Moon Phase Calculator
The Moon completes a full cycle of phases approximately every 29.5 days — a period called the synodic month. It begins with the New Moon when the lunar disk is invisible, then grows through Waxing Crescent, First Quarter (half illuminated), and Waxing Gibbous before reaching the Full Moon at maximum brightness. It then decreases through Waning Gibbous, Last Quarter, and Waning Crescent back to New Moon. Our calculator uses the Meeus astronomical algorithm, accurate to within a few hours for any date from 1900 to 2100.
Enter any date to instantly see the lunar phase, the illumination percentage, and the exact dates of the next four key phases — New Moon, First Quarter, Full Moon, and Last Quarter. The built-in calendar also lists the upcoming 6 New Moons and 6 Full Moons, making it easy to plan around lunar events. The calculator works entirely in your browser with no external data needed.
Frequently Asked Questions
What are the 8 moon phases in order?
The 8 moon phases in order are: New Moon (0% lit), Waxing Crescent, First Quarter (50% lit, growing), Waxing Gibbous, Full Moon (100% lit), Waning Gibbous, Last Quarter (50% lit, shrinking), and Waning Crescent. The cycle then repeats with the next New Moon.
How is the moon phase calculated?
Moon phase is calculated from the Moon's age — the number of days since the last New Moon. This uses the Julian Day Number and a known New Moon reference date (January 6, 2000). The age is divided by the synodic period (29.530589 days) to find where in the cycle the Moon currently is.
How long is one complete lunar cycle?
One complete lunar cycle (synodic month) lasts approximately 29 days, 12 hours, 44 minutes, and 3 seconds — or 29.530589 days. This is the time from one New Moon to the next. The sidereal month (one orbit relative to the stars) is shorter at 27.3 days.
What is the difference between a New Moon and a Full Moon?
During a New Moon, the Moon is between Earth and the Sun, making its illuminated side face away from us — the Moon appears dark or invisible. During a Full Moon, the Earth is between the Sun and Moon, so the Moon's entire illuminated hemisphere faces us, appearing as a bright complete disk.
What is a supermoon?
A supermoon occurs when a Full Moon (or New Moon) coincides with the Moon being at or near its closest point to Earth (perigee). Because the Moon's orbit is elliptical, its distance varies from about 356,500 km to 406,700 km. A perigee Full Moon appears up to 14% larger and 30% brighter than an apogee Full Moon.
What is a Blue Moon?
A Blue Moon most commonly refers to the second Full Moon in a single calendar month. Since a lunar cycle is ~29.5 days and most months have 30–31 days, this happens roughly every 2–3 years — hence the phrase "once in a blue moon." A second definition is the third Full Moon in a season that has four Full Moons.
Does the moon phase affect human sleep?
Several studies suggest that sleep quality can be slightly poorer around the Full Moon, with people taking longer to fall asleep and sleeping about 20 minutes less on average. The proposed mechanism is that even in the absence of visible moonlight, our internal circadian rhythm may retain a lunar component from evolutionary history. The effect is subtle and varies significantly between individuals.
What is a lunar eclipse and when does it happen?
A lunar eclipse occurs when the Earth passes directly between the Sun and the Full Moon, casting Earth's shadow onto the Moon. A total lunar eclipse makes the Moon appear deep red ("Blood Moon") because Earth's atmosphere refracts and filters sunlight. Lunar eclipses only happen at Full Moon and require the Moon to be near a lunar node.
How does the moon phase affect ocean tides?
Tides are primarily caused by the Moon's gravitational pull on Earth's oceans. During New Moon and Full Moon, the Sun, Earth, and Moon align, creating stronger "spring tides" with higher highs and lower lows. During First and Last Quarter, the Moon and Sun pull at right angles, producing weaker "neap tides" with smaller tidal range.
Can I use this calculator for past and future dates?
Yes. The Meeus algorithm is valid for any date from approximately the year 1000 to 3000, though results are most accurate for dates within a few centuries of the present. You can check the moon phase for any historical date or plan ahead for any future date using the date picker.