What’s the phase of the Moon during next year’s Perseids?

Perseid in Moon-flooded sky
The full Moon's light flooded the ground and sky on August 13, 2011, wiping out the usually dependable Perseid meteor shower. A solitary streak appears over the observatory of S&T senior editor Dennis di Cicco.

You can figure this out in your head! To find the Moon’s phase on
any month and day in the years 2000–2009:

· 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. For example, consider August 12, 2005. 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 full Moon, and 22 last

This remarkable method was devised by Sean Barton, maker of
Lunawheel (March issue, page 62), and it is given with a minor
refinement on his Web site, www.moonstick.com. “I was astounded by its simplicity,” writes Jim Foxworth of Fort Lauderdale, Florida.“
Also, there is a simple way to extend 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.

Now comes the kicker. Foxworth points out that you can easily find
the Moon’s 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 2000–2019 range. If it’s after 2019, subtract a multiple of 19
instead. Then apply the method given above. It is accurate to one or
two days.

— Roger W. Sinnott


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