What is a blue moon?
To understand the celestial event - and the story behind the name - we've prepared a brief Q&A, with a little help from James Applegate, a professor of Astronomy at Columbia University.
(This post was originally published on Aug. 31, 2012, which is why you will see this date throughout the following Q&A.)
BI: What is a blue moon?
JA: The term blue moon refers to the second full moon in a single calendar month. The first full moon was on Aug. 2. The second full moon will occur on Friday, Aug. 31.
BI: I know the phrase "Once in a blue moon" means something very rare. How rare is a blue moon?
JA: Blue moons happen every two to three years. Typically, a single month only has one full moon. But sometimes another gets squeezed in because our calendar months and our lunar months aren't an exact match. The time between full moons (the time it takes for the moon to orbit the Earth and cycle through all its phases) is 29.5 days. Our calendar months, on the other hand, usually have 30 or 31 days in them. This means each calendar year has about 11 more days than a lunar year. "The fact that our months are about a day longer than the time between full moons means that if the 1st or 2nd (for a 31-day month) is a full moon, then we get a second full moon (the blue moon) in the same month," says Applegate. This happens roughly every 30 months, or every two to three years.
BI: Is a blue moon really blue?
JA: No, it will not be blue in color. It looks like a regular full moon.
BI: Why is it called a blue moon if it is not the color blue?
JA: The origin of the term "blue moon" as it refers to the second full moon in a single calendar month is tied to modern folklore. Sky & Telescope has a lengthy article that uncovers the mystery behind the name. Here are the Cliff Notes.
There is typically one full moon a month, which means 12 full moons a year. This also means that each three-month season - winter, spring, fall and summer - has three full moons.
Originally, each moon had a name that represented the season they appeared in (i.e Flower moon in June during the late spring or Harvest moon in October during the early fall). As early as 1932, the Maine Farmers' Almanac used the term "blue moon" to describe the third full moon in a season that had an extra fourth full moon so that the seasonal-appropriate names didn't get out of whack.
In March 1946, astronomer James Hugh Pruett wrote an article called "Once in a blue moon" that appeared in Sky & Telescope. In the article, the author incorrectly used the Farmers' Almanac's definition of blue moon to refer to the second full moon in a single month.
This definition for blue moon gained widespread adoption after Deborah Byrd from EarthSky used Pruett's 1946 definition of a blue moon to describe the second full moon in a month on a radio show in 1980.
The second-full-moon-of-the-month definition for blue moon is now commonly accepted in popular culture.
BI: When is the best time to see the blue moon?
JA: "The full moon is visible in the eastern part of the sky near sunset, high in the southern sky near midnight, and low in the west near dawn," says Applegate.
BI: If I miss this blue moon, when will I be able catch the next one?
JA: There won't be another blue moon for three years. Mark your calendars for July 2015, the next time two full moons will occur in the same month.
BI: Does the Moon ever actually appear blue in color?
JA: Yes, but it's a rare occurrence. The moon can appear blue (or green) after a volcanic eruption that emits particles into the air, which scatter red light. People saw blue moons for years after the 1883 eruption of Krakatoa in Indonesia. The same phenomenon was reported after the eruptions of Mt. St. Helens in 1980 and Mount Pinatubo in 1991.
But again, the blue moon you will see on Friday refers to the second full moon in August.