How accurate is the lunar phase calculation?

The tool uses the Julian Day Number method with a known new moon reference date (January 6, 2000) and the mean synodic period of 29.53059 days. It is accurate to within about 1 day for dates within a few centuries of the present.

What is a synodic month?

A synodic month is the time it takes for the Moon to return to the same phase, roughly 29.53 days. This differs slightly from the calendar month, which is why the full moon occurs on different dates each month.

How is illumination percentage calculated?

Illumination is estimated from the moon's age in its cycle. It rises from 0% at new moon to 100% at full moon, then falls back to 0%. The formula uses a cosine curve fitted to the synodic period.

What date range does this tool support?

The calculator works for any date from year 1 onward. For very ancient dates (before 1582) the Gregorian calendar did not exist, so treat results as approximate.

Why does the moon have phases?

Moon phases occur because the Moon orbits Earth while Earth orbits the Sun. The fraction of the Moon's sunlit half visible from Earth changes throughout the ~29.5-day cycle, producing the eight named phases.

Lunar Phase By Date

Name: Lunar Phase By Date
Availability: InStock
Author: Nham Vu

Enter any date to instantly see the moon phase, illumination percentage, and days until the next major moon event.

Select a Date

Date

Quick access

Enter a date and click Calculate

The 8 Lunar Phases

New Moon

0% illuminated — Moon is between Earth and Sun

Waxing Crescent

1–49% and growing — right side lit in Northern Hemisphere

First Quarter

50% illuminated — right half visible

Waxing Gibbous

51–99% and growing — nearly full

Full Moon

100% illuminated — fully lit disk

Waning Gibbous

99–51% and shrinking — just past full

Last Quarter

50% illuminated — left half visible

Waning Crescent

49–1% and shrinking — approaching new moon

Summary

Enter any date to instantly see the moon phase, illumination percentage, and days until the next major moon event.

How it works

Enter any date using the date picker.
The tool calculates the Julian Day Number for that date.
It determines the moon age within its ~29.53-day synodic cycle.
Based on the age, it assigns the correct phase name and emoji.
Illumination percentage and days to the next new/full moon are displayed.

Use cases

Find what moon phase you were born under.
Plan outdoor activities, gardening, or fishing around the lunar cycle.
Check the moon phase for a past or future historical date.
Understand how the moon cycle works with a visual guide.
Verify moon phase for astrological or cultural purposes.
Determine how many days until the next full or new moon.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu