I've done some "tongue-in-cheek" research before about how there's been proof that it does rain more on the weekends around here (well, using a small sample size of about a year in 2001, the two wettest days of the week were Saturday and Sunday.)
But what about full moons?
I decided to check after catching this blog entry by Tom Gwilym, the president of the Eastside Astronomical Society and frequent helper to my astronomy-themed blog topics.
He says you can always tell when it'll be clear in Seattle -- when the full moon is about to come out. See, full moons are the enemy of star gazers as the bright moonlight washes out the sky. So it'd be sadly ironic if the few days the skies clear are the days when the moon ruins any chances to stargaze anyway.
I decided to check his theory over the past few months and found that he might have a point, although I mostly confirmed that Seattle is a lousy place to stargaze anyway no matter the moon since clear skies are hard to come by.
Going back each month to January 2008, I looked at the three days that bookend a full moon (as in, the day before, the day of, and the day after) and did the same with the new moon to compare.
On each day, I recoded the observed cloud cover for the day, with the National Weather Service rates on a scale of 1-10, 1 being clear and 10 being completely overcast. (1-3 is considered a sunny day, 4-7 is a partly cloudy day, and 8-10 is a cloudy day.)
Drum roll please....
The average cloud cover around a full moon was 6.53 -- not the greatest; that's on the cloudy side of partly cloudy. On the other hand, when it was a new moon, the number was 7.80 -- pretty much always cloudy.
On the day Tom wrote the blog (March 14th), the day rated a '5'. That's averaged over the entire day, so it's possible it was cloudy early then clearing at night.
Of the 45 days (15 months), there were 8 clear days around the full moon, while just 2 clear days around the new moon.
Guess that's why most of the big telescopes don't find their way to the Northwest.
(By the way, just in time for the overcast layer from the approaching weekend storm, we're now in a new moon period.)
If you are eager to get out there and stargaze, be sure to check out this great skygazing weather chart.
It's updated each day and is specifically geared toward atmospheric conditions for stargazing.