How accurate is the moon phase shown for a given date?

The phase is calculated using the average synodic month length (about 29.53 days) rather than precise orbital mechanics, so the phase name and illumination percentage are accurate to within roughly a day of the true phase.

What time zone is used to determine the moon phase for a date?

The calculation treats the entered date as a calendar day and compares it against a fixed reference new moon, so results may shift by up to a day near a phase boundary depending on your local time zone.

Moon Phase Calculator

Find the moon's phase and illumination percentage for any date.

How this is calculated

Phase fraction = (days since reference new moon mod 29.53) ÷ 29.53

Assumptions used in this calculation

Algorithmic approximation: Uses the average synodic month length rather than precise orbital mechanics, accurate to within about a day.

About this calculator

The moon phase calculator finds the moon's phase name and illumination percentage for any date, using a reference new moon and the average synodic month length.

Worked example

25 January 2024

Result: Near-full moon, ~98% illuminated

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Frequently asked questions

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Convert a Gregorian date to the Islamic (Hijri) calendar date.

Moon Phase Calculator

About this calculator

Frequently asked questions

How accurate is the moon phase shown for a given date?

What time zone is used to determine the moon phase for a date?

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