A blue moon is a semi-rare occurrence. It’s not as cool as a blood moon (lunar eclipse), but it’s a good enough reason to look up at the stars tomorrow night.

The blue moon is defined a couple of different ways. The most popular definition today is a second moon within one calendar month. The last full moon came on July 1 and the next one happens tomorrow, July 31. Simple, right? Except that wasn’t always the case. This definition came about from a mistake in a Sky & Telescope article in 1946.

A Sky & Telescope article in 2004 explains what happened:

S&T writer James Hugh Pruett (1886–1955) made an incorrect assumption in 1946 about how the term had been used in the Maine Farmers’ Almanac.

So, what’s real definition of a blue moon? It’s still the second moon in a calendar month. There’s no turning back from that definition now. But technically, a blue moon is the third full moon in a three-month season having four full moons.

If we use the Maine Farmers’ Almanac definition of a blue moon, tomorrow’s moon won’t make the cut. The next blue moon would not happen until May 21, 2016.

The next time two full moons will occur during a single month will be in January 2018.

A real blue moon

Tomorrow’s blue moon isn’t actually blue. But, there have been several instances of people witnessing blue moons. These events are even more rare and need a bit of help. The moon can appear blue when smoke or dust particles are in the atmosphere.

In 1950 and 1951, residents in Sweden and Canada witnessed a blue moon after raging wildfires sent plumes of smoke into the atmosphere. Volcanic eruptions have also been known to make the moon appear blue.

Particles from the wildfires, volcanic eruptions or even dust have to be just the right size to scatter red and yellow light.

A similar effect was captured by NASA’s Curiosity Mars rover earlier this year.

blue sun on mars

“The colors come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently,” said Mark Lemmon, the Curiosity science-team member who planned the observations.

“When the blue light scatters off the dust, it stays closer to the direction of the sun than light of other colors does. The rest of the sky is yellow to orange, as yellow and red light scatter all over the sky instead of being absorbed or staying close to the sun.”

Tomorrow’s moon won’t look any different than your average full moon. Grab a chair and a pair of binoculars and head outside anyways. It’s never too late to become an amateur astronomer.

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