Moon Phase Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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The Moon Phase Calculator is most useful when you want a quick result that still makes its assumptions visible. It helps you estimate one of eight moon-phase labels from the selected date using a fixed synodic-cycle approximation, letting you test a second set of inputs and see immediately which change moved the output.

What this calculator does

For the Moon Phase Calculator, the primary output is the page’s approximate moon phase. Supporting values include Lunar age, Illuminated fraction. Every value shown by the Moon Phase Calculator comes from the visible fields used by this rule. In practical terms, it is best used to estimate one of eight moon-phase labels from the selected date using a fixed synodic-cycle approximation.

How to use it

Enter Date. For the Moon Phase Calculator, use the exact calendar dates you want to compare. When comparing two Moon Phase Calculator scenarios, change one input at a time and keep the other entries unchanged so the source of the difference stays clear.

How the calculation works

For the Moon Phase Calculator, the model compares the selected date with a fixed reference new moon on January 6, 2000, cycles by a 29.530588853-day synodic month, and rounds the cycle position to one of eight named phases.

Example

For the Moon Phase Calculator, using September 25, 2026, the current approximation places the lunar age at about 13.65 days and labels the phase Full Moon, with an estimated illuminated fraction near 98.6%. Exact astronomical timing can differ from this mean-cycle model.

How to interpret the result

For the Moon Phase Calculator, read the approximate moon phase as the result of the specific inputs and rule described above. It is most useful when you want to estimate one of eight moon-phase labels from the selected date using a fixed synodic-cycle approximation. For a Moon Phase Calculator comparison, keep the unchanged fields identical and vary only the value you actually want to test; that prevents a second assumption from being mistaken for the effect you are measuring.

Limitations and notes

For the Moon Phase Calculator, the phase is an approximation from a mean synodic cycle. For the Moon Phase Calculator, exact phase instants, rise/set times, local visibility, libration, and high-precision illumination require astronomical ephemerides.