“Blue Moon” refers to a very infrequent and hard to pinpoint event. This meaning is still commonly used now as in the past. There are a variety of Blue Moon definitions. The most current usage for the term Blue Moon is the second full moon occurring in the same month. Therefore, Blue Moons can only occur at the very end of a calendar month. In old almanacs, that is not where Blue Moons occur. The definitions for a Blue Moon have changed over time.

Four hundred years ago, a Blue Moon was based on the Christian ecclesiastical calendar. Each year had 12 named full moons, three per season. Sometimes a year can have 13. When a fourth full moon fell in one season, the third moon was called the Blue Moon. It helped keep the Christian holidays, like Easter, on track. Blue Moons could only occur from the 20th to the 23rd of May, August, November, or February. These Blue Moons can be found in editions of the Maine Farmer’s Almanac from 1819 to 1962.

There are examples of the moon actually appearing to be blue in color at different times in history. Debris in the atmosphere can make the moon appear different in color. One example is when the volcano Krakatoa erupted in Indonesia in 1883. So much dust and debris went into the atmosphere, sunsets turned green and the moon looked blue for almost two years afterward.

In 1927, the monsoon season began late in India. The extra long dry season before the monsoons blew up enough dust to make the moon appear blue. A huge forest fire in western Canada threw up enough smoke particles to turn the moon blue in northeastern North America in 1951.