Moon Phase Calculator

How to use it

1. 1

   Pick a date

   Choose any past or future date to see the moon for that day.

2. 2

   Read the phase

   See the named phase, from new moon to full and back, with its symbol.

3. 3

   See the detail

   Check how much of the disc is lit and how many days into the cycle the moon is.

When it comes in handy

Frequently asked questions

How is the moon phase calculated?

The calculator measures the time since a known new moon and divides it by the average length of a lunar month, just under 29.53 days, to find the position in the cycle. From that it works out the named phase and the percentage of the disc that is lit. The maths runs in your browser using astronomical figures, with no lookup needed.

How accurate is the result?

It uses the mean length of the lunar month, which keeps the result within about a day of the true phase. That is close enough to know whether a night will be dark, bright or somewhere between. The named phase and the illumination percentage match what you would see in the sky.

What do the eight phase names mean?

The cycle runs new moon, waxing crescent, first quarter, waxing gibbous, full moon, then waning gibbous, last quarter and waning crescent before returning to new. Waxing means the lit part is growing, waning means it is shrinking, and the quarters and full and new are the turning points.

Does this work offline and is anything sent to a server?

The calculation runs entirely in your browser, so nothing you type is sent anywhere, and once the page has loaded it keeps working with no connection. There is no sign-up and no limit on how many dates you check.