Moon Phase Calculator

Use the Today's Moon tab above to see the exact current moon phase, illumination percentage, and phase name for today's date. The calculator uses precise astronomical algorithms to determine the Moon's position in its 29.5-day synodic cycle. The result includes the phase name (New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, or Waning Crescent), the exact illumination percentage, the Moon's age in days since the last new moon, and the distance from Earth in kilometers.

The lunar cycle — from one new moon to the next — is approximately 29 days, 12 hours, 44 minutes, and 3 seconds (29.53059 days). This is called the synodic period. It's slightly longer than the Moon's actual orbital period around Earth (27.3 days, the sidereal period) because the Earth itself moves around the Sun during that time — so the Moon has to "catch up" to align with the Sun again from Earth's perspective. The discrepancy means the lunar calendar gradually drifts out of sync with the solar calendar, which is why lunar holidays (like Ramadan and many Chinese festivals) fall on different dates each Gregorian year.

A supermoon occurs when a full moon (or new moon) coincides with the Moon's closest approach to Earth in its elliptical orbit — a point called perigee. Because the Moon's orbit is elliptical rather than circular, its distance from Earth varies between about 356,500 km (perigee) and 406,700 km (apogee). A full moon at perigee appears approximately 14% larger in angular diameter and 30% brighter than a full moon at apogee. There is no single official definition of "supermoon" — most astronomers use the threshold of the Moon being within 90% of its closest approach. By this definition, there are typically 3–4 supermoons per year.

There are two common definitions of a Blue Moon. The more popular modern definition is the second full moon in a calendar month — this happens approximately every 2.5 years because the 29.5-day lunar cycle doesn't divide evenly into calendar months. The older, more traditional definition is the third full moon in an astronomical season (spring, summer, fall, or winter) that contains four full moons, instead of the usual three. A Blue Moon does not actually appear blue — the name likely comes from an archaic English usage of "blue" meaning "betrayer" (a moon that doesn't fit the expected seasonal pattern). Actual blue-colored moons can occur due to wildfire smoke or volcanic ash in the atmosphere.

Scientific research has found mixed evidence. A 2021 study in Science Advances found measurable effects on human sleep patterns correlated with lunar phases — people fell asleep later and slept less around full moons, even when the moon wasn't visible. The proposed mechanism is that ancient humans evolved sensitivity to moonlight (which affects nighttime visibility and predator/prey behavior), and this sensitivity persists even in modern artificial lighting environments. However, effect sizes in published studies are generally small (10–20 minutes of sleep difference), and many other studies find no significant effects. The connection between the full moon and unusual behavior or "lunacy" has not been consistently supported in controlled research.

The best nights for astrophotography (stars, Milky Way, nebulae) are the nights near new moon, when the sky is darkest. Specifically, the 5–7 nights centered on new moon — from about 3 days before new moon to 3–4 days after — provide the darkest skies. During this window, the moon either rises and sets close to the sun (crescent phases) or isn't visible at all (new moon). The Milky Way's galactic core is best visible from late spring through early fall from mid-latitudes. For moon photography specifically, the nights near full moon offer the most dramatic moonrise and moonset shots — the moon rising above the horizon at sunset, still showing warm orange colors, is one of the most sought-after shots in nature photography. Use our Astrophotography Planner tab to find the optimal nights for any month.

The "Moon Illusion" is one of the most studied perceptual phenomena in psychology. The Moon appears significantly larger when near the horizon than when high in the sky — but this is entirely a psychological illusion. Measurements confirm the Moon's angular size is actually slightly smaller near the horizon (because you're slightly farther away from it when it's at the horizon — you're on the surface of a spherical Earth rotating toward it). The most widely accepted explanation is the Ponzo illusion: the brain interprets the horizon moon as being far away (because it's near trees, buildings, or terrain features that cue depth perception), and therefore sizes it up to account for apparent distance. The same illusion can be "broken" by looking at the horizon moon through a cardboard tube or rolled-up paper, which removes the contextual cues.

Yes on tides — definitively. The Moon's gravitational pull (and the Sun's, to a lesser degree) is the primary driver of ocean tides. The highest tides (spring tides) occur at new and full moons when the Earth, Moon, and Sun are aligned, causing their gravitational effects to combine. The lowest tides (neap tides) occur at quarter moons when the Sun and Moon pull at right angles. Tidal amplitude varies enormously by geography — the Bay of Fundy in Canada sees tidal ranges exceeding 16 meters; the Gulf of Mexico sees less than 1 meter. On plant growth: the scientific evidence for "moon gardening" (planting by lunar phase) is limited and inconsistent. Some studies find marginal effects; most controlled studies find no significant impact. The practice is ancient and widely followed, but it lacks the robust scientific support that tidal effects have.