Science
Once in a blue moon: rare phenomenon expected in Saturday night sky – The Guardian
Blue moons – the second full moon in a month – occur only every few years, and the name is misleading

On Saturday night, Democrats yearning for a blue wave on election day may choose to look to the skies for an omen: a blue moon.
Blue moons, typically defined as the second full moon in one month, are rare, arriving every two to three years. According to Earthsky.org, the last was on 31 March 2018.
Confusingly, an older definition of blue moon refers to the third of four full moons in a season which, because seasons usually last three months, is one more full moon than expected. The next such seasonal…
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