A Celestial Double Dose
Stargazers will have something extra to look forward to Friday night, and may be confused as to why there is a full moon in the sky when we just saw one on July 2nd. Friday, July 31, 2015 will see a blue moon. Strangely, there are two common definitions for a blue moon, one being scientific and the other being colloquial understanding. First, the astronomical term “blue moon” refers to the fourth full moon in an astronomical season (as there are usually three). The second definition overshadowed the popularity of the first after a miscalculation was made by James Hugh Pruett in Sky and Telescope magazine in 1946. This definition (which we will call a Pruett blue moon) labels any second full moon in a month as a blue moon. Friday’s spectacle will be a Pruett blue moon, meaning it will be the second full moon in July. Even though the concept was created from a miscalculation, a Pruett blue moon is still a rare event, only occurring roughly every two to three years. The calculations for a typical blue moon are slightly more regular, occurring in three year intervals.
Friday’s event shouldn’t be confused with when the moon actually looks blue in coloration. This is a significantly rarer event, as the blue coloration is caused by particles in the air that are of sufficient size. Usually, particles big enough to cause a blue colored moon are sent into the air by volcanic eruptions, making seeing a blue colored moon a truly rare sight. The next typical blue moon is predicted to be on May 21, 2016, while the next Pruett blue moon is expected on January 31, 2018.
