Blue Moon: the next blue moons and what the name means

Two definitions of a Blue Moon

Most years have twelve full moons, but the cycle of phases is about 29.5 days. Twelve cycles fall about 11 days short of a calendar year. The shortfall builds up, and every few years a year has thirteen full moons. “Blue Moon” is the name for the extra one. The two definitions disagree about which full moon counts as extra.

The calendar (monthly) rule. A Blue Moon is the second full moon in a single calendar month. A month can only hold two if its first full moon comes within the first day or two. That rule means February can never have one. This is the definition most news stories use, and NASA notes it has been in use since the 1940s.

The seasonal rule. A Blue Moon is the third full moon in an astronomical season, such as the stretch from the June solstice to the September equinox, that happens to contain four full moons instead of the usual three. NASA’s guides call this the older definition. Sky & Telescope found that, by the almanac’s reckoning, seasonal Blue Moons fall only in February, May, August or November, about a month before an equinox or solstice.

The two rules rarely pick the same full moon. In a year with thirteen full moons, the calendar rule usually flags one month and the seasonal rule a different one. The table above marks each kind separately.

How the seasonal rule began

The most detailed history comes from Sky & Telescope, in an article by Donald Olson, Richard Tresch Fienberg and Roger Sinnott (posted online in 2006). Folklorist Philip Hiscock had traced the calendar meaning of “Blue Moon” to the Maine Farmers’ Almanac for 1937. The Sky & Telescope authors then gathered more than 40 editions of the almanac, from 1819 to 1962. Those editions list more than a dozen Blue Moons, and not one is the second full moon in a month. Under editor Henry Porter Trefethen, the almanac listed Blue Moons consistently from 1932 to 1957.

The almanac’s system had some twists. Its year ran from one winter solstice (“Yule”) to the next, and most such years hold twelve full moons, three per season, each with a seasonal name. It set the seasons with a “mean” Sun, an imaginary Sun moving at a steady rate that gives four seasons of equal length. Its spring names followed church rules for Lent and Easter. When a season had four full moons, the almanac called the third one blue, because only then did names such as the Moon Before Yule and the Moon After Yule stay in their proper places. Modern calculators usually use the actual equinox and solstice instants, so they can occasionally disagree with the almanac’s old mean-Sun dates.

How a misreading became the modern meaning

The calendar-month rule began as a mistake, as Sky & Telescope’s history explains. In March 1946, the amateur astronomer James Hugh Pruett wrote an article in Sky & Telescope about the Maine almanac. He took it to mean that a Blue Moon was the second full moon in a month, which the almanac never said. The almanac’s actual 1937 Blue Moon fell on August 21, which is plainly not a second full moon in a month. Sky & Telescope nevertheless repeated Pruett’s version in a May 1950 note.

The misreading spread slowly. On January 31, 1980, the radio program StarDate used the second-full-moon definition. Sky & Telescope’s authors found that the script, written by Deborah Byrd, cited Pruett’s article in a footnote. A 1986 edition of the game Trivial Pursuit spread it further. By the time Sky & Telescope’s authors traced the error back to their own magazine, the calendar definition was the one most people knew. Their conclusion was to treat both as Blue Moons, and this site does the same.

Does a Blue Moon look blue?

Not normally. Neither rule has anything to do with color, and the Farmers’ Almanac points out that the Moon does not normally look blue. A Blue Moon is an ordinary full moon on the calendar. The phrase “once in a blue moon,” meaning rarely, is much older than either rule. NASA notes the earliest recorded English use of “blue moon” in 1528.

How to watch a Blue Moon

Treat it like any full moon. The table above lists the date and the moonrise time for major cities. The best moment is moonrise near sunset, when the Moon is low in the east and tinted yellow or orange by the long path through the air. It will look larger there, too. NASA explains that this Moon illusion is a trick of perception, since photographs show the Moon is the same width on the horizon as overhead. A Blue Moon can also be a supermoon, and the table says when. For the names of the regular monthly full moons, see the full moon names overview.