Moon Calculator
Calculate moon phase, position, illumination, and upcoming lunar events for any date and location.
Input Parameters
Moon Phase
Understanding the Moon
Moon Phases
The Moon completes a full cycle of phases — the synodic month — in approximately 29.53 days. Phases arise from the changing angle between the Sun, Earth, and Moon. At New Moon the Sun and Moon are in conjunction; at Full Moon they are in opposition. The illumination fraction varies sinusoidally with the phase angle.
Libration
Although the Moon is tidally locked (same face always toward Earth), libration allows us to see about 59% of the surface over time. Libration in longitude results from the Moon's elliptical orbit, while libration in latitude comes from the 6.7° tilt of its orbital plane relative to the ecliptic.
Observing Lunar Features
Craters, mountains, and rilles are best observed along the terminator — the line between the lit and dark hemispheres — where low Sun angles cast long shadows that reveal topographic detail. The Full Moon, while bright, appears flat and featureless for surface observation.
Eclipse Conditions
Eclipses occur when the Moon is near a node — where its orbit crosses the ecliptic. Solar eclipses happen at New Moon, lunar eclipses at Full Moon. The Saros cycle (≈18 years 11 days) governs the repetition of nearly identical eclipses, as it represents a common multiple of the synodic, draconic, and anomalistic months.