Moon Phase Calculator

Calculate the current moon phase, illumination percentage, and upcoming lunar events for any date.

Updated August 2, 2026

What is the Moon Phase Calculator?

Discover the exact phase of the moon for any date with our free, highly accurate Moon Phase Calculator. Whether you are an amateur astronomer, a photographer planning a night sky shoot, a gardener tracking lunar cycles, or simply curious about the night sky, this tool provides instant, precise lunar data. Instantly calculate the current moon phase, illumination percentage, moon age, and zodiac position. The tool also predicts the exact UTC dates and times for upcoming New Moons, First Quarters, Full Moons, and Third Quarters, complete with traditional full moon names like the Wolf Moon or Harvest Moon. All calculations are performed locally in your browser using proven astronomical algorithms, meaning the tool works completely offline and keeps your data entirely private.

This tool relies on established astronomical mathematics to determine the moon's position and appearance without needing external APIs or network requests. Date Conversion: The selected calendar date is converted into a Julian Day Number, a continuous count of days since the beginning of the Julian Period, which simplifies astronomical time calculations. Ecliptic Longitude: Using Jean Meeus's astronomical algorithms (from Astronomical Algorithms, 1998), the calculator computes the apparent geocentric ecliptic longitude of both the Moon and the Sun for the given moment. Elongation and Phase: The angular distance (elongation) between the Sun and the Moon is calculated. This angle directly determines the phase fraction (from 0.0 to 1.0) and the illuminated percentage of the lunar disc visible from Earth. Event Extrapolation: To find upcoming or previous phase events, the tool solves for the exact moments when the elongation equals 0° (New Moon), 90° (First Quarter), 180° (Full Moon), and 270° (Third Quarter), applying periodic correction terms for high precision. Lunar Calendar Generation: For the monthly view, the phase is sampled at noon UTC for each day of the selected month, mapping exact phase transitions to their respective calendar days for easy visual reference.

Check today's moon phase and illumination

August 2, 2026

Waxing Gibbous, 87.4% illuminated, Moon Age 14.2 days

Find the exact date and time of the upcoming full moon

Any date in August 2026

Full Moon on August 29, 2026, at 06:34 UTC (Sturgeon Moon)

How accurate is this moon phase calculator?

This tool uses Jean Meeus's astronomical algorithms, providing phase timings and illumination percentages accurate to within a few minutes for any date between 1900 and 2100.

What is "Moon Age"?

Moon age is the number of days that have passed since the most recent New Moon. A complete lunar cycle (synodic month) lasts approximately 29.53 days, so the moon age ranges from 0 to 29.53.

Why does the moon phase look different in the Southern Hemisphere?

The visual representation in this calculator defaults to the Northern Hemisphere orientation, where the moon waxes on the right side. In the Southern Hemisphere, the illuminated portion appears reversed (waxing on the left). However, the phase name, illumination percentage, and exact timing remain identical globally.

Does this tool work offline?

Yes. All astronomical calculations are performed locally in your browser using pure JavaScript. No data is transmitted to any external server, ensuring complete privacy and full offline functionality once the page is loaded.

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