Moon phase calculator
Pick a date to see whether the Moon is a crescent, full or at a quarter, and how many days have passed since new moon.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Moon age = (Julian day − 2451550.09766) mod 29.530588861 days (Meeus mean synodic month). Illumination = (1 − cos(2π × age ÷ 29.53)) ÷ 2.
Example
Inputs: Date = 2024-04-23. Result: Moon phase: Full moon; Moon age since new moon: 14.7 days; Illuminated share: 100%.
Limits and caveats
- Uses the Moon's mean motion, so real new and full moons can differ by up to about 14 hours, sometimes a day on the calendar.
- It does not predict crescent visibility or the start of a Hijri month, which depend on sighting and location.
Questions about this tool
How long is a lunar month?
On average 29.53 days from new moon to new moon, which is why Hijri months have 29 or 30 days.
What is the difference between new moon and crescent?
New moon is when the Moon lines up with the Sun and cannot be seen; the crescent is the first thin sliver a day or more later.
Does this tool set the start of Ramadan?
No. The month start depends on crescent sighting or each country's official visibility rules.