Moon Phase Calculator

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

Updated September 21, 2026

What is the Moon Phase Calculator?

Estimate the Moon's phase, illuminated fraction, and age for a selected date. The calculator also lists estimated dates and UTC times for upcoming major phase events and shows a monthly phase calendar. The results are calculated in your browser using simplified astronomical algorithms. Treat phase-event times and other displayed values as estimates rather than precise ephemeris data. Traditional full-moon names are included as cultural labels and can vary by source.

This tool uses approximate astronomical calculations to estimate lunar phase information. It does not retrieve live astronomical data. 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. Approximate positions: The calculator uses a subset of formulas based on Meeus's Astronomical Algorithms to estimate the Moon's and Sun's ecliptic longitudes. Phase estimate: Their angular separation is used to estimate the phase and illuminated fraction. This is a simplified geometric estimate, not an observation of the Moon's appearance at a particular location. Phase-event estimates: Periodic corrections are applied to estimate major phase-event times. These are not exact predictions and may differ from a high-precision ephemeris. Monthly calendar: The displayed phase is sampled at noon UTC each day. A phase event is assigned to its UTC calendar date.

Check today's moon phase and illumination

August 2, 2026

Waning Gibbous, 85.2% illuminated, Moon Age 18.5 days

View the calculator's estimate for an upcoming full moon

Any date in August 2026

An estimated full-moon date and time in UTC, with a traditional month name.

How accurate is this moon phase calculator?

The calculator uses simplified formulas based on Meeus's astronomical algorithms. It is intended for general reference; event times and phase values are estimates and are not guaranteed to match a high-precision ephemeris. The supported date range is 1900-2100.

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 defaults to the Northern Hemisphere orientation. The apparent lit side is reversed in the Southern Hemisphere. The phase estimate is based on the selected date and UTC calculations, not a location-specific view.

Does this tool work offline?

The calculations run in your browser and the tool does not send the selected date to an astronomical data service. Reopening the page offline depends on your browser's cache.

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