What Is a “Blue Moon”?

Tonight marks a fairly rare astronomical occurrence: a blue moon. What exactly is a blue moon? Well, according to NASA, it is defined as any time there is a second full moon that happens in the same calendar month. Most typical years have 12 full moons. In 2015, we have 13 and therefore, a blue moon.

Image Credit: NASA

The last time the U.S. experienced this phenomenon was back in August of 2012. We won’t be treated to another blue moon until January of 2018. So while this doesn’t happen often, it’s not as rare as say, Halley’s Comet.

The term “blue” moon has nothing to do with color, so don’t expect to see a different hued moon when you look up to the sky tonight. However, a true blue colored moon is possible. Usually, there has to be something (ash, ice, dust, etc.) in the atmosphere larger than the wavelength of red light (0.7 micron) to drastically change the appearance of the moon. Those tiny particles refract, or bend, the moon’s light as it is traveling towards Earth. When the light finally reaches our eyes, it appears blue. The same principle applies to the brilliant red colors that are seen at sunset when smoke from wildfires enters the atmosphere. At sunrise or sunset, the light from the sun has a longer distance to travel to our eyes and it also must go through smoke, which has a wavelength smaller than that of red light, so the shorter wavelengths (blues and greens) get “filtered” out, which leaves the bright orange and red hues to create a magical scene in the sky.

A moon that appears blue will most often require a volcanic eruption. One of the more popular instances of blue moon sightings happened in 1883 after Krakatoa, an Indonesian volcano, exploded sending tons of ash high into the Earth’s atmosphere. These volcanic-caused blue moons also occurred after Mt. St. Helen’s erupted in 1980. Since ash can stay in the atmosphere for months, sometimes years, these rare colored moons can last for quite some time.

Image Credit: NOAA and NSF

Another less common way for the moon to be viewed as blue is if wildfires are burning close to your home. It is pretty much the same cause as volcanoes: ash from the fires enters the atmosphere which causes the bending of light and, therefore, a moon that appears blue.

Image Credit: NOAA

Tonight’s moon will likely look grayish or white at your house, but if you are lucky enough to see a true blue moon, be sure to snap a picture! For more information, be sure to check out http://science.nasa.gov/science-news/science-at-nasa/2015/27jul_bluemoon/.

-Meteorologist Ashley O’Connor