The term "blue moon" has become an idiom meaning "rare." So naturally, a double blue moon, which will grace the sky Saturday night, is even rarer.
As explained by NASA, the common definition of a blue moon is a second full moon in a single month. This happens about every three years. However, some calendar years get two blue moons. This is much less common, occurring just a handful of times each century. And 2018 is one of those lucky times.
The previous double blue moon took place in 1999; the next will be in 2037, Travel and Leisure reports.
If it looks like any other full moon, there's a reason for that: It is. Though the moon can appear blue under certain conditions involving dusty skies or volcanic eruptions, this phenomenon has no correlation to the above definition or the moon's phases.
Still, if you want a look at the first double blue moon of the 21st century, forecasts indicate clear skies in Sacramento and much of Northern California, making for great viewing conditions.
The moon has taken on plenty of different names, colors and appearances over the past several months—since last August, we've had an "awesome" solar eclipse, a "supermoon" a "super blue blood moon" that was enhanced by a lunar eclipse.