"Once in a Blue Moon" Happened Friday Night
"Blue" means second full moon of the month, in this case
The last chance to see a blue moon until 2015 was on Friday night, but there was no azure hue in the sky.
In fact, "blue moon" refers not to color, but to rarity. Blue moons are defined as either the fourth full moon in a season, or, more recently, as the second full moon in a month. It's the second definition that covers August's blue moon; the month's first full moon was on Aug. 1.
Blue moons happen because our calendar months don't precisely sync with the moon's orbit. It takes the moon 29.5 days to wax and wane from full to new to full again. With the exception of February, months are longer than that, meaning that once in a while the timing works out so there are two full moons in one month.
Friday night's blue moon was on the same day of a private memorial service for Neil Armstrong, the first man to walk on the moon.