Today is a blue moon: What does that really mean, anyway?

Today, people the world over are learning the origins of the expression “once in a blue moon.”

That’s because the second full moon in a month, known as a “blue moon,” will be appearing in the Americas this evening (a little after 8pm ET, although technically it already revealed itself at 6:43am ET).

Normally there is one full moon per month (the first full moon this month was on July 2), but because the lunar calendar and human calendar aren’t exactly synced, about every three years or so the moon’s calendar yields two full moons in the same calendar month. (The calendar year has 365.24 days; while the lunar month has 29.53 days, yielding 11 leftover days after the moon’s twelfth cycle.) The last time this happened in the Americas, for instance, was in Aug. 2012. The next time it’s expected is in 2018.

True to the expression, this makes it a rare occurrence, as does the fact that today’s moon will be a notably large and bright ‘super moon.’ That happens when the moon reaches its closest distance from Earth.

What the moon won’t be is blue. So why do we call it that?

The origins of the term date back to the 16th century, historian Phillip Hiscock told the Christian Science Monitor. In the writings of Cardinal Thomas Wolsey, advisor to Henry VIII, he slighted his intellectual peers by saying they “would have you believe the moon is blue.”

By the early 1800s, historians note that, judging by its usage in writings from the period, the term shifted meaning from an absurdity to something that happened infrequently (i.e. “I haven’t seen you in a blue moon”). Later in the US, the term cropped up in the Maine Farmer’s Almanac, which defined a “blue moon” as the third full moon in a three-month season that is meant to have four full moons.

It may be cold comfort to those disappointed at today’s white “blue moon,” but there is such a thing as a blue-hued moon. It just requires volcanoes to spew ash into the sky. Here’s a NASA video explaining how a true blue moon comes about:

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