You may have used the expression “once in a blue moon” at some point or another, but do you actually know what it means? This phrase refers to something that is uncommon, or in some cases, rare. The term blue moon comes from the Farmers’ Almanac. The Farmers’ Almanac defined each season as having three full moons. If four moons occurred within one season, they referred to it as a blue moon.

Such is the case in August this year, when two full moons appear within the same calendar month. And don’t be fooled; the moon isn’t actually blue! On the first of the month, the initial full moon occurred. Friday marks the second and last chance to see a blue moon until 2015.


The reason a blue moon occurs is because the length of each month does not accurately line up with the moon’s orbit. It takes the moon approximately 29.5 days to go from full, to new and back to full again. Most months, with the exception of February, are longer than 29.5 days. This means when the timing is right, you have the chance to see two full moons in one month!

Ever heard the phrase “twice in a blue moon?” An event even rarer than just one blue moon is two happening within the same calendar year. It has been more than 12 years since two blue moons have occurred. The next time the sky will deliver two blue moons will be in 2018, when they fall within January and March.

Bookmark and Share

777 E. Princeton Street • Orlando, Florida 32803 • Phone: 407.514.2000 • TTY: 407.514.2005 • Toll Free: 888.OSC.4FUN • Email:
  Orlando Science Center is supported by United Arts of Central Florida, host of and the collaborative Campaign for the Arts.
This project is funded in part by Orange County Government through the Arts & Cultural Affairs Program. Privacy Policy • Accessibility