The earliest reference to the concept of a Blue Moon appeared in 1528, where it simply meant something absurd or impossible. Later, because of a difference between the physical world and the Catholic church’s calendar, an extra full moon occasionally appeared at the “wrong” time. They referred to this as a “betrayer” moon, and in Middle English, the word for “betray” sounds similar to “blue.” By the 19th century, a Blue Moon was associated to the seasons. Each season—summer, fall, winter, and spring—generally lasts three months, meaning that there are usually three full moons during a season. Because a solar year does not coincide with the lunar year, occasionally a season will have four full moons. When this occurs the third full moon of the season is called a Blue Moon. In 1946, Sky and Telescope magazine ran an article that misinterpreted the seasonal understanding, and stated that in any month of the calendar year that has two full moons, the second full moon was called a “Blue Moon.” This error caught on and is now the most popular interpretation for the source of the expression. Metaphorically, “once in a blue moon” means something that rarely happens.