Usually, “once in a blue moon” means something rare or unusual. This week, it means Tuesday. But tonight’s blue moon may not be the kind of blue moon you’re familiar with.

Tuesday’s full moon will not change color, nor is it the second full moon in a month. It is, however, a “seasonal blue moon,” the third of four full moons we’re expected to get this summer. This moon fits the long-standing definition of a “blue moon” that has only recently been usurped by the popular one.

In 2006, Sky and Telescope writers Donald Olson, Richard Fienberg and Roger Sinnott tracked down the origin of the definition of "blue moon." The traditional definition of a blue moon is based on guidelines in the Maine Farmers’ Almanac, which relies on the tropical year -- from one winter solstice to the next -- rather than the traditional calendar year. Usually a tropical year will feature 12 full moons, with three in each season, but occasionally, a tropical year will contain 13 full moons, and one season will have an extra.

And while we mark the seasons today by tracking the sun’s apparent position in the sky, the Maine almanac uses an older method called the “dynamical mean Sun” that produces much more regular seasons. The almanac also follows Christian ecclesiastical rules that fix the spring equinox on March 21, irrespective of where the sun is, and which also prescribe how various full moons throughout the season relate to the Easter season. So the names of full moons near the spring equinox are tightly constrained.

“Why is the third full Moon identified as the extra one in a season with four?" Olson, Fienberg and Sinnott wrote. "Because only then will the names of the other full Moons, such as the Moon Before Yule and the Moon After Yule, fall at the proper times relative to the solstices and equinoxes,” 

So where did the idea of a blue moon being the second full moon in a month come from? The error, they found, originated in a 1946 article in their own magazine by an amateur astronomer named James Pruett. In 1980, the popular science radio program StarDate cited Pruett’s article, and from there the erroneous definition spread through the popular imagination.

It’s probably impossible to remove the more recent definition from the public’s mind. But Sky and Telescope is looking on the bright side of this little lunar snafu.

“Rather than argue over whether to celebrate the dawn of the new millennium on January 1 in 2000 or 2001, those with the sunniest outlooks will celebrate twice,” Olson, Fienberg and Sinnott wrote. “Why not treat Blue Moons the same way, marking both the second full Moon in a calendar month and the third full Moon in a season with four?”

While we won’t be seeing an azure-tinged moon this week, there are some circumstances that can give rise to a truly blue moon. Volcano eruptions or forest fires that belch ash high into the sky can give the appearance of a blue moon soon after the event. That’s because some of the tiny particles of ash or dust are 1 micron wide, the same size as the wavelength of red light. When the moonlight passes through the dust-choked atmosphere, the red light scatters, but the shorter wavelengths of blue light can pass through.

Actually, what might be more likely is that tonight’s full moon will take on a rosy hue. When full moons hang lower in the sky, they redden for the same reason that sunsets do: Earth’s atmosphere is full of very tiny particles, even smaller than the ash and dust from volcanoes. Just as the larger particles scatter red light, these tiny aerosols scatter blue light and let the red pass through.

“For this reason, red Blue Moons are far more common than blue Blue Moons,” NASA scientist Tony Phillips wrote last year.