What is a Blue Moon? Several different explanations describe this infrequent occurrence.
August 31, 2012 is the second full moon of the month. This phenomenon is known as a “Blue Moon” in current discourse. This is a rather recent formulation of the term.
Sky & Telescope magazine has traced the history of the term in at least two articles and it has even been part of the change of how the term is used.
Here is a summary of what the articles describe:
- The original usage of the term was like the modern statement “When pigs fly.” (That will happen when the moon is blue.)
- Another described an infrequent event related to volcanic eruptions. The dust ejected high into the atmosphere, can give the moon and sun a bluish hue when seen through the dust. While infrequent, blue moons do occur.
- A usage closer to the popular modern concept can be traced to the Maine Farmers’ Almanac that related the term an extra full moon during a season. Each season normally has three full moons. When a season has four, the third one is called “Blue Moon.” Historically, the full moons had season names, such as Harvest Moon, Egg Moon or Lenten Moon. Because those full moons were related to specific events related to the seasons, there came a time when a season had an extra full moon without a name; the third month in that series was named “Blue Moon,” a sort of unnamed full moon for that infrequent occurrence.
- Sky & Telescope also stated that it contributed to the popular notion with articles in 1946 and 1950 that cited the Maine Farmers’ Almanac, but added that a second full moon in a month was a “Blue Moon.” This is the term that has been popularized today.
With that written, I will put here that I will not write about this topic until the next Blue Moon and let the reader determine the usage.
April 20, 2021: Venus is very low in the west-northwest after sunset. The gibbous moon is in Cancer, between Regulus and Pollux. Mars, above the horns of Taurus, approaches the star cluster Messier 35.
April 20, 2021: Bright Jupiter and Saturn are in the southeastern sky before sunrise, gently moving eastward compared to the starry backdrop of Capricornus.
April 19, 2021: The first evening appearance of Venus for this apparition occurs this evening. Look for it low in the west-northwest about 20 minutes after sunset.
April 19, 2021: Venus begins to appear in the west after sunset. The moon lines up with Pollux and Castor, while Mars is above Bull’s horns in the western evening sky.
April 19, 2021: The bright morning planets, Jupiter and Saturn, are in the southeastern sky before sunrise. Capricornus is the starry background for this giant planet duo.