Moon Phase Calculator
Enter any date to instantly find the moon phase, how much of the moon is illuminated, how many days it is into its current cycle, and when the next new moon and full moon will occur. The calculator uses the same synodic-month formula astronomers use, accurate to within one day for any date from 1800 to 2100.
How the moon phase is calculated
The moon orbits Earth in a repeating cycle called the synodic month, which averages 29.53058770576 days. This calculator anchors on a well-known reference new moon (January 6, 2000, 18:14 UTC) and counts how many days have elapsed since then. Dividing by the synodic month and keeping the remainder gives the lunar age, the number of days the moon is past its most recent new phase. The illumination percentage uses a simple cosine formula: (1 - cos(2 * pi * age / cycle)) / 2, which closely matches the observed brightness pattern. The phase name is then assigned based on which of the eight named ranges the lunar age falls in.
Why the moon appears to change shape
The moon does not produce its own light. We see only the portion of its sunlit half that faces Earth. As the moon orbits from new (between Earth and sun) toward full (on the opposite side of Earth from the sun), the illuminated fraction grows, or waxes. After full moon the illuminated portion visible from Earth shrinks, or wanes, back toward new. The shape appears to go from a thin crescent on the right side (waxing in the Northern Hemisphere) through a half-lit quarter, a nearly-complete gibbous, to a full disk, and back again in reverse. The cycle takes about 29 and a half days.
Principal phases vs. intermediate phases
Astronomers distinguish four principal phases (New, First Quarter, Full, Last Quarter) from four intermediate phases (Waxing Crescent, Waxing Gibbous, Waning Gibbous, Waning Crescent). The principal phases are exact moments in time, the instant the moon reaches 0, 90, 180, or 270 degrees of angular separation from the sun. The intermediate phases are ranges between those instants. This calculator identifies which named phase the moon is in on a given calendar day. Because the moon moves continuously, the boundary between phases may occur at any hour of the day; the result here applies to local midnight on the chosen date.
Moon phases and tides, gardening, fishing, and folklore
The gravitational pull of the moon drives Earth's tides. The strongest tides (spring tides) occur at new moon and full moon, when the sun, Earth, and moon are roughly aligned. Weaker neap tides occur at the quarter phases. Many fishing guides treat full and new moon days as the best for activity because fish feed more aggressively when tidal currents are strong. Traditional almanac gardening schedules planting and harvesting by moon phase, recommending that crops grown for leaves or fruits be planted while the moon is waxing, and root crops when it is waning. Scientific evidence for these gardening effects is limited, but the traditions are ancient and widespread across cultures.
The 8 moon phases
| Phase | Lunar day range | Approx. illumination | Visibility |
|---|---|---|---|
| π New Moon | 0 - 1 and 28.5 - 29.5 | 0% | Not visible (rises and sets with the sun) |
| π Waxing Crescent | 1 - 6.4 | 1 - 49% | Western sky after sunset |
| π First Quarter | 6.4 - 8.4 | 50% | High in the sky at sunset, sets around midnight |
| π Waxing Gibbous | 8.4 - 13.8 | 51 - 99% | Rises afternoon, high by evening |
| π Full Moon | 13.8 - 15.8 | 100% | Rises at sunset, visible all night |
| π Waning Gibbous | 15.8 - 21.2 | 51 - 99% | Rises after sunset, visible into morning |
| π Last Quarter | 21.2 - 23.2 | 50% | Rises around midnight, high at dawn |
| π Waning Crescent | 23.2 - 28.5 | 1 - 49% | Eastern sky before sunrise |
Each named phase spans a range of lunar days within the 29.53-day synodic cycle. Illumination is approximate.
Frequently asked questions
What is the difference between waxing and waning?
Waxing means the illuminated portion of the moon is growing larger each night, moving from new moon toward full moon. Waning means it is shrinking, moving from full moon back toward new moon. The word waxing comes from Old English for "grow," and waning from "diminish."
How long is a lunar month?
The synodic month (new moon to new moon) is approximately 29 days, 12 hours, 44 minutes, or 29.53059 days. This differs slightly from the sidereal month (27.32 days), which is the time for the moon to orbit 360 degrees relative to the stars; the difference exists because Earth is also moving around the sun, so the moon must travel a bit farther to "lap" the sun from our perspective.
Why is the moon not always visible on a clear night?
Near new moon the moon rises and sets close to the same time as the sun, so it is above the horizon mainly during daylight hours and too close to the sun's glare to see. A waning crescent moon rises a few hours before dawn and may only be visible for a short time. The position in the sky changes roughly 50 minutes later each day as the moon moves through its cycle.
What is a blue moon?
The most widely used modern definition of a blue moon is the second full moon in a calendar month. Because the synodic month is slightly shorter than most calendar months, two full moons occasionally fit within a single month, happening about once every two to three years. An older definition calls the third full moon in a season that has four blue moons, but the newer calendar definition is now more commonly used.
What is a supermoon?
A supermoon is a full (or new) moon that occurs when the moon is near perigee, the closest point in its slightly elliptical orbit around Earth. At perigee the moon can be about 14% larger and 30% brighter in the sky than at apogee (the farthest point). There is no official astronomical definition and thresholds vary by source, but the term is widely used to describe full moons that appear noticeably larger or brighter than average.
How accurate is this calculator?
The calculator uses the mean synodic month and a fixed reference epoch, giving results accurate to within about one calendar day for dates from 1800 to 2100. For exact phase times to the nearest minute, an ephemeris-based calculation (such as from the U.S. Naval Observatory) is needed, because the true synodic month length varies slightly due to gravitational interactions with the sun and other planets.