No doubt you've all heard the saying "Once in a blue moon." The astronomical definition of a blue moon has nothing to do with the moon having a tinge of the color blue. It all has to do with mathematical odds. The synodic period of the moon, an astronomical term, is 29.5 days. That's the time it takes the moon to go through all of its phase shape changes. In other words, it’s the time interval between full moons.

Since the average length of a calendar month is a little over 30 days, it's bound to happen that we'll have two full moons in a month every now and then. On average, there's a blue moon about every two and a half years.

Having a blue moon on Halloween is much rarer. In the past 100 years, it's only happened five other times since 1900. The last time it happened in Rochester was in 2001. It seems everything else has happened in this crazy year of 2020, so why not have a blue moon on Halloween?

No one really knows exactly when the term "blue moon" originated. In literature, it first showed up around 1600 A.D. in England during the time of William Shakespeare. It wasn't defined as it is today as the second full moon in a calendar month, but rather, as a visibly appearing blue moon, with a little imagination and possibly some added paranoia. Many believed it was a bad omen of global calamities in the near future.

Even back then, the Y2K and 2012 Mayan end-of-the-world buffs must have been in business. A blue moon's real or imagined appearance also interfered with the scheduling of church festivals and feast days.

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A blue moon was also seen as a symbol of sadness and loneliness. Even music of the past century reflected this in songs like "Blue Moon," written in 1934 by Richard Rodgers and Lorenz Hart, and recorded by tons of artists.

My favorite version of the song was recorded in 1961 by the Marcels, a doo-wop band that really added some kick to the old standby.

The blue moon joins other celestial Halloween characters roaming the night sky next Saturday evening. Halloween will hang on in the night skies all this week and over the next few weeks.

There's no great pumpkin in the night sky, but this year, we do have a very bright planet with a distinct orange-red glow. That's Mars, shining brightly in the eastern sky as darkness sets in and the spooks start lurking about. Mars is the closest and brightest it's been in over two years, and it won't be this bright again until 2035. On Halloween, Mars will only be about 42 million miles away.

There are some sinister shiners on the rise in the northeast sky. It's the famous, or some would say infamous, Pleiades Star Cluster, also known as the ";Seven Little Sisters." This time of year it's also referred to as the "Halloween Cluster." That's because it rises in the northeast and climbs high in the sky around the midnight hour. You can't miss it, as it's easily visible to the naked eye, resembling a miniature Big Dipper.

Astronomically, the Pleiades is actually a group of over 100 young stars, more than 410 light-years away, that were born together out of a vast cloud of hydrogen gas about a hundred million years ago. By the way, if you're new to this column, just 1 light-year equals nearly 6 trillion miles!

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications. Send questions to mikewlynch@comcast.net.

The Rochester Astronomy Club welcomes new members and puts on public star parties. Their website is rochesterskies.org.