Blue Moon: Science in tradition

The full moon on May 31 will be called a ‘Blue Moon’. When two full moons occur within the same calendar month, the second full moon is popularly known as a Blue Moon. Despite its name, this Moon neither appears blue nor is it connected to the expression “once in a blue moon,” a phrase that entered the English language in the 17th or 18th century to describe something rare.

The term Blue Moon, in its astronomical sense, does not originate from classical Latin or traditional British English. Instead, it developed in North America in connection with seasonal cycles and agricultural practices.

In Europe and North America, the year is traditionally divided into four seasons – spring, summer, autumn, and winter – each lasting about three months. Normally, there are twelve full moons in a year, roughly one occurring every month. Farmers gave each full moon a distinct name associated with seasonal activities or natural events. Thus, the April full moon is often called the Pink Moon, the May full moon the Flower Moon, and the June full moon the Strawberry Moon, marking the season when strawberries ripen.

However, the lunar cycle does not fit perfectly into the solar calendar. One lunar month lasts approximately 29.531 days, while the solar year is 365.2422 days long. This means that a year contains about 12.37 lunar months. As a result, the “extra” fraction accumulates over time, and occasionally a year contains 13 full moons instead of the usual 12.

When this happens, one season ends up with four full moons instead of the normal three. Traditionally, in such a season, the third full moon was called the Blue Moon, while the fourth retained its customary seasonal name. This was the original meaning of the term.

The modern definition of a Blue Moon as the second full moon in a calendar month arose from a misunderstanding. In March 1946, amateur astronomer and science writer James Hugh Pruett published an article in Sky & Telescope magazine, in which he mistakenly described the Blue Moon as the second full moon in a month. Although technically incorrect according to the older tradition, this simpler definition became widely popular.

Public awareness of the term grew significantly after it was mentioned during the popular radio programme StarDate on January 31, 1980. Since then, science communicators and amateur astronomers have enthusiastically used the Blue Moon phenomenon to encourage public interest in astronomy.

Scientifically, a Blue Moon is simply the result of the mismatch between the lunar cycle and the calendar month. This creates several interesting peculiarities.

Most Blue Moons occur in months that have 31 days. The last Blue Moon in a 30-day month occurred on June 30, 2007, while the next such event will occur on September 30, 2050.

February can never have a Blue Moon because even in a leap year, the month has only 29 days – shorter than a complete lunar cycle. Interestingly, if a Blue Moon occurs on January 31, the next full moon may fall on March 1 or 2, allowing March itself to have a second full moon and therefore another Blue Moon.

Blue Moons are also sensitive to time zones. The Blue Moon of January 31, 1980, for example, was visible as a January Blue Moon mainly in North America. In many other parts of the world, the date had already advanced to February 1.

Similarly, when much of the world observed a Blue Moon on August 31, 2012, it was already September 1 in New Zealand. As a result, observers there experienced their Blue Moon on September 30 instead.

Although a Blue Moon does not normally appear blue, there have been rare occasions when the Moon acquired a bluish tint. This can happen when volcanic ash or smoke particles from large forest fires scatter red light in the atmosphere, allowing more blue light to pass through.

So, if weather conditions permit, do take a moment to observe the Blue Moon on May 31 this year. Most likely, the Moon rising near the horizon will appear reddish or golden rather than blue. Still, the occasion remains a fascinating reminder of the subtle interplay between celestial motions and the calendars by which we measure time.

(The writer is Director, Nehru Planetarium, Nehru Centre, Mumbai)