The Blue Moon (And the Love of Hyperbole)

Hey! There is a Blue Moon this Friday, July 31! But wait, what does that mean? Perhaps you have seen images like this one being passed around social media: 

So is the Moon really going to look blue on the night of the Blue Moon? No. The Moon will look the same as ever, gracefully reflecting sunlight back to the Earth. 

A Blue Moon is simply a traditional term that refers to an "extra" full moon, either when there are 13 full moons in a year, or when there are two full moons in one month. How does this happen? It all has to do with our calendar and the lunar cycle. 

Can you see the bunny rabbit in the full moon? Image Credit: Wikipedia Commons

The moon takes 29.5 days to go from full moon to full moon on average. The first civilizations based time on the lunar cycle - from full moon to full moon was one unit of time. Our word month comes from the word moon. Gradually, people shifted to telling time by the Sun. The Earth takes just over 365 days to orbit the Sun. Because the lunar cycle of 29.5 days does not divide evenly into 365 days, the Romans added days to each month to to equal 365 days in a year. Our modern calendar is still based on these original calendars (with the addition of leap years and leap seconds), and so still tied to the cycle of moon phases. Because each calendar year contains roughly 11 more days than the number of days in 12 lunar cycles, our months are a little bit longer than a lunar cycle. The extra days accumulate, so every two or three years we see 13 full moons in a year, and two full moons in the same month. 

So if the Moon will not look blue on a Blue Moon, why do people love to share images like the one above? It is a lot easier and a bit more fun to imagine a blue-colored moon than to take the time to think about lunar cycles and the number of days in a month. It is the same reason news media talks about the "super moon" when the Moon is just slightly closer to the Earth than average, or the blood moon during a lunar eclipse. It gets attention, it sounds exotic and slightly dangerous. The slower, more accurate message often gets smothered by the shouting hyperbolic headlines.

Apollo 15 astronaut Jim Irwin is seen with the Lunar Roving Vehicle, with Mount Hadley in the background. July 31, 1971. Image Credit: NASA

While the Moon will not look blue, it will look beautiful on Friday. When you gaze up at it, perhaps you will be more impressed if you remember that it is the only world besides Earth that humans have walked on! It is currently being explored by the Lunar Reconnaissance Orbiter, which take high resolution images like the one below:

Tycho Crater (6,600 feet from the crater floor) on the Moon. Image taken by LRO in 2014.

Will we ever go back to the Moon? NASA does not have current plans for manned missions to the Moon. They are focusing on an asteroid capture mission, or sending humans to Mars, but there are no hard deadlines or money dedicated for those missions. Google is trying to spur innovation to get back to the Moon with its Lunar X Prize. You can learn all about it in our new fulldome planetarium show, Back to the Moon for Good. 

Enjoy the beautiful bonus full moon on Friday! Remember, you can always learn more about the Moon, planets, stars and science in Peoria's Dome Planetarium. 

P.S.  You are all invited to my free lecture, this Thursday, about my Astronomy in Chile trip!