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Moon Phase On A Date Calculator

Work out moon phase on a date instantly with clear inputs, formula shown and shareable results.

About Moon Phase On A Date Calculator

Moon age is the time since the last new moon within the 29.53 day synodic month, and illumination follows a cosine of that fraction. The phase names divide the cycle into eight segments. Because the orbit is elliptical the true illumination can differ from this mean-motion estimate by a few percent.

Formulas

Moon age

age = days since a known new moon, modulo 29.530589

Illuminated fraction

illumination = (1 - cos(2 pi x phase fraction)) / 2

Worked example

With the default values below, Moon Phase On A Date Calculator returns Illuminated Fraction of 100.0%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Date
2026-10-26
UTC offsetaccepts -12–14 h
1 h

Results

Illuminated Fraction
100.0%
Moon Age Since New
14.62 days
Phase Name
Full moon
Days To The Next Full Moon
0.2

Step by step

  1. 1Values used
  2. 2Moon age
  3. 3Illuminated fraction
  4. 4Illuminated Fraction= 100.0
  5. 5Moon Age Since New= 14.62 days
  6. 6Phase Name= Full moon
  7. 7Days To The Next Full Moon= 0.2

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the illumination not linear with age?

Because the terminator sweeps across a sphere. Illumination changes fastest around the quarters and slowest near new and full.

What causes a blue moon?

Two full moons in one calendar month, which happens roughly every 2.7 years because the synodic month is shorter than most months.

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