Weather Blog

"Once in a blue moon" pretty frequent these days

"Once in a blue moon" pretty frequent these days
File Photo of blue moon courtesy: Vic Winter ( via NASA.
It is extremely rare to see an actual blue moon around the world but it can happen.

Case in point, the residents of Iran, where a blue moon is now almost a nightly occurrence says a severe dust storm so large that it is visible from space is blowing across Iran. Government officials have closed schools, cancelled flights, and warned the elderly and children to stay indoors.

But on the other hand, it is turning the moon blue. (See image here)

The blue tinge is due to that dust storm scattering the light particles to where just the blue survives. With the lingering dust storm, blue moons could be quite regular there for a while.

That flies in the face of the term "once in a Blue moon" which is supposed to be roughly every year or so. The current mainstream definition is when you have two full moons in a month. Since moon phases are about 29 days apart, it's rare to squeeze two in a month. According to, which has more blue moon stats than you can shake a stick at (really, I tried!) there are 15 blue moons scheduled in the next 20 years, with 2011, 2014, and 2017 among the years missing out.

And roughly every 19 years, you get the blue moon double where you get two blue moons in a year. The last time that happened was in 1999.

You can find out when the blue moons are with this handy Blue Moon Calculator by One caveat -- blue moons can be different months depending on your time zone.

For example, if a full moon is on August 1st at 4 a.m. in London, that would be 9 p.m. July 31st in Seattle. So for London, the blue moon is in August, while in Seattle, it's in July.

How we define a blue moon now is actually not how it was originally intended. It was supposed to be the name for a moon when you have four full moons in a three-month period -- the third full moon was then known as the blue moon. An article in Sky & Telescope magazine in 1943 had an error in the original interpretation and gave it the definition we know of today.

The error was discovered by the magazine's editor in 1999, but the cat is way out of the bag now, (and has likely used up eight of its nine lives in the years' since), so the two in a month usage has since trumped the four in a season definition as popular use.

By the way, the next blue moon is in December, 2009, at least for the United States. However, with those quirky time zones, it's actually January, 2010 in India and Vietnam, and both January *and* March (but not December) for Australia, Japan and China.