Moon Phase Calculator
Find the moon's phase and illumination percentage for any date.
Assumptions used in this calculation
- Algorithmic approximation: Uses the average synodic month length rather than precise orbital mechanics, accurate to within about a day.
About this calculator
Knowing the moon's phase on a specific date is useful for planning stargazing, photography, fishing or hunting trips (many anglers and hunters track moon phase for activity patterns), or just for curiosity about what the sky looked like on a particular night, and it isn't something you can figure out from a regular calendar. This calculator finds the moon's phase name, from new moon through full moon and back, and its illumination percentage for any date, by measuring elapsed days since a known reference new moon and taking that count modulo the average synodic month length of about 29.53 days. Enter any date, past or future, and get an instant phase name and illumination reading without needing an astronomical almanac or planetarium app. Because it uses the average lunar cycle length rather than full orbital mechanics, it's a lightweight approximation well suited to everyday planning rather than precision astronomy.
Worked example
25 January 2024
Result: Near-full moon, ~98% illuminated
Was this helpful?