On Friday, much of the world will have the opportunity to observe a Blue Moon: A somewhat rare occurrence that doesn't have anything to do with the moon's color.
During most years, the Earth experiences 12 full moons, one in each month. But some years, such as 2015, have 13 full moons, and one of those "extra" lunar displays gets the label of Blue Moon.
The lunar or synodic month (full moon to full moon) averages 29.530589 days, which is shorter than every calendar month in the year except for February. Those extra one-half or one-and-one-half days accumulate over the year, causing some years to have 13 full moons rather than 12. [Video: What's a Blue Moon, Is It REALLY Blue?]