If something happens on a rare occasion, you might have heard someone describe is as a "once in a blue moon" situation. The phrase describes occurrences that don't come around often, making them ...
but soon he had the blue orb of his favourite planet in his sights. Sure enough, up to the right was the silver streak; hurtling toward him. Apollo 11. Still a few thousand miles away, but closing ...
Only when the moon, Earth and the sun are perfectly aligned is the moon 100% full. And sometimes — once in a blue moon — the moon is full twice in a month (or four times in a season ...