A Blue Moon, while not exactly blue in color, is the term for the second full moon to occur in a calendar month.

The reason for this anomaly is explained by Columbia University professor of Astrology James Applegate: "The fact that our months are about a day longer than the time between full moons means that if the 1st or 2nd (for a 31-day month) is a full moon, then we get a second full moon (the blue moon) in the same month."

This rarity won't be seen again until January 31, 2018, but sky-gazers around the world took plenty of photos of last night's Blue Moon in case you missed out.