🌙 Moon Phase Calculator

What Is the Moon Phase Today?

Enter any date and the calculator returns the moon's phase, its age in days since the last new moon, and the illuminated percentage. October 16, 2026: a waxing crescent, 4.62 days old, 22.2 percent lit. The method is pure astronomy: days since a known new moon, modulo the 29.53-day synodic month.

The Method

The reference new moon of January 6, 2000 is the standard anchor; the synodic month (new moon to new moon) averages 29.53 days. Eight phase buckets of ~3.7 days each give the familiar names. Illumination follows the cosine curve: 0 at new, 100 at full. Accuracy is within a day for any date, plenty for planning and curiosity.

Worked Example

October 16, 2026: 9,779.24 days since the reference new moon; modulo 29.53 gives age 4.62 days. Phase index: floor(4.62/29.53 x 8 + 0.5) = floor(1.75) = 1, waxing crescent. Illumination: (1-cos(2 x pi x 4.62/29.53))/2 = 22.2 percent. Thirteen days later, October 29, the age is 17.62: waning gibbous at 91.1 percent.

The Eight Phases

New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, then back to new. "Waxing" means growing toward full; "waning" means shrinking toward new. The quarters are the half-lit milestones, named for the moon being a quarter through its cycle, not for looking like a quarter.

Why the Phase Matters

Photographers chase new moons for dark skies and full moons for night photography. Gardeners plant by folk calendars tied to phases. Anglers swear by solunar tables built on them. And tide ranges peak near new and full moons, when sun and moon pull together. Whatever your reason, the phase is worth knowing before you go.

Accuracy Notes

The simple algorithm here is accurate to about a day, which covers every everyday use. Precision astronomy accounts for the moon's elliptical orbit and perturbations, shifting exact new/full moments by hours. For eclipse prediction or telescope pointing, use dedicated ephemeris software; for everything else, this is plenty.

Supermoons and Micro Moons

A supermoon is a full moon near perigee, the moon's closest approach, appearing about 14 percent larger than a micromoon at apogee. The phase calculator does not show distance, but the effect is real: perigee full moons look noticeably bigger on the horizon. Tides run slightly larger then too. If a full moon looks unusually dramatic, check whether it coincided with perigee: it probably did.

Moon Phase and Sleep Folklore

People have blamed the full moon for bad sleep for centuries, and small studies do find slightly shorter, lighter sleep around full moons, though the effect is modest and debated. Light is the plausible mechanism: a bright full moon through thin curtains is real light exposure. If you sleep worse at full moon, blackout curtains will do more for you than any lunar theory.

Worked Example: Planning a Stargazing Trip

Dark skies need a new moon. On October 16, 2026, the moon is 4.62 days old, so the new moon was about 4.6 days earlier, around October 11. Add one full cycle of 29.53 days: the next new moon falls around November 10. Plan the trip for November 8 to 12, when the moon is a thin crescent or absent and the Milky Way owns the sky. The full moon, worst for stargazing, lands around October 26, two weeks after the new.

Check moonrise times too: a crescent moon sets early, leaving the rest of the night dark even a few days from new. The calculator gives the phase; a moonrise table gives the schedule. Together they pick the night.

The Moon Illusion and Other Tricks

The moon looks enormous on the horizon and modest overhead, but it is the same size: the moon illusion is a perceptual trick, not astronomy. Harvest moons and supermoons are real but subtle: a supermoon is about 14 percent larger than a micromoon, noticeable only in side-by-side photos. Blue moons, the second full moon in a calendar month, are just calendar trivia.

The tides are the phase effect that actually matters. Spring tides, the largest ranges, arrive at new and full moons when sun and moon align; neap tides, the smallest, at the quarters. Surfers, fishermen, and coastal builders plan by it. The phase on the calculator is the same number the tide tables start from.

Worked Example: Planning a Full-Moon Hike

You want a night hike under a full moon on the weekend of November 21 to 22, 2026. The calculator shows the full moon falls on November 24, 2026, three days after your target weekend. A full moon is effectively full for about a day on either side (illumination stays above 98 percent), but three days off means a waning gibbous at roughly 85 percent. Still plenty bright for a trail, but not the iconic full disc over the ridge.

The better move: shift to the following weekend or accept the gibbous. Moonrise timing matters too: a full moon rises around sunset, giving light all night, while a waning gibbous rises hours later, leaving the early evening dark. For a 7 PM trailhead start, the full moon weekend gives you light from the first step; the gibbous weekend starts dark. Check moonrise, not just phase, when planning.

The 29.5-Day Cycle: Why Phases Drift Through the Calendar

The synodic month (new moon to new moon) is 29.53 days, which does not divide evenly into any calendar month. So full moons drift about 11 days earlier each calendar year, and every 2 to 3 years the drift accumulates into a 13th full moon: the famous blue moon. This is also why Ramadan, Easter's computus, and Chinese New Year all wander against the solar calendar; they are tied to the moon's schedule, not the sun's.

The drift has practical consequences. A "full moon party" on the same calendar date each year will sometimes land on a gibbous. Tide tables, which follow the moon closely, need lunar math, not calendar habits. And the reason moon-phase calculators exist at all is this drift: the pattern is perfectly predictable but impossible to track by feel, so the algorithm does what intuition cannot.

Moon Phase Calendars vs. Moon Phase Apps: What to Trust

Printed moon calendars give the phase for each date, usually accurate to the day, and they never run out of battery. Apps give phase plus moonrise, illumination percentage, and augmented-reality sky views, and they update for your exact location. For casual curiosity the calendar is enough; for photography, hiking, or fishing, the app's rise times and percentages earn their place.

Accuracy differences between good sources are tiny: reputable calculators agree within minutes on phase times because they use the same astronomical algorithms. Where apps differ is presentation and extras like golden-hour overlap or dark-sky maps. Pick one trusted source and stick with it rather than averaging three; the variation between them is noise, not information.

Pro Tips for Photographing the Moon

Shoot the moon when it is near the horizon and include foreground: buildings, trees, or hills give scale and trigger the moon illusion effect that makes it look huge. A bare moon in a black sky photographs as a flat white disc no matter the phase. The day or two after full (waning gibbous) is actually better for detail, because the slightly angled sunlight throws crater shadows.

Use a tripod and manual settings: start around 1/125s, f/8, ISO 100 for a full moon. The moon is sunlit, so it needs daylight exposure, not night exposure; auto mode will overexpose it into a blob. And arrive before moonrise. The best shots happen in the first 20 minutes, when the moon is big, golden, and near something interesting.