Skywatchers take note: The last Blue Moon of 2018 is just around the corner. If you miss it, you’ll have to wait to 2020 for the next one.
The upcoming Blue Moon — the name given to the second full moon to occur in a single calendar month — rises on Saturday (March 31). It’ll be the second Blue Moon of the year; the first occurred on Jan. 31, when we experienced the “Super Blue Blood Moon Lunar Eclipse.”
Blue Moons aren’t actually blue, and they don’t look different from any other full moon in the sky. The term, which has been around for hundreds of years, apparently originally signified something that’s absurd, but then shifted over time to refer to exceedingly rare events, Philip Hiscock wrote in a 2012 article for Sky & Telescope. (Interestingly, a Blue Moon previously meant the third full moon in a season that had four of them. This sense of an “extra” full moon morphed into the definition most people recognize today. Language is a slippery and changeable thing!)
But Blue Moons aren’t all that rare, really: On average, they occur about once every 2.7 years. Blue Moons are possible because it takes Earth’s nearest neighbor 29.5 days to circle our planet, but each calendar month (except February) contains 30 or 31 days.
Editor’s note: If you capture an amazing photo of the Blue Moon or any other celestial sight and would like to share it with Space.com for a story or gallery, send images and comments to managing editor Tariq Malik at firstname.lastname@example.org.