Tonight, the night sky will be dominated by a "blue moon."
Most people know that doesn't refer to the moon's color.
But, "blue moon" also doesn't mean what most people think it means, either.
The original meaning of a "blue moon" is not, technically, the second full moon in a month.
Instead, it's original meaning, goes back to the number of full moons in a quarter of a year.
Normally, there's one full moon a month, three a quarter.
But, if there are four full moons in a quarter, the third moon was traditionally called a blue moon.
These days, the more common meaning is the second full moon in a month.
Tonight will be the second full moon this month.
The first full moon this month was on Tuesday, August 2nd.
Whatever the meaning, the next blue moon won't happen until July, 2015.