Online Tool Store
Moon Phase Calculator
Calculate the moon phase, cycle age, and approximate illumination percentage for any date. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Defaults to today — pick any date to see that day's moon phase.
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- Moon age
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- Cycle length
- 29.53 days
- Illumination
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An approximation based on a fixed synodic month length, not precise astronomical ephemeris data.
How it works
- Pick a date, or leave it on today.
- See the moon phase, its age in the current cycle, and approximate illumination.
The formula
Days since a known new moon (2000-01-06) ÷ 29.53 (synodic month)
Remainder = position in the current cycle (0 = new moon, 14.8 = full moon)
Illumination ≈ (1 − cos(2π × position ÷ 29.53)) ÷ 2
FAQ
How accurate is this?
It uses a fixed average synodic month (29.53 days) measured from a known new moon reference date — accurate to within about a day for most dates, but not a substitute for precise astronomical ephemeris data used in navigation or research.
What is a synodic month?
The time from one new moon to the next — about 29.53 days on average. It differs slightly from the moon's actual orbital period because Earth also moves around the sun during that time.
What does "illumination" mean here?
The approximate percentage of the moon's visible face lit by the sun, from 0% at new moon to 100% at full moon, modeled as a smooth curve across the cycle.
Can I check a past or future date?
Yes — pick any date and the phase, age, and illumination update for that day.
How we compare
| Feature | Online Tool Store | Calendar app moon widgets | Astronomy software |
|---|---|---|---|
| Check any past or future date instantly | ✓ | Shows today only, usually | ✓ |
| No install | ✓ | ✓ | Often a desktop app |
For planning a night photo shoot, a stargazing trip, or just satisfying curiosity about a date, this is a quick check without opening a dedicated app.