🌙 Moon Phase Calculator

About the Moon Phase Calculator

This tool computes the phase of the moon for any date using a standard astronomical algorithm based on synodic months. A lunar cycle — from one new moon to the next — lasts about 29.53 days. The calculator measures how many days have passed since a known new moon reference, then maps that onto the lunar cycle to find the current phase, illumination (the fraction of the moon's visible disc lit by the sun), and moon age.

The eight phases of the moon

• New Moon — 0% illumination, the moon is between Earth and the sun.
• Waxing Crescent — growing sliver of light on the right (Northern Hemisphere).
• First Quarter — 50% lit, right half visible.
• Waxing Gibbous — more than half lit, still growing.
• Full Moon — 100% illumination, Earth is between the sun and moon.
• Waning Gibbous — shrinking from full, light on the left.
• Last Quarter — 50% lit, left half visible.
• Waning Crescent — shrinking sliver before the next new moon.

How to use it

1. Choose the date you want to inspect.
2. Press Calculate (or change the date — it recalculates automatically).
3. Use Prev day / Next day to step through a lunation and watch the phase change.

Frequently asked questions

How accurate is this? The result is typically within a few minutes of full astronomical ephemerides for the phase angle and illumination. Small deviations come from the moon's elliptical orbit, which this simplified model averages out.

Why does the illumination not match what I see? The percentage is the fraction of the moon's disc lit by the sun, not the brightness you perceive. Atmospheric conditions, the moon's altitude, and your hemisphere also affect the view.

Does the phase look reversed in the Southern Hemisphere? Yes. The moon's illuminated side grows from the left rather than the right south of the equator. The phase name stays the same, but the visual disc is mirrored.

Does this tool upload anything? No. All math runs in your browser.