A decades old mistake became a colourful phrase.
Whenever there’s a second full Moon in a month, the phrase “Blue Moon” gets a real workout in the popular press and on-line. Interestingly, that definition of a Blue Moon is of surprisingly recent vintage. What’s more, it’s rooted in a mistake.
The short version of the story is the definition originated in an article appearing in the March 1946 issue of Sky&Telescope magazine. That article’s author misunderstood how the Maine Farmer’s Almanac used the term. The Farmer’s Almanac described the third full Moon in a season that had four, as a Blue Moon. But their rule was so convoluted that the S&T article simply got it wrong. The story would have likely ended there had Deborah Byrd not found the S&T article while doing research for a 1980 broadcast of her radio program Star Date. That show seemed to have been the spark that lit the fuse. Since then, the “2nd full Moon in a month” definition for Blue Moon has only gained popularity and, rightly or wrongly, is now the most widely accepted. For the full story, you can go right to the source and check out this article from S&T.
It’s ironic that a Blue Moon should attract so much attention since the full Moon is arguably the least rewarding time to view the lunar surface in binoculars or with a telescope. The reason observers give the full Moon a pass is because there’s no shadow contrast, and as a result, most lunar features appear flat and lifeless. Even big, prominent craters can be a tremendous challenge to find under these conditions. That said, the full Moon is an excellent time to view the feathery rays of craters like Tycho and Copernicus as well as subtle tonal gradations in the lunar Maria. This article will tell you more about what you can see when the Moon is full, blue or not.