The phrase* “once in a blue moon”* means something that happens very rarely. But there are different ideas about what a blue moon is. For a long time folklore* and farmer’s almanacs* said that blue moons occur in years with 13 full moons. Then, you find the season of that year with four full moons. Finally, you find the month that has two full moons. The second full moon in that month is the “blue moon.” Very complicated!*Almanacs are books that have been around for hundreds of years. Each year a new one is printed, and has information like the time of the tides*, the rising and setting of the sun, and even predictions* about what the weather will be like all year long! Benjamin Franklin started publishing Poor Richard’s Almanac in 1732. Farmers used almanacs to help them decide when to plant crops. Many believed folklore like “corn planted in the dark of the moon will have large ears low on the stalk*,” so knowing facts about the moon was important.But in 1946 Sky & Telescope magazine printed an article that said a blue moon is just whenever there is a second full moon in a month. The writer of the article had read the more complicated explanation in the Maine Farmer’s Almanac, which didn’t explain it correctly in his article. So a lot of people now think that his explanation is right. It’s much simpler, so maybe it’s a much better idea. Whatever meaning of “blue moon” you like, it is just a made-up rule and a fun thing to think and talk about. So, pick your own definition and catch a “blue moon,”they only happen “once in a blue moon.”(SD-Agencies)(Difficulty: junior)