English Level: Advanced

“Once in a blue moon” is an English expression used to describe something that happens very rarely, but have you ever thought of its etymology? A moon may visually appear blue for a multitude of factors, including lighting, smoke, and/or dust particles that can superficially alter its hue. Outward appearance, or our visual perception of color, is not the reason why the phrase exists. The term “blue moon” is used to describe the second full moon that occurs in one month. As the full lunar cycle (29.53 days) is less than the average calendar month (30.4375 days), on an exceedingly rare occasion, a blue moon occurs, approximately every 2.72 years (2 years and 8 – 9 months) mathematically speaking. Using hertz as the unit of frequency, its reciprocal reveals the period of recurrence in seconds. The calculation of this is shown below.

Another way to calculate this is by using synodic months and the number of days in a year. Given that a synodic month is 29.53 days and there are 365.25 days a year, 365.25/29.53 = 12.369 synodic months per year. This means there are 0.369 extra synodic months in each 12-month calendar year. The results are the same using this method: 1/0.369 = 2.72 years.

 

While a blue moon occurs fairly regularly, astronomically speaking, a Halloween blue moon is even rarer (and a full moon on Halloween is ALWAYS a blue moon, given that Halloween is always on October 31st). The last Halloween blue moon that was visible in all U.S. times zones occurred in 1944 or 76 years ago! 

 

The COVID-19 pandemic may have canceled your Halloween plans to go out and party or go trick-or-treating, but don’t let it deter you from moon gazing this Saturday. Whether you’re an astronomy enthusiast or a moon gazing neophyte, this is undoubtedly a rare occasion. If you miss your chance this year, the next Halloween blue moon is forecasted to be in 2039 (you’ll have to wait 19 years!), so don’t miss your chance to see the Halloween blue moon that comes around,  well, only once in a blue moon!