Moon Phase Calculator

Find the moon phase for any date

The moon cycles steadily through its phases, and knowing the exact phase for a particular date is useful for both planning and simple curiosity. This calculator finds the moon phase and the illumination percentage for any date you choose, whether in the past or the future, following the known lunar cycle rather than only working for today.

Why the phase follows a cycle

The moon’s phase is determined by the relative positions of the moon, the Earth and the Sun, and it cycles smoothly from new moon to full moon and back again over a period of about 29.5 days as the illuminated portion of the disc that we can see steadily grows and then shrinks. Calculating the phase for a given date, together with the exact percentage of the disc that is lit, tells you whether a particular night will be dark or brightly moonlit, which genuinely matters for activities like stargazing and night photography, as well as for satisfying simple curiosity about the moon on a past or future date. Because the calculation follows the well-understood lunar cycle, it works reliably for any date, not merely the present one.

A calculation, not advice

It runs entirely in your browser, so nothing you enter is uploaded and it works without an account, keeping your figures on your own machine and available on any device immediately.

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