Calculate moon phase

Estimate lunar phase age, illuminated fraction, and the next full and new dates with a fixed synodic-cycle model.

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ToolMoon Phase Calculator
Input
Output
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Preview

How it works

UTC noon is reduced modulo 29.530588 days from a 2000-01-06 18:14 UTC new-moon epoch. The nearest cycle eighth supplies a name, a circular-orbit cosine estimates illumination, and phase offsets estimate the next full and new dates.

  • Coordinates are absent because this is a global low-precision phase estimate rather than an observing ephemeris.
  • Eight names label ranges around phase instants; they do not make illumination jump between eight fixed values.

Worked example

The day after a new moon
Input
											2000-01-07
Output
												Phase: New Moon
Illumination percent: 0.6
Age days: 0.74
Next full moon: 2000-01-21
Next new moon: 2000-02-05

When to use this

Calendar mockups label phases, astronomy lessons visualize a synodic cycle, and photography plans perform a rough check before consulting an ephemeris.

Edge cases

  • The Full Moon label covers the nearest eighth even though astronomical full moon is one instant.
  • A fixed average month accumulates error because real lunations vary and the lunar orbit is not circular.
  • UTC dates do not provide moonrise, moonset, visibility, or the observer’s local calendar date.

References