There's a reason the phrase "once in a blue moon" means something rare, but what about two blue moons happening in the same year.

Is twice in a blue moon even a thing?

While a blue moon — the second of two full moons occurring in the same month — happens roughly every 2½ years, the phenomenon of two blue moons in a calendar year appears only a handful of times a century.

It's happening in 2018.

Full moons on Jan. 1 and 31 will be followed by full moons on March 1 and March 31. The last time two blue moons happened in the same year was 1999.

"It's because every 19 years the sun and moon match up again," said Bob Bonadurer, director of the Milwaukee Public Museum's Daniel M. Soref National Geographic Dome Theater & Planetarium.

"It's not going to turn blue. That's just an expression that came up in the 1940s by a writer in Sky and Telescope magazine and it kind of stuck," Bonadurer said.

There's an added bonus for stargazers, astronomy nerds and telescope aficionados — January's blue moon on the 31st will feature a total lunar eclipse. Plus, both the January full moons will be super moons, appearing about 7% bigger than normal.

And because the heavenly orb will turn a reddish/orange color, which makes it look like the moon is rusting, a total lunar eclipse is often called a blood moon. In Bonadurer's monthly newsletter to Milwaukee Public Museum planetarium visitors and astronomy fans, he's calling the Jan. 31 event a "super, blue, blood moon."

The phenomenon of the moon turning into a blood orange is caused by red light bending as it passes through the Earth's atmosphere and reflecting on the moon's surface.

Full moons materialize once every 29.5 days or so, which makes it unusual for two to fit in one month.

New and full moons realign on or near the same dates once every 19 years. There are 235 full moons during the 228 months of the 19-year period which means at least seven of those months will have two full moons.

When a February within that 19-year period has no full moon, which happens in 2018, that's when two blue moons can happen in the same calendar year. But not always. It happened in 1999 and 1961 and won't occur again until 2037 and 2094.

Each full moon features a name, such as Wolf Moon for January, Snow Moon for February and Harvest Moon for September. Blue Moon was used in years with 13 full moons. That led to an article in Sky and Telescope Magazine in the 1940s which dubbed the second full moon in a month as a blue moon.

For the Jan. 31 total eclipse of the blue moon, viewers will be able to see it early in the morning though in Wisconsin it will be low in the western sky. In Milwaukee the eclipse will start at 5:48 a.m. and totality will begin at 6:51 a.m. and last until the moon sets at 7:06 a.m., said Bonadurer.

The best viewing will be from a tall structure because the moon will be so low on the horizon. But there's no need to move into the countryside away from light pollution.

"The best thing about this, with lunar eclipses it doesn't matter where you are because the moon is so bright and visible," Bonadurer said.

If skies are clear, Bonadurer will bring the museum's telescopes to the roof of the Urban Ecology Center at Riverside Park, 1500 E. Park Place, from 5:30 a.m. to 7 a.m. on Jan. 31 for anyone who wants to see the blue moon total eclipse.

Read or Share this story: