Full Moon: How rare is it on Feb. 29?

# Wait. What About a Full Moon on Feb. 29?

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June 13 2014 12:55 PM

# Follow-up: Full Moons, Leap Days, and Friday the 13th

After this morning’s post about a full Moon occurring on Friday the 13th, I got a few comments wondering how much more rare a full Moon on Feb. 29 would be.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

Actually, the math on that isn’t hard, and the number may surprise you. So let’s take a look.

Feb. 29 occurs once every four years, on leap year (if you want the details on that, oh boy do I have ‘em). That means it happens once every 48 months. Every month has a 13th in it, so in that same time period the 13th happens 48 times.

So just to start, a full Moon on any given 13th of the month will happen 48 times more often than on Feb. 29.

But now we want to constrain the 13th, looking only when that date falls on a Friday. Statistically speaking, that should happen one time out of seven (because there are seven days in a week). So out of the 48 times the 13th occurs in four years, 48 / 7 = roughly 7 should be Fridays.

That means Friday the 13th happens about seven times as often as Feb. 29. Since the full Moon can fall on any random date, the full Moon occurring on Feb. 29 should happen about one-seventh as often as one on Friday the 13th.