You’ve heard the phrase “Once in a blue moon,” but where does it come from? And what’s a blood moon or a super moon? Let’s explore the strangest, most unusual moons that light up the night sky.
A blue moon isn’t really that rare in astronomical terms. By one definition, it happens when there’s a second full moon in a single calendar month. By another, more technical definition, a blue moon is the third full moon in a single season with four full moons total. Some astronomers call the first type a monthly blue moon and the second type a seasonal blue moon.
These full moons aren’t actually any bluer than usual. The first use of the term “blue moon” to mean something extremely rare seems to be in 1821, in a book called Real Life in London by Pierce Egan. The seasonal blue moon dates back to the 1940s when James Hugh Pruett published an article in Sky & Telescope magazine about the phenomenon.
The next monthly blue moon will happen on Halloween of 2020—spooky!
Speaking of spooky, “blood moon” sounds sinister, doesn’t it? The lunar eclipse in January of this year treated us to the overhyped phenomenon of a super blood wolf moon—in other words, an unusually large moon in January that underwent a lunar eclipse.
The term is relatively recent, but you can see where it comes from. During the eclipse, the moon takes on a deep reddish-orange hue. Lunar eclipses are pretty commonplace, with anywhere from 4-7 occurring every year. Calling it a blood moon is trendy right now, but it doesn’t have any real astronomical significance.
Micromoons and Supermoons
The moon doesn’t change size over time. But it can appear larger or smaller depending on its relative position to the Earth. That’s because its orbital path is slightly skewed. When a full moon happens at the closest point in its orbit (called the perigee), it appears larger and brighter than usual. The word “supermoon” has been around for about 40 years, but like “blood moon” it has become more widespread thanks to the internet.
A micromoon is the exact opposite of a supermoon. A full moon at the farthest point in its orbit (or the apogee), it seems dimmer and smaller than usual.
If you’re around in November of 2034, you’ll get to see the largest supermoon in more than 100 years!