Moon Phase Calculator

What this generator does

Counts days from a known new moon to the date, reduces that by the synodic month to get the moon's age, and derives the illuminated fraction from the angle through the cycle. The phase name, the waxing flag and both countdowns all follow from that one number.

How to use this tool

1. Enter a date.
2. Read the phase and how much of the disc is lit.
3. Note whether it is waxing or waning.
4. Check the countdown to the next new and full moons.

Understanding the controls

Date

Any date from 1900 to 2200. The phase is given for the date rather than a moment, so it will be a little out for a date where the phase changes during the day.

Common use cases

• Finding the moon phase for a date in the past or future
• Planning stargazing around a dark sky
• Working out when the next full moon falls
• Explaining why the phase drifts against the calendar
• Dating a scene in a story consistently

How this generator works

Days are counted from a reference new moon using a Julian day number computed arithmetically, so no timezone can shift the result. Dividing by the synodic month of 29.530589 days and taking the remainder gives the moon's age. The illuminated fraction follows from the cosine of the angle through the cycle, which is why the moon appears to brighten slowly near new and quickly near quarter. Before display the illumination is recomputed from the age, the waxing flag is checked against the half-way point, both countdowns are re-derived, and a phase labelled new or full is confirmed to be dark or bright accordingly.

Randomness and fairness

Nothing here is random. A date gives exactly one phase — this computes where the moon is in its cycle rather than drawing anything.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

• This uses a mean lunar cycle, so the real moon can be up to about half a day ahead of or behind these figures — the orbit is elliptical and the cycle is not perfectly regular.
• Phases are given for the date rather than a moment, so a date on which the phase changes is approximate.
• No location is taken into account, and the phase is effectively the same worldwide even though the moon's orientation in the sky is not.
• Eclipses, which need the moon's orbital tilt, are outside what a phase calculation covers.

Privacy and your data

The calculation runs in your browser and the date you enter is never transmitted, stored or included in analytics.