Iowans may notice a big, bright full moon in the sky during their New Year’s Eve partying, but it’s much more than that. Tonight’s full moon will be the second full moon this month. The first was on December 2nd.

That makes tonight’s full moon a blue moon. Amateur astronomer Lynn Zeleski says blue moons are rare. Zeleski says, “The lunar cycle is like 29-and-a-half days so with the calendar that sometimes has 30 days and sometimes 31, it occurs a little more often, about every two and a half years.”

But tonight’s blue moon is also on New Year’s Eve, a much more rare occurrence. She says, “They happen only about every 19 years so they’re going to have to wait until 2028 for the next one.” Zeleski says a blue moon is all in the timing.

Early, like winter, spring, summer, fall and then we have our mid-full moon and our late one,” she says, “so the blue moon throws our calendar off so we just call it a blue moon.” The first written reference to a blue moon dates back to 1883, when a volcano named Krakatoa exploded in Indonesia.

The moon, seen through the ashes the eruption, had a blue hue. The phrase blue moon was first used to refer to the fourth full moon in a three-month season in the Maine Farmer’s Almanac in 1932.