 Due to analysis of an inadequate number of old issues of the "Farmer's Almanac" the author of an article in the March 1946 issue of "Sky & Telescope" magazine falsely concluded that a "blue moon" had been used in the sense of "the second full moon in any month which contains two full moons".This mistaken rule has been admitted to be "erroneous" in a 1999 "Sky & Telescope" article, which gave the corrected rule based on order in seasons.According to either definition, "blue moons" occur with the average frequency of intercalary months, seven times in 19 years, the "Farmers' Almanac" system of full moon names effectively defining a lunisolar calendar.