Approximate Moon phase calculator
Choose a UTC date to estimate the lunar phase, age, and illuminated fraction.
Estimate a phase
Approximate mean-cycle result at 12:00 UTC.
Enable JavaScript to calculate. You can also consult the NASA phase tables linked below.
How the estimate works
The calculator counts elapsed days from the reference new Moon on 6 January 2000 at 18:14 Universal Time, using a fixed mean synodic month of 29.530588 days. It evaluates your chosen calendar date at 12:00 UTC. Illumination uses a simplified cosine model; lunar age means position within this model cycle, not elapsed time since a calculated recent new Moon.
Understand the limits
Actual lunations vary. This mean-cycle model does not calculate orbital perturbations, so its phase label, age, and illumination can differ from an astronomical ephemeris, particularly near phase boundaries. It is an educational approximation throughout the supported range 1900–2100, with no guaranteed accuracy. It does not calculate Moon signs, rise/set times, eclipses, or local visibility.
Read the labels
The eight labels use equal model sectors centered on new, quarter, and full phases. A boundary belongs to the next sector. This is a presentation convention, not the exact moment of an astronomical phase event. Use the NASA tables for precise phase times rather than planning observations from this estimate.
Sources and further reading
NASA: Moon phases explains the cycle. NASA: phase tables 1901–2000 supplies the reference new Moon and mean lunation used here. NASA lists times as Universal Time; this educational tool uses UTC at minute precision.