What’s the phase of the Moon during the Perseids? Use this simple calculation.

You can figure this out in your head! To find the Moon’s phase on
any month and day after the year 2019:

• Subtract a multiple of 19 from the current year until you have a number between 2000 and 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.
• Subtract multiples of 30 until the result is less than 30.

You’ve got the age of the Moon in days, accurate to one or two days.

For example, consider the date August 12, 2024. Subtract 19 from the year to get 2005. Then, the digits of the year add up to 7, and when multiplied by 11 the result is 77. Adding 8 for the month and 12 for the day makes 97, and subtracting 30 three times gives 7. So the Moon is 7 days past new, or first-quarter phase, at the peak of next year’s Perseid meteor shower. (An age of 15 would have indicated a full Moon, and 22 a last-quarter Moon.)

This remarkable method was devised by Sean Barton, maker of Lunawheel. “I was astounded by its simplicity,” writes Jim Foxworth of Fort Lauderdale, Florida.

The original equation Barton wrote, which included all the above steps except for the first one, was good for the year range 2000 to 2009. But a simple addition extended its validity to the year 2019: Treat the year’s last two digits as a single number instead of individual digits.” Thus, the sum for the year 2015 is 2 + 0 + 15 = 17.

The first step of the calculation comes in for years after 2019. Foxworth pointed out that you can easily find the Moon’s phase for years outside the 2000-2019 range 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 2000–2019 range. If your target year is after 2019, subtract a multiple of 19 instead.