Sky & Telescope Moon Phase Calculator

Whatís the phase of the Moon during next yearís Perseids? You can figure this out in your head! To find the Moonís phase on ANY MONTH and DAY in the years 2000 to 2009:

Add up the digits of the year and multiply their sum by 11. To this add the number of the month and the day. Then subtract multiples of 30 until the result is less than 30. Youíve got the age of the Moon in days. Also there is a simple way to extend the validity to the year 2019. Treat the yearís last two digits as a single number instead of individual digits. Thus the sum for the year 2015 is 2 + 0 + 15 = 17.

Now the kicker. You can easily find the Moonís phase for other years using the Metonic cycle, whereby any Lunar phase recurs on the same date every 19 years. If your target year is before 2000, add 19 repeatedly until you find the equivalent year in the 2000 to 2019 range. If itís after 2019, subtract a multiple of 19 instead. Then apply the method given above. It is accurate to one or two day. 

User Date / / user input

This form displays information on the moon, calculated with the above algorithms. Fill in the desired date and press [Calculate].

Note Moon images provided by Ernie Wright.

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This web page was last updated 01/21/16