuminated portion is growing (new moon → full moon). "Waning" means it's shrinking (full moon → new moon). First/Last Quarter means half the moon is illuminated — not a quarter of the cycle elapsed.
**Calculation method** The algorithm uses the known epoch of a new moon (e.g., January 6, 2000 at 18:14 UTC) and the mean synodic period (29.530589 days) to compute the moon's age (days since last new moon) for any date. Phase = moon age / 29.530589. Values: 0–0.125 (new moon), 0.125–0.375 (waxing crescent), 0.375–0.625 (first quarter/gibbous), 0.5 (full moon), 0.5–0.875 (waning), 0.875–1.0 (waning crescent).
**Illumination percentage** Illumination ≈ (1 − cos(2π × phase)) / 2 × 100%, reaching 100% at full moon and 0% at new moon.
**Supermoons** A supermoon occurs when a full moon coincides with the Moon being near its orbital perigee (closest approach to Earth), making it appear 14–30% larger and 30% brighter than average. The Moon's orbit is elliptical: perigee ≈ 356,500 km, apogee ≈ 406,700 km.
Privacy: all calculations run in the browser. No date or location data is transmitted.
Frequently Asked Questions
- The calculation uses the Julian Day Number of the target date minus the Julian Day Number of a known new moon (e.g., January 6, 2000 = JD 2451549.5), divided by the mean synodic period (29.530589 days). The fractional part (0.0–1.0) gives the phase: 0.0 = new moon, 0.25 = first quarter, 0.5 = full moon, 0.75 = last quarter. This is an approximation accurate to within ~12 hours; precise calculations require perturbation corrections for the Moon's elliptical orbit and Earth–Moon–Sun geometry.
- This is one of the great coincidences of our solar system: the Sun is approximately 400 times larger than the Moon in diameter, but also approximately 400 times farther from Earth. These ratios cancel out, giving both bodies an angular diameter of about 0.5 degrees as seen from Earth. This coincidence makes total solar eclipses possible — the Moon just barely covers the Sun's disc. In about 600 million years, the Moon will have drifted far enough away that total solar eclipses will no longer occur.
- The 'lunar effect' (full moons causing unusual behaviour, more emergency room visits, more crime) has been extensively studied and consistently found to be a myth. A 1996 meta-analysis of 37 studies by Ivan Kelly found no reliable correlation between lunar phase and mental illness admissions, homicides, violence, or emergency calls. The belief persists due to confirmation bias (we remember the full moon night when something unusual happened; we don't notice the many full moon nights when nothing unusual happened). The gravitational tidal effect of the Moon on individual humans is negligible compared to a nearby building.
- There are two definitions: (1) The second full moon in a calendar month — this occurs roughly every 2.5 years because 12 synodic months (354.4 days) is shorter than a calendar year (365.25 days), creating approximately one extra full moon per 2–3 years. (2) The third full moon in a season with four full moons (the older astronomical definition). The phrase 'once in a blue moon' (meaning rarely) predates the astronomical usage — 'blue' was historically used to mean 'absurd' or 'impossible', as in suggesting the moon was blue.