How accurate is this calculator?

The Julian Date algorithm used here is accurate to within a few hours for dates between 1900 and 2100. For precise astronomical predictions (to the minute), a dedicated ephemeris is recommended.

What is a synodic month?

A synodic month is the time between two identical moon phases — approximately 29.53059 days. This is different from a calendar month.

What does illumination percentage mean?

It is the fraction of the moon's visible surface that is lit by the sun as seen from Earth. 0% = New Moon, 100% = Full Moon.

Why does the full moon appear on slightly different dates each year?

Because the 29.53-day synodic month does not divide evenly into the 365.25-day year, full moons shift by about 11 days each year relative to the calendar.

Can I look up historical moon phases?

Yes. The Julian Date algorithm works reliably for any date from the 18th century onward and reasonably well further back.

Moon Phase Calculator

Name: Moon Phase Calculator
Availability: InStock
Author: Nham Vu

Enter any date to see the moon phase name, illumination percentage, and a visual diagram.

Select a Date

Moon Phase Diagram

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Select a date and click Calculate to see the lunar phase.

Phase Reference

New Moon0%

First Quarter50%

Full Moon100%

Last Quarter50%

Summary

Enter any date to see the moon phase name, illumination percentage, and a visual diagram.

How it works

Enter or pick any date using the date picker.
The tool converts the date to a Julian Date Number.
It calculates the age of the moon in days within the ~29.53-day synodic cycle.
The phase name and illumination percentage are derived from that age.
A visual diagram updates to reflect the current shadow/light geometry.

Use cases

Plan outdoor photography sessions around the full moon.
Find the next new moon for stargazing or astrophotography.
Check the lunar phase on a past date (birthday, anniversary, historical event).
Gardeners following lunar planting calendars.
Fishermen and hunters who time activities by moon phase.
Writers and game designers who need accurate lunar lore.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu